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    HeNe Laser Testing, Adjustment, Repair

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    HeNe Problems and Testing

    How Can I Tell if My Tube is Good?

    A variety of faults can result in a HeNe tube not working properly. These can be the result of a large number of problems including a bad tube, a bad power supply, incorrect input voltage (or configuration), incompatibility between the tube and power supply, bad or incorrect ballast resistor, or defective wiring.

    If all you have is a HeNe tube but no power supply, see the section: Testing a HeNe Laser Tube Without a Compatible Power Supply for ways to determine if the tube is good. The following applies to both bare HeNe tubes and laser heads though some of the inspection and/or tests will require removing the tube from any enclosure.

    Several types of problems can prevent a HeNe tube from lasing properly or make it hard to start:

    Testing a HeNe Laser Tube Without a Compatible Power Supply

    First time laser enthusiasts are often confronted with this problem: Becoming the proud owner of a HeNe laser tube but no HeNe laser power supply and possibly no specs for the tube or assurance that it is even good. What to do? How can a quick inexpensive test be done to determine if the tube works?

    First, you need to determine the tube's power connections. See the section: Identifying Connections to Unmarked HeNe Tube or Laser Head if you aren't sure.

    There are many ways to power a HeNe tube for the purposes of seeing if it produces a beam. Almost anything that can provide enough voltage to get a few mA through the tube will result in at least a momentary flash of laser light out the end if the tube is good. There won't be any way of determining output power or whether the tube meets specs, but the knowledge that it lases at all may be enough to take the next step - the purchase or construction of a proper power supply.

    It is easy to use the family microwave to see if the tube is gas-intact if the tube will fit inside. See the section: Using a Microwave Oven to Evaluate and Revive HeNe Laser Tubes. While this won't tell you if the tube lases, if it fails this test, there is no need to go further.

    To test for lasing, current must be passed through the bore of the tube. A couple of options for a quick test power supply are:

    Even a high voltage AC supply with appropriate current limiting can be used safely for a few seconds only. And even with the rectifier voltage, the tube will be restarting once per cycle which is hard on it so don't run that for too long either. None of these are suitable to operate a HeNe tube continuously unless proper filtering and starting circuitry is added to turn it into a proper HeNe laser power supply.

    Don't go overboard though: Too high a voltage applied in the wrong place can arc straight though the glass at which point you have a rather boring high-tech sculpture. :( A very high current can also damage the tube very quickly, thus the need for the current limiting ballast resistance.

    With these power supplies driving the tube, if there is any output beam, even if it is weak or in the form of short flashes, the tube is probably good. However, there is no way to tell if it meets specs since HeNe laser output power is only maximum over a narrow range of tube current and these quick test power supplies are at most controlling only average current, not instantaneous current as would be the case with a real HeNe power supply. But, at least you know the tube isn't dead.

    Quick and Dirty Universal HeNe Laser Test Power Supply

    The following simple device can be used to confirm that a HeNe laser tube or head is not totally dead. It won't be able to help with output power or be safe to run for more than a few seconds but should produce some coherent output from any HeNe laser from 0.5 to 40 mW or more.

    It consists of the following components in series:

    Wire the output of the transformer in series with the rectifier(s) and ballast resistors. The positive output goes to the anode of the HeNe head or tube; the negative to the cathode. It doesn't matter whether the laser has an internal ballast resistor. Insulate everything VERY well. :)

    Powering the laser should result in flashes of coherent light, probably at the power line frequency (60 or 50 Hz). The amount of light will not be that impressive even with a perfectly good high power laser since the current is nowhere near optimal for any length of time, if ever. However, the presence of laser output would confirm that there is life.

    WARNING: Since centertap of transformer secondary should be grounded, both outputs of the power supply will be floating with respect to ground. Take care.

    HeNe Tube Lases but Color of Discharge Changes Along Length of Bore

    I've only seen this on a couple of HeNe tubes but the appearance is quite strange. At the anode-end of one, which I call the "Northern Lights" tube, the discharge color is perfectly normal - white-ish red-orange (salmon color), just as would be expected in any self respecting HeNe tube. However moving toward the cathode-end of the tube, the color changes to what appears to be a much more blue-ish white color (relative to the normal white-ish red-orange, though in absolute terms it may be more of a pink), typical of some sort of contamination. However, having a tube with the behavior provided a way (as you shall see below) of relating power output to the color of its discharge.

    In this condition, the tube still lased at a power level which relative to its rated output, is approximately proportional to how much of the bore has the correct color. In this sample, about 2 mW for a tube specified at 4 to 5 mW. I don't believe the starting or operating voltage has been affected very much.

    The explanation that makes the most sense is that due to the discharge current in the bore, the few N2 and O2 atoms (and any other party poopers that may have entered without an invitation) are being ionized and pushed toward the cathode of the tube leaving the desired helium and neon atoms to play at the anode-end. The contamination, whether due to a manufacturing problem or an air leak, is so marginal that nearly all of the unwanted atoms are swept from about half the length of the bore. However, the other HeNe tube I have like this had the color change in the exact opposite direct - correct at the cathode but blue-ish-pink at the anode, also reduced power. I now suspect that it may have been internal contamination. More research is needed. :)

    Another unusual characteristic of the Northern Lights tube was that the output power (what of it there is) peaked at a current somewhat higher than expected (8 mA as opposed to the 6 or 6.5 mA typical of this size tube). I don't know whether this is simply due to the overall contamination or that more of the nasty unwanted ions being swept from the bore when running at a higher current.

    This tube had an unfired getter which provided a means of cleaning up the contamination without a refill. A few weeks later, I got around to making the attempt. And the results are.... See the section: Repairing the Northern Lights Tube.

    Starting Problems and Hard-to-Start Tubes

    Some tubes seem to practically start on their own. Other won't perform even when you stand on your head, hold your breath, and provide the proper chants and sacrifices. :-) Note that this may not be a problem under your control. Apparently, even the Ph.D. physics types at major laser companies may not understand why apparently identical HeNe tubes from the same production run may vary in their ability to start by a large amount. See the section: About Hard Start HeNe Tubes. HeNe tube itself isn't to blame.

    However, it could also be that your power supply operating voltage, ballast resistor, and other factors may need modification. Of course, if the system used to work reliably and suddenly died, an actual power supply or wiring problem is most likely though a dead HeNe tube is also possible especially if the system has been unused for several years. The discussion below is somewhat oriented to the situation where a HeNe tube or laser head is being assembled with a power supply (or parts have been replaced) and the combination just doesn't want to work properly. However, some of it also applies to actual failures as well. Where the power supply itself is suspect, see the section: Power Supply Measurements, Testing, Repair.

    There are several types of possible behavior depending on how well the power supply, ballast resistance, and HeNe tube are matched up, and if any of these as well as the wiring, are faulty. You first need to determine if the discharge is being initiated at all. If the starting voltage is adequate, there will be momentary flashes that may be extremely short and weak and only visible in a darkened room but operating current may not actually follow. Under marginal conditions, operating current will flow in response to the starting voltage but won't be maintained. These flashes will be brighter and longer in duration. The result may be a nice flashing laser. In fact, this progression is exactly what will be seen when operating a HeNe laser tube from a power supply on a Variac as the voltage is increased: Short flashes followed by longer flashes and at some point, a steady beam.

    WARNING: If your HeNe tube doesn't start after a reasonable length of time (like a minute), don't leave the power supply on overnight in a futile attempt to get it going. Starting is a stressful time for power supply components, especially some wide compliance designs, and an extended period with the very high starting voltage on parts of the circuitry may result in total failure. It could also result in electrical breakdown (arcing) inside the laser head or cable. If the laser is flashing, this may be ultimately bad for the tube as well. Turn it off, step back, and try to determine what is wrong.

    Where the power supply components and/or wiring is exposed and subject to dirt and grime, first, carefully clean everything to eliminate possible sources of electrical leakage, which can affect operation, particularly the very low current starting circuit. As an experiment, try warming up the unit (which drives off conductive moisture) with a hair dryer or heat gun on the 'low heat' setting. This may enable it to start more easily confirming the need for some housekeeping. :)

    First, vacuum and/or dust it off with a soft brush, then use mild detergent and water followed by isopropyl alcohol (rubbing or medicinal is fine as long as there are no additives). Give it ample time to dry completely. The hair dryer or heat gun can be used to help it along. You may now find that your starting problems have disappeared!

    If your tube or head has an external starting loop (not common, see the section: Power Requirements for HeNe Lasers), it must be cleaned thoroughly as well (or maybe it has become disconnected, is broken, or has shorted to the case!).

    There is also a possibility that something else is shorting out the power supply, possibly only when enough voltage is applied so it won't show up with an ohmmeter test. Sometimes, the ballast resistor inside cylindrical laser heads will arc to the case. This can be checked with an HV insulation tester or more easily for most people, by removing the end-cap(s) and visually inspecting (as well as smelling!) for evidence of arcing, or by disconnecting the anode wire and driving the tube directly from the power supply with an external ballast resistor.

    Assuming none of this helps, there are three types of behavior: (1) No action of any kind, (2) an occasional flash possibly at random intervals, and (3) a periodic flashing laser which never settles down to normal steady operation. However, the behaviors and their causes are not really always independent so read through all of the possibilities before replacing components or ripping your system apart!

    1. Tube does not fire at all. This means that the tube itself appears totally dead with no flashes inside and no evidence of a beam, even for an instant.

      This generally means that the starting voltage is inadequate for the tube or isn't reaching it, there are other circuit problems, or the tube is bad. Tubes with longer and narrower bores (capillaries) will generally require greater starting voltage and your power supply may just not be up to the task. While tube manufacturers generally specify a starting voltage of 7 to 10 kV (or higher), typical tubes will fire with 3 to 5 times their operating voltage. Thus, a tube that runs on 1,700 VDC will probably start on 5,400 to 8,500 VDC.

      • There may be too much leakage in the anode circuit preventing the buildup of adequate starting voltage. The problem may be in the power supply itself or in the wiring to the HeNe tube. Corona discharge or arcing can result from inadequate insulation or component spacing as well as sharp points in the wiring or connections. Dirt and grime may also reduce the insulation resistance. A sizzling and/or ticking sound along with the aroma of ozone are indications of this sort of HV breakdown. Highly humid conditions may make the situation worse. For pulse (trigger) type starters, there may be too much capacitance as well.

        In the case of an enclosed laser head with a HV (e.g, Alden) connector, HV cable, and internal (potted) ballast resistor, there may be a breakdown in one of these components and it may only show up when starting voltage is applied (not with an ohmmeter). Here are two ways of testing for this situation:

        • Disconnect the anode of the HeNe tube and substitute your own ballast resistor and wiring.

        • Remove the negative connection from the ballast resistor assembly entirely - float it so the starting voltage cannot arc to anything. Connect the negative directly to the cathode of the HeNe tube or laser head case as appropriate.

        If the tube now starts, one of the original components was faulty (most likely the potted ballast resistor assembly if the negative connection runs through it) and this will need to be replaced.

      • If you need to increase the input to start or obtain any sort of response but then must back it off substantially to reduce the tube current to the proper value, low starting voltage or one of the other related problems is indicated.

        Assuming the power supply and wiring check out and the tube is good, the only solution is to boost the starting voltage or use a different type of starting circuit (inverter instead of voltage multiplier, for example).

      • You may have an extremely hard-to-start tube. Whether this is just normal for your particular model tube or this particular sample, is due to it being old or unused for a long time, it is just tired with many hours under its belt, or some other problem, the result is that the specified starting voltage does not have any effect.

        Note that newly manufactured tubes requiring more than a second or so to start using a compatible power supply are usually rejected as defective and may end up in the hands of surplus dealers who may sell them as 'new' even though they don't meet specs. Thus, you may be more likely to end up with one of these hard starting tubes!

        • If a particular tube doesn't start right away because the tube itself is hard starting, cycle the power supply on and off a few times at about 1 second intervals. If it's going to start, it usually will by the third or forth try. I suppose the rapid rate of rise of the starting voltage does something useful. :) How well this works will depend on the actual tube and power supply but most power supplies should survive the abuse). While probably not a long term solution, if the tube starts, at least you know it isn't dead.

        • If the voltage doesn't discharge in a reasonable amount of time after powering down due to a lack of bleeder resistor, it may help to discharge the capacitance of the tube and power supply manually with a well insulated high value resistor (a few hundred k to a few M ohms) before attempting to restart. Again, the rapid rate of rise of the starting voltage helps to ionize the gas. Where the tube starts reliably when this is done, it may be possible to add a high value (e.g., several hundred M ohm) resistor permanently across the power supply output.

        • For HeNe laser tubes with exposed bores like Spectra-Physics side-arm tubes, touching various glass parts of the tube (NOT the high voltage connections!) might also provide a capacitive path for the starting pulse. This applies more to pulse starters or those with s substantial component of high voltage AC, not the typical voltage multiplier with a DC output. Older Spectra-Physics exciters tend to have up to 50 percent of the starting voltage in the form of an AC waveform. Double check that the laser head case is grounded - a missing ground may result in starting problems. (More on this below.)

        • Another test is to try the tube with reverse polarity on its input. Connect the positive output of the power supply to the ballast resistor (don't omit this!) and then to the cathode (can electrode) end of the HeNe tube. Connect the negative of the power supply to the anode of the tube. You are only doing this for testing! Do not be tempted to leave the tube wired this way permanently should it actually start.

          Based on tubes I have tested, the starting voltage is much lower with the anode and cathode connections interchanged. However, the voltage drop across the tube when running with reverse polarity is much higher than with correct polarity. Thus, the tube may not run within the normal operating voltage range of your power supply even if the discharge is initiated - it may just pulse.

          Nonetheless, even if it just pulses, at least you know the tube is not totally dead. If the tube is otherwise undamaged, there should also be an indication of (at least weak) laser output from the business end of the tube. Perhaps, all you need is a power supply with higher starting and/or operating voltage. An inverter type starter using a flyback transformer appears to be particularly good for hard-to-start tubes. Unfortunately, I do not know of any reliable way of determining the likelihood of success without actually trying it.

          I have one 5 mW HeNe tube that requires (depending on its mood) as much as 15 to 20 kV to start (it should be less than about 10 kV). However, once started, it runs with a normal operating voltage of about 1,800 VDC.

          WARNING: Do not let the HeNe tube run for any length of time with reverse polarity as damage may occur due to heating and sputtering at the anode end of the tube.

        • Carefully heating the tube (with a hair dryer, NOT a propane torch!) may help to get it started in some cases. I've only heard of one instance of this and have not tried it myself but it should be easy to experiment. A tube that responds to this treatment is probably one what just has trouble starting the first time after being off for awhile but restarts easily when warm. However, what may really be going on is that you are heating the power supply circuit board nearby reducing leakage in the anode circuit (see above) or bringing a ground in close proximity to the tube (see below).

    2. Tube flashes occasionally for the very briefest of instant, possibly at random intervals. There may or may not be a laser beam accompanying the flashes. Depending on the situation, it may just appear to give up and revert to (1), above. This may be a protective feature of certain power supplies or an indication that the starting pulse is being diverted by an electrical leakage path, corona, or arcing somewhere.

      This sort of behavior is probably more likely with a pulse type starter but can occur with other types as well. What is likely happening is that the energy is insufficient to fully ionize the gas inside the bore of the HeNe tube so the discharge doesn't 'catch'.

      In addition to the other possibilities listed above and below:

      • Check for missing grounds. Where a pulse starter is used or a marginally compatible power supply where the high rate of rise of the starting voltage is needed to start it is possible that like many fluorescent lamp fixtures, a grounded metal plate is needed near or in contact with the tube to provide a capacitive path to aid initial ionization of the gas. Operating the laser (or fixture) on an ungrounded outlet, not installing a foil shield or not properly grounding it, or replacing the HeNe tube with one that doesn't have such a 'feature', might result in erratic starting - perhaps no action at all or maybe an occasional flash but not actually remaining on or taking a long and random time to 'catch'. If putting your hand on the tube (carefully avoiding the high voltage terminals!) results in consistent starting, lack of proper grounding would be confirmed. (This applies to uncooperative fluorescent fixtures as well.) Add a wrap of aluminum or copper foil and attach it to earth ground. I have only heard of one case that might have been due to a missing ground so I suspect this is quite an unlikely scenario!

      • Make sure the anode leads are as short as possible to minimize stray capacitance.

    3. Tube flashes momentarily - longer than an instant possibly up to a second or two - but does not 'catch'. During the time the discharge is on, there is a laser beam.

      What happens is that the discharge is initiated but the voltage drops too much at the tube anode and the discharge goes out. This cycle repeats resulting in a flashing HeNe laser.

      To produce a stable discharge, the following must be satisfied:

      • The sum of the effective resistance of the power supply and the ballast resistor and the (incremental) negative resistance of the tube (dV/dI at the operating point) must be greater than 0.

      • The voltage across the tube must be above the minimum for the tube at the operating current.

      • The current must be above the minimum for the tube/power supply/ballast resistor combination.

      These factors are not independent. Since the negative resistance and sustaining voltage of the tube are not normally specified and depend on current, some amount of trial and error may be required to achieve consistent stable operation but in most cases it really is very easy.

      Cycling behavior can be due to several factors:

      • Poor power supply voltage regulation or excessive ripple. Until the tube fires, there is essentially no load on the supply resulting in much greater voltage than under load. Except for a high compliance type of design where this is needed to produce the starting voltage, minimizing this difference will improve stability and reduce the voltage needed for stable operation.

        If the transformer or inverter drops too much under load, the tube voltage may fall below the minimum for the tube/ballast combination as soon as it starts. This cycle will repeat continuously or it occasionally may catch.

        Use a higher voltage and larger ballast resistor, and/or increase the uF value of the main filter capacitor (and/or the one in the DC supply to an inverter type supply as well if it isn't regulated).

        Minimum capacitor values for less than 5 percent voltage ripple (typical voltage and current requirements):

        • Line operated supplies: .5 to 1 uF (2000 V, 5 mA).
        • Inverter output: .005 to .01 uF (10 kHz, 1,800 V, 4 mA).
        • Unregulated inverter input: 15,000 to 20,000 uF (12 V, 1 A).

        Actual ripple in the current to the tube may be several times greater than this since it depends on the change in voltage with respect to the total effective resistance of the PS+tube+ballast resistor combination). However, the resulting ripple in the optical output power will be 2 to 10 times lower than the ripple in the current depending on operating point. The lowest will occur around the tube's optimal current specification.

      • Ballast resistor too large for the operating voltage. The operating current falls too low resulting in increased (magnitude of) negative resistance. Once the total system resistance goes negative, the discharge becomes unstable and goes out. The result is a flashing laser like a neon bulb relaxation oscillator.

        For an unregulated power supply, increase the operating voltage and/or decrease the ballast resistance.

        For a regulated power supply, decrease the ballast resistance so that the voltage for the desired operating current falls within its compliance range.

      • Too much stray capacitance and/or inductance in anode circuit. The system is behaving like a relaxation oscillator as the capacitance charges and then discharges through the tube. The wiring inductance causes the current from the main supply to lag too far behind the starting current and the discharge goes out.

        Shorten the wiring - minimize the distance between the power supply and ballast resistor, the ballast resistor, and tube anode, and don't use long runs of high voltage coax (which may have higher capacitance). Increasing the energy of the starting circuit slightly may help as well.

      • For laser heads in particular, the additional capacitance resulting from the metal case may increase the minimum stable tube current by up to 1 mA or more - and thus require changes in the power supply and/or ballast resistor. So, if you tested the HeNe tube and power supply on your workbench but the enclosed system is unstable, this may be the reason.

      • Power supply polarity is reversed. The voltage drop across a HeNe tube operated with the cathode and anode interchanged is higher than under normal conditions. However, required starting voltage is much lower. The result is likely to be a pulsing laser. Double check your wiring and terminal connections. I have also seen commercial power supplies mislabeled! See the section Making Measurements on HeNe Laser Power supplies if you need to actually test for reverse polarity.
    Also see the sections: How Can I Tell if My Tube is Good?, About Hard Start HeNe Tubes, Testing a HeNe Laser Power Supply, and Power Supply Construction Considerations.

    About Hard Start HeNe Tubes

    As noted elsewhere, apparently identical HeNe tubes made on the same production line may differ widely in their starting performance. While one sample may start absolutely instantly, another one that is indistinguishable may take several minutes to wink on. A newly manufactured HeNe tube that requires more than a second or so to start using a compatible power supply is generally rejected as defective. Guess who may end up with these - surplus dealers, and ultimately, you! The condition may get worse with use so some high mileage tubes could indeed be a lot worse in this department - possibly to the point of being virtually impossible to start even if they were within spec when new.

    As far as I can determine, the fundamental physics behind this phenomenon may not even be well understood by the major laser companies. The only meaningful data is statistical, because even a give tube with a given power supply will have dramatically different start times from attempt to attempt, as will tubes built side-by-side through the entire production process.

    Tubes that are kept in dark cold environments for long periods of time don't tend to start well. But, once one of these tubes is started successfully, restarts will likely be instantaneous, or at least reasonably quick. However, left overnight, they will revert to being uncooperative.

    Also lower fill pressures and cleaner tubes make for hard starting - not to mention power supply variables.

    Some manufacturers (e.g., Melles Griot) use a conductive 'start-tape' running the length of the tube attached to the anode electrode to aid in starting. It's not even really proven that this improves performance (and I've found that it can be a source of electrical breakdown problems. I've never noticed any difference in the speed of starting after removing the start-tape). Uniphase had a pointed electrode inside the anode mirror sleeve to aid in starting but it isn't obvious that it made any statistical difference either. There has even been talk of using a trace of radioactive gas (as used to be common in neon indicator lamps and glow tube fluorescent starters), but this of course would probably not be a popular idea today!

    A given production line may still have hard-start related yield problems from time to time (which kind of suggests the Ph.D physicists don't understand it). Funny thing is, no one can tell anything that's different on a hard-starter versus a regular one.

    Possible Causes of No Output with a Normal Discharge Glow

    Where the HeNe tube starts - there is a stable glow discharge and it is the correct color (bright white-ish red-orange, see the section: How Can I Tell if My Tube is Good?), there are a few possibilities that are not due to a bad HeNe tube or laser head:

    And, for other-color HeNe tubes which have much lower gain for a given length than red HeNes, all of the above may apply. The following comments were prompted by questions about a non-lasing short green HeNe tube (similar to a Melles Griot 05-LGR-024, 215 mm in length:

    (From: Lynn Strickland (stricks760@earthlink.net).)

    Those things are touchy, touchy, little SOBs. They usually have an almost flat HR and OC combination. If it does lase, it will probably be a few tenths of a mW at best. Probably have to walk the beam AND tweak both ends for any hope. Try some magnets too, for 3.39 micron suppression. In general, low power greens are a bitch to tweak.

    Note that the green discharge is more 'pink' (red tubes more 'orange'). Fill mixture is a little different, but the different color mostly due to lower fill pressure - which is why greens have shorter lifetimes than red.

    Possible Causes of Low Output with a Normal Discharge Glow

    Where the discharge color is normal and the tube current is known to be reasonably correct but the output is weak, there can be a variety of causes. (Of course, most of this also applies to no output but this is covered in the section: HeNe Tube Problems and Testing.) The following applies when the output power just isn't as great as expected:

    Unstable or Flickering HeNe Tube

    Where your HeNe tube starts and lases normally but is unstable, flickering or going out and then restarting whenever the power line voltage dips slightly or for no apparent reason, the problem may be power supply or HeNe tube related. It is also not unusual for this to start happening after the system has been on for awhile due to characteristics of one or more of the components changing slightly. A different power supply or slight adjustments or modifications may make your HeNe tube happy, at least temporarily. However, where the HeNe tube is an inexpensive vanilla flavored variety, replacement may be the easiest solution if it turns out to be marginal. :-)

    Sputtering or Erratic HeNe Tube

    This behavior is somewhat similar to the some of the compatibility problems described above. However, it could be much more random and may only occur during warmup.

    The symptoms are that the tube may start normally but then go off and restart, possibly quickly and unpredictably. One possible cause is a bad internal connection between the cathode can and its attachment to the mirror mount where the negative lead of the power supply is hooked up. The type of construction susceptible to this malady is where a 'nipple' on the end of the cathode can is swaged (pressed/squished) into the mirror mount rather than actually being attached by spot welding or via a spring contact. After many thermal cycles, the swage can loosen resulting in intermittent contact especially as the tube heats and parts expand. Any sort of high resistance increases the required tube voltage since the mirror mount has a higher 'cathode fall' voltage drop. The discharge will likely go out and the power supply will then attempt to restart. In some cases, the discharge may strike to the mirror mount itself (look for a glow near the mirror) and if this persists, will eventually destroy the mirror. (See the section: Damage to Mirror Coatings of Internal Mirror Laser Tubes) After the tube warms up sufficiently, since aluminum expands faster than steel or Kovar, the problem may disappear once the connection tightens. However, until then, the intermittent contact and many restarts is hard on the power supply and possibly the mirror.

    Assuming the power supply and tube are properly matched and the power supply isn't defective, this is a defective HeNe tube. No cure is possible. This is a relatively unusual problem (I've only seen it in two (2) HeNe laser tubes so far) so first check external connections and make sure your HeNe tube and power supply are properly matched. If its maximum voltage is marginal, as the tube heats up, the voltage drop may increase just enough to result in erratic behavior. However, one possible difference between this and a bad cathode connection is that with the latter, the condition may clear up once the tube heats up since the expansion of the aluminum cathode will improve contact. The marginal voltage situation will just get worse. The power supply itself could also be defective. The easiest way to determine which is at fault is to swap the PSU and/or tube with known good units.

    Also see the section: Unstable or Flickering HeNe Tube.

    Cyclical Variations in Output Beam Power

    A HeNe laser that is in good condition will produce an output beam that is quite stable and will have no visible (at least by eye) variations in output power though a laser power meter will show fluctuations over various time scales of a few percent even after warmup unless it is high performance amplitude stabilized laser. A typical laser tube will have a "mode sweep" specification of between 2 and 20 percent depending on type and size (smaller tubes typically have poorer performance). As the tube warms up, this cycling will start out slowly, peak, and then taper off as the system stabilizes thermally. For longer HeNe lasers, in addition to the mode cycling at the output wavelength, there may be a slower power variation due to power stealing by the unwanted 3.39 um line if it isn't adequately suppressed by magnets or bore/mirror design. This would occur at a rate of 0.6328/3.39 as fast as the 632.8 nm mode cycling (for a 632.8 nm laser). If the laser output power is recorded over time, one would see the longer term variation superimposed on the shorter one.

    Note that if the discharge is actually going on and off, the cause is entirely different - an incompatibility with the power supply, incorrect ballast resistor, low line voltage, etc. See the section: Unstable or Flickering HeNe Tube.

    However, sometimes you will find a laser that exhibits significant periodic variations in output intensity even where the discharge is perfectly stable. There are two types of phenomena depending on the period of the power cycles:

    Reasons for Short HeNe Tube Lifetime

    As noted, sealed HeNe should last many thousands of hours with no noticeable degradation in performance.

    If you are experiencing excessively short life (e.g., a month instead of years), the first things to check are operating current and polarity. See the section: Making Measurements on HeNe Laser Power Supplies. Of course, if you omitted the ballast resistor, life will likely be very short. :-(

    If the HeNe tube and power supply are mismatched, one can damage the other. For example, running a 1 mW HeNe tube on a power supply designed for a 35 mW HeNe tube may not only result in too high a current by design (e.g., 8 mA instead of 3 mA) but may also result in much higher current if the compliance range of the power supply is exceeded (i.e., the voltage across the HeNe tube is much lower than the power supply can handle). Conversely, attempting to power a 5 mW HeNe tube using the power supply from a barcode scanner (designed for a .5 to 1 mW HeNe tube) will likely result in a blown power supply. Just because the high voltage connectors mate and/or the tube lights up doesn't imply anything about compatibility! Also note that maximum optical output occurs at the optimum operating current - too high or too low and it goes down. (Operating current for yellow, orange, and green HeNe tubes is even more critical than for the common red variety so setting these up with an adjustable power supply or adjusting the ballast resistance for maximum output is recommended.)

    New and even used HeNe tubes and power supplies from reputable surplus dealers will generally last a long time if not abused. But, much of what you get at swap meets and hamfests has been pulled from equipment for one reason or another. So, the problems you are experiencing may have nothing to do with your setup!

    (From: Lynn Strickland (stricks760@earthlink.net).)

    Speaking as a non-physicist....

    There are so many variables in a gas discharge, it's a game of averages. That's why the power supply business can be so tricky - and why, for the power supplies you can look inside of, you see so many modifications. That, and the rate at which electronic components go obsolete keeps it in a continuous state of flux (no pun intended).

    Reasons for the variability in lifetime and failure mechanisms from design to design revolve around design fill pressure and gas mix, operating current, distance from capillary bore-end to cathode, optical design (some designs are more sensitive to misalignment than others). Also power supply variability, ballast resistor value differences, operating current tolerances (often set at, say, +/-0.2 mA).

    Gas lasers can be a pain, but for a lot of applications, they're still the most cost effective solution -- in some cases the only solution.

    HeNe Tube Use and Life Expectancy

    You often hear that lasers like to be run to keep them healthy and maximize life.

    For both types of HeNe tubes (as well as other lasers), power and beam quality will peak only after some warmup period. So it makes sense to keep the laser energized continuously over the course of an application where these are critical but this has no bearing on any need to turn the laser on just to keep it healthy.

    Brown Deposits Inside HeNe Tube Bore

    Many surplus HeNe laser tubes - both hard-seal and soft-seal - will have various amounts of a brown material coating portions of the inside of the bore, usually toward the anode-end. The presence of these unsightly deposits has no significant impact on operation or power output but is an indication that the tube has seen a lot of use. On high power lasers with IR suppression magnets, the brown stuff will generally collect near the magnets with obvious effects of N and S polarity. I've yet to see an explanation for this phenomenon in any laser reference. One suggestion from someone from a major HeNe laser manufacturer was that it was material sputtered off the anode but the one below makes more sense.

    (From: Chris Leubner (cdleubner@ameritech.net).)

    The usual cause is silicon being freed from the oxygen in the glass due to the intensely hot plasma on it. The ionized oxygen ends up reacting with the getter or cathode leaving elemental silicon film behind causing that brown look. In some tubes it will make a zebra or tiger stripe pattern on the bore that is a dead giveaway of both long use and plasma oscillation. On larger tubes that use magnets for IR suppression (Zeeman splitting), the magnetic fields smash the plasma into the tube wall and increases the rate of dissociation of the glass. The oxygen, which is a gas, will disperse throughout the tube and combine with the more reactive materials in it, namely the getter or cathode. The silicon will remain behind wherever it was separated because it is not volatile and relatively difficult to ionize. I do not know why it appears first on the anode end. My guess is probably due to the larger number of negative ions there reacting with the silica in the glass via this reaction: SiO2+2Ne-1=SiO+O-2+2Ne. Then SiO+2Ne-1=Si+O-2+2Ne.

    Care of HeNe Laser Tubes

    All modern internal mirror HeNe laser tubes use hard-seal construction where everything but the mirrors (where the required high temperatures would destroy the coatings) use glass-to-metal seals. Mirrors are either sealed with frit (low temperature glass powder which acts as a sort of solder for glass), optical contacting, or are fully enclosed inside the glass envelope. None of these seals leak on any time scale that matters unless the processing was defective. Melles Griot quotes a 12 year shelf life but in reality, it's virtually unlimited.

    Note that frit is quite soft compared to even optical glass so don't unnecessarily abuse the mirror seals. Those with large amounts of frit like Melles Griot and Siemens are fairly robust. But the mirrors on those with only a thin frit line like Aerotech and Uniphase may pop off if whacked the wrong way. Unless your intent was to salvage the mirrors, this would be bad news.

    However, there are still many external mirror HeNe lasers that use soft-seals for the Brewster window(s) and these show up surplus with varying degrees of leakage. Tubes of the same age may differ greatly in their condition. apparently due to large variations in the rate of leakage. Where the discharge color is still a pastel but quite bright - somewhat more pink than normal, even with a bluish tinge - just running the tube for a few hours or days may clean it up irrespective of the condition of the getter because the cathode itself acts as a getter - a very slow one but good enough to scavenge a small amount of contamination. The typical discharge color that is still salvageable would be the "Minor" examples in Color of HeNe Laser Tube Discharge and Gas Fill, perhaps slightly worse. Even a HeNe tube that doesn't lase at all may benefit from this simple treatment. Periodically running soft-sealed HeNe laser tubes without getters or with exhausted getters is recommended. A few hours every month is probably adequate and this will extend their life considerably, possibly indefinitely. Note that any detectable (by eye) change in discharge color will be accompanied by a significant drop in output power. As the tube is operated, the discharge color will gradually approach the correct one. The last place where a normal color appears will be the expanded regions of tubing (e.g., in the glass tube that joins the side-mounted cathode to the bore in a Spectra-Physics laser). Here, the normal color is a nice orange but will tend toward pink or pinkish-blue with contamination.

    Remarkably, for a soft-seal tube, the bottom of the "Minor" samples may actually be easier to salvage by running for a few hours. I've revived both a very old SP-130B as well as a not quite so old SP-120 using this simple treatment. Both these lasers were discarded because based on the color of the discharge, the original owners thought they were too far gone for there to be any hope. The SP-130B only recovered to about one third its rated power (but it is over 30 years old!). Running it every few days for a couple minutes appears as though it will maintain that power indefinitely. The SP-120 was restored to essentially new specifications.

    However, if the discharge color is highly saturated red or blue (the bottom two examples in the above diagram) and/or there are visible striations of the discharge in the expanded regions of tubing, all hope is probably lost as no amount of operation or getter reactivation will make enough difference to matter. But there is nothing to lose by running the tube for awhile to see if a miracle occurs. :)

    When powering an off-color HeNe tube, keep in mind that the discharge voltage may be quite different than normal especially initially and may overstress the power supply if it doesn't have enough compliance. A brute force unregulated power supply on a Variac can also be used, adjusting the Variac to maintain a more or less constant current at the rated value for the tube. It's also nice to monitor the laser's output (assuming there is any eventually!) with a laser power meter to keep track of how the patient is responding to treatment. What may happen is that the power will initially increase, then decrease as the tube heats up and internal parts outgas, then gradually decrease again as the cathode acting as a getter scavenges the contaminants, and then level off. This process may take several hours or days. Powering the laser on successive occasions may result in increasing power levels if the process wasn't complete. In any case, it won't hurt to try.

    Hard-seal tubes generally - but not always - will not respond to these sorts treatments since there should be essentially no leakage over any time scale that matters but I imagine there are exceptions. I did have a modern Melles Griot internal mirror HeNe tube that had an off-color discharge and low power. Running it for several hours didn't help at all but activating the getter with my Solar furnace rig completely cured permanently (it's been over a year now with no degradation in discharge color or output power so this tube isn't a "leaker" but must have not have been properly processed at the factory). See the section: Repairing the Northern Lights Tube.

    I've found some hard-seal HeNe laser tubes where the gas fill was obviously contaminated on the shelf. One example was an HP 5501 two-frequency (Zeeman split) laser tube that hadn't been used in about 15 years. It wouldn't lase at all when first powered up. After running for a total of about 12 hours, it has recovered probably to essentially normal output. This type tube is of very high quality construction and no doubt was very expensive with glass-to-metal seals for electrical connections and mirrors fully enclosed inside the glass envelope. Leakage is unlikely so it must have been internal outgassing over time. Thus, even hard-seal tubes can suffer from soft-seal maladies! :)

    Note that end-of-life tubes will often show an off-color discharge which may be mistaken for leakage. Output power will be low or zero and there will often be evidence of shiny metallic sputtering deposits on the glass near the cathode can - a dead give away that the tube is end-of-life. These will not respond to any known treament.

    Troubleshooting an External Mirror HeNe Laser

    In addition to all of the problems of internal mirror HeNe tubes, external mirror lasers are subject to dirty optics and much more prone to have misaligned mirrors.

    Like their internal mirror counterparts, the general appearance of the output when non-lasing will be a diffuse blue, blue-green, or purple spot but no red light. If there is any evidence of a red beam, something may be marginal but it is lasing.

    If it won't start, then the tube could be up to air or there could be a power supply problem. Try another power supply if available. Or, see the section: How Can I Tell if My Tube is Good? for info on using a low level RF or microwave source to check for ionization.

    Assuming the tube lights up, follow the steps below to narrow down the cause:

    1. If possible, check the tube current. Though unlikely, a discharge current much much greater than optimum will result in low or no output beam (as well as overheating of the tube, ballast resistor(s), and power supply components.

    2. Compare the color of the discharge in the narrow bore/capillary with Color of HeNe Laser Tube Discharge and Gas Fill. The comments about output apply to red and maybe orange HeNe tubes; yellow and green HeNe tubes will likely produce no output at all unless the gas fill is nearly perfect. The normal appearance is a white-ish red-orange generally described as "salmon color" though there can be a fair range from more orange to more pink for a good tube depending on the exact gas fill He:Ne ratio and pressure. Lower pressure tends toward pink and may be normal for low gain non-red HeNe tubes to boost gain at the expense of tube life. Some of these may also be even more of a white-ish shade. To confirm, check the discharge spectrum, preferably with a spectroscope but a diffraction grating or prism may be adequate. It should be similar to the combination of the helium and neon spectra in Bright Line Spectra of Helium and Neon. If the color inside the bore appears normal, check the color of the discharge where it isn't as constricted - the color should be quite orange. (This can be seen in the funnel area near the anode on most internal mirror tubes or the expanded tubing sections on those like Spectra-Physics side-arm cathode tubes with exposed capillaries.)

      • Where both these colors are correct (salmon and orange), gas fill is probably not the immediate problem.

      • If the discharge color in the expanded areas is the same as the bore or more towards white or blue, the gas fill is somewhat contaminated and marginal - lasing may not occur (definitely not for yellow and green HeNe tubes).

      • A slightly more pinkish discharge in the bore but with the normal orange color elsewhere may indicate low gas pressure and near/at end of life. There is no cure (but as noted above, this may be normal for some tubes).

      Firing the getter (if any) or just running the tube for an extended period of time may clean up any slight contamination (but won't help low gas pressure). However, if it is very pink, blue, purple, or white, a significant amount of air has leaked in over time, probably via the soft-sealed Brewster windows, and the only cure is likely to be a tube transplant. This is probably the most common problem with older external mirror HeNe lasers. Unfortunately, it isn't cost effective to refill them and replacement tubes are likely to be very expensive - if they are available at all.

    3. Check the mirrors, Brewster windows, and any other intra-cavity optics for damage and clean them if necessary using the proper optics cleaning technique.

      See the section: Cleaning of Laser Optics for the recommended procedure.

    4. The only remaining cause of a non-lasing laser with a proper discharge color and clean optics is mirror alignment. If the laser was dropped (Ack!) or someone decided the alignment screws were loose and tightened them, see the section: >Sam's Approach for Aligning an External Mirror Laser with the Mirrors in Place or the more general procedures starting in the section: External Mirror Laser Cleaning and Alignment Techniques.

      Of course, this assumes that the optics are correct for the laser or that someone didn't remove a mirror for use in their science fair project! Note that alignment is super critical, especially for a long HeNe laser. Thus, if misalignment is found to be the problem, it may require a lot of patience, determination, and the proper jigs, to remedy it. You won't succeed by luck alone (though luck may play a part)!

    External Mirror HeNe Laser Health Checklist

    When considering the acquisition of a large-frame HeNe laser, here are some specific things to look at or questions that can be asked of the owner which will help to determine if the laser is likely to be functional near original specifications. The following applies directly to external mirror Spectra-Physics HeNe laser tubes with minor modifications for other manufacturers/models:

    Once the laser can be powered up, check the discharge color in the bore. It should be similar to the bright white-ish red-orange or 'salmon' color at the top of Color of HeNe Laser Tube Discharge and Gas Fill, or of any other fully functional HeNe laser tube. If it does not, either the tube is soft-seal and has leaked, or it has been very totally abused. See the sections starting with: HeNe Tube Use and Life Expectancy. If the discharge color looks good, then very likely mirror alignment is all that is needed to achieve at least a substantial fraction of full power.

    Can I Increase Output Power Using the Waste Beam from the HR?

    When operating a bare HeNe laser tube, you've no doubt noticed the weak beam that exits from the supposedly totally reflecting mirror (the HR or High Reflector). (This assumes it isn't covered with tape or paint.) So, could the power from the output end of the laser be increased by putting another mirror behind the HR?

    The quick answer is that this might be possible in theory.

    The practical answer is: forget it.

    The long answer is too involved to go into here but if the extra mirror were properly aligned AND an exact multiple of 1/2 wavelength of 632.8 nm from the other mirror AND if there were no losses from the non-AR coated HR surfaces, part of the wasted power might appear at the output.

    But, in the end, all you would gain at most would be the couple microwatts that escapes out the HR. :) The lost power isn't much on most tubes. For those occasional tubes where the output is significant from the HR (either because of a mistake in manufacture or by design), there might be more benefit but as a practical matter, there is no way to satisfy all the conditions in a stable manner without a fancy feedback loop, if at all.

    (From: Steve Roberts (osteven@akrobiz.com).)

    Assuming it's a standard TEM00 mode HeNe and not a multimode laser, you'd see little tiny increases and decreases in the power on a very sensitive power meter as the mirror was translated toward and away from the existing rear mirror. But you would not really recover any of the rear beam, in fact you'd confuse the lasing going on inside the main cavity somewhat, and at certain possible "magic" combinations of external reflector and distance, cause lasing to actually cease. In practice, HeNe lasers tend to run by default at their maximum possible gain for a given combination of tube optics.

    If you want to see one wink out or flicker, precisely anchor it to a stable bench and then use a third flat mirror some distance away on a precision mount to reflect the output back down the bore. When the reflected beam is 180 degrees or so out of phase with the wave in the cavity, it will wink and flicker.

    (From: Sam.)

    I wonder about this...

    To actually interfere with lasing in a typical HeNe laser may be more difficult than Steve claims. While flickering and apparent instability will be seen if this experiment is done with a common HeNe tube, it may only be a result of the output beam interfering with itself outside the cavity when reflected back to the OC. This could appear to be confusing lasing but may actually not cause any substantial effect inside the cavity. Monitoring the waste beam (as noted below) can be used to determine whether the behavior is due to external or internal interference. If it's only external, the waste power will be almost unaffected (just the portion of the reflected output beam that gets back through both the OC and HR). This is likely to be less than 0.1 percent of the output power or a couple percent of the waste beam power at most. However, if actual lasing is being affected, the waste beam power will fluctuate significantly - up to (as Steve suggests), total wink-outs. :)

    (From: Bob.)

    On a somewhat related side note, there is at least one commercial instrument I know of that focuses the output of a HeNe laser onto a surface, and has a highly sensitive photodetector behind the HR of the laser (the arrangement Steve mentioned, but in reverse). As the surface the light is focused on moves back and forth in relation to the laser, the photodiode detects changes in output power out the back end. Basically, this is a form of a Fabry-Perot interferometer which can be used to very precisely measure small distances.



  • Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

    Connections, Electrical Characteristics, Output Power

    Identifying Connections to Unmarked HeNe Tube or Laser Head

    Where you can visually inspect the wiring, this is trivial as we all know that the large aluminum 'can' electrode is the cathode (negative) terminal.

    CAUTION: While most modern HeNe tubes use the mirror mounts for the high voltage connections, there are exceptions and older tubes may have unusual arrangements where the anode is just a wire fused into the glass and/or the cathode has a terminal separate from the mirror mount at that end of the tube. Take note of the cathode arrangement in particular because the tube will still lase perfectly if you attach to the mirror mount but instead of the actual cathode but that will result in sputtering near the mirror which is about the worst place for this - similar to running the tube on reverse polarity. (Miswiring the anode might result in no or weak lasing but probably no permanent damage.)

    Or, if the connector is the standard male 'Alden' type, the shorter (narrower) side goes to the anode (positive) and the longer (fatter) side goes to the cathode (negative). When such a connector is present, there will also be a ballast resistor (typically about 75K ohms) built into the HeNe tube assembly or laser head between the Alden's positive terminal and the anode.

    
                                  ________-__
                     Anode (+) ==|________|  |---_______
                             _____________|  |  |_______ HV Cable   
              Cathode (-) ==|_____________|__|---
                                          -
    

    Or see High Voltage Cable with Male Alden Connector. This one is built with separate wires and appears to have a ballast resistor built into the anode (red) lead (or maybe it's just a wart!). Many use coax similar in thickness to RG58U for the HV cable instead.

    Note: Genuine Alden brand connectors will have the name stamped on the plastic. Some power supplies may come with Alden compatibles without identification. This probably doesn't matter in any way, shape, or form, except as an indication that the power supply manufacturer installed the connector onto existing wiring or saved a few cents. :)

    Some Spectra-Physics lasers use a special 3 pin round connector (view is looking toward power supply):

                   O  Positive (Anode)
                   1
          GND  O 3   2 O  Negative (Cathode)
               
                   o  Interlock Prong
    
    

    The GND may not actually be present on some power supplies. In most cases, it is already connected to the negative elsewhere. The interlock prong activates a microswitch in the power supply to complete the primary-side circuit only if the power supply and laser head are securely attached. This provides protection for the power supply but isn't present on all models. (If your laser refuses to lase and there is no interlock prong, it's possible that the power supply requires it. It's either fallen or broken off, or the power supply isn't the one intended for your laser head.)

    Some larger HeNe lasers (mostly from Siemens and Spectra-Physics) use a somewhat similar but more rectangular connector but with 3 pins instead of 2. This connector typically has anode (positive, head will include ballast resistor), cathode (negative, head may include a small ballast resistor here as well), and case ground. It's usually fine to tie the cathode and ground pins together. I've also found that for some of these laser heads that have a cathode ballast resistor, bypassing it will reduce operating voltage requirements and still work fine though the claim is that stability will be better with it when used with the recommended power supply.

    However, suppose the whole thing is sealed and all we have are some dangling wires or an unusual unmarked connector? Here are some guidelines. Try to obtain agreement on several of the following tests as no single one is necessarily a guarantee of correct identification:

    Getting the HeNe Tube Out of a Laser Head Intact

    If the tube is bad and won't be salvaged, more drastic means can be used. However, assuming you want to extract it intact and with a chance of being reused, the choices narrow dramatically.

    With rectangular laser heads, the actual HeNe tube will probably be mounted in a sane fashion - with screws and clamps for example. So, no problem if you have the correct screwdrivers.

    However, for cylindrical laser heads, the tube may be mounted in a variety of ways. Just getting the end-caps off can be a fun experience as well. They may be mounted with screws or set screws (for which Murphy's law states you won't have the correct hex wrench), rivets (some drilling required), just glue (which will likely be hard and brittle by the time you need to do this - probably an advantage). As for the tube, there may be (plastic) set screws at 3 or 4 points around the outside in two locations - front and back. In this case, loosening the set screws should allow the tube to be slid out of the housing. If it still doesn't move, check for additional anchors or wiring connections at either end. If it still doesn't move, there may be some RTV, hot melt glue, or other adhesive in a hidden locations still securing it.

    Often, you will find that the tube itself has been set in place with Silicone RTV forced through holes on the side to keep it there. Unfortunately, removing these tubes intact appears to be right up there with dropping bare eggs from 10th story windows and having them survive unbroken in the level of difficulty department. :) However, it can be done without dynamite. (But, before going through any of the following RTV removal gymnastics, determine if the adhesive is actually something less stable than RTV, see below.)

    As we all know, Silicone RTV, a.k.a. GE Bathtub Caulk, be it white, black, or clear, is impervious to virtually everything but a good sharp blade. If there is enough clearance around the tube, it may be possible to slip a thin strip of metal in there and carefully slice the RTV from each end. I've done this to extract a couple of HeNe tubes intact. The first was dead (up to air) so I wasn't too worried about breaking it. I used thin aluminum strips (e.g., roof flashing) from either end and through the fill holes to grind away at the RTV until the tube could be removed - surviving with just a few scratches as aluminum is softer than glass! This literally took HOURS! However, there is often not even enough clearance for this to be possible. For my laser head, this was the case on *opposite* sides at each end even for the .015" aluminum. Only when enough RTV had been removed on the side with more clearance could it be worked loose. (In addition to the tube being dead, it had been mounted skewed in its cylindrical prison - someone must have had a really bad day when this thing was put together!) The second tube was weak (putting out only about 1/3 mW when it should have been 2 mW). It came out quite easily (still putting out only 1/3 mW) as the adhesive was localized and could be sliced with a single pass of my 'tool' for each small glob of RTV.

    Sometimes a hard non-RTV type adhesive is used in a similar manner to the RTV. For this, a narrow coping saw or model maker's saw blade between the tube and housing should work quite well.

    If you don't care about saving the housing, very carefully use a hacksaw to remove it as close as possible to the adhesive clumps (near the ends of the HeNe tube). This will make it easier to get at the glue with a thin knife, saw, razor blade, or that roof flashing. A copper tubing cutter may even work for this but go real slow or the distortion of the housing may crunch the tube. :(

    One might think a chemical exists capable of dissolving RTV that isn't totally toxic and disgusting. Such a substance would make this task a whole lot easier. Is there?

    (From: Mark Schweter (schweter@mail.bright.net)).

    Short of ashing the assembly (which will strip your wires for you too!), not really. (Considering the NON_toxic, NON_disgusting requirements - assuming you mean Silicone RTV, fuming HF or HNO3 comes to mind!)

    Fully cured RTV is fairly stable, unfortunately.

    You might try a NaOH solution to digest the RTV, if nothing else, it'll take the aluminum 'can' off! (NO smoking in the area PLEASE - H2 is released!)

    A thought occurs to me.... Get a 'slitting saw' or 'burr' and slice the aluminum can lengthwise, several times. Use a hot-knife to peel away the RTVed sections. Then use the hot-knife to pare RTV off glassware. My Weller soldering gun used to have one.

    (From: Mark Shipley (mark@startrek.com).)

    I have successfully removed an old Hughes HeNe tube from such a head by using an old piano wire (violin, cello, etc., as long as the wire was wound, it would work). (You hated the practicing, anyhow! :) --- Sam.)

    Pass the wire down the side of tube, anchor the end, say in a vice and slowly work the tube back and forth pressing the caulking against the wire. The wound wire cuts away at the caulking and after not too much time you should free the tube.

    (From: Dave (ws407c@aol.com).)

    I have yet to have a problem removing end-caps from the Melles Griot HeNe laser heads I have had after my tried and true tested method. :-)

    Fill a coffee cup about 3" high with BOILING hot water and let the head sit in it for about 10 min's. Repeat 3 times and the cap pops off by hand no problem. After it is removed, run your thumb around the inside to remove the remaining glue. Use a hair dryer to clear up the condensation inside the head from the process.

    Repeat for the other end. This has worked for most of the PMS and Uniphase heads as well.

    Removing a tube from ANY head is a cinch (if you're willing to sacrifice the aluminum cylinder) by using a hacksaw. There is no need to remove the end-caps in this case. First remove any set-screws. In Melles Griot heads there are usually two sets of 3 (alternating with glue-only holes). Use a sharp blade or Dremel(tm) tool to cut a slot in the plastic and then just unscrew them (COUNTER-CLOCKWISE!). Next roll the head across a table while making a mark around the middle of the head to follow with the hacksaw. Saw slow and carefully as not to nick the tube. The metal is soft and wont take too long to cut. When the cut is finished, squeeze some liquid dish-washing detergent (Ivory, etc...) into the head followed by some water. Give it a shake and then twist one way with the left hand and twist the other way with the right and the glue will give way most easily. :-) Make sure there are no set-screws hidden in the RTV or whatever it is. Once one end of the case frees up, cut the wires and pull it off. From here do the same for tube in one hand and half of head in the other. Once the tube is free and still soapy, pick off the rest of the glue and "starter tape". Then wash off the tube with fresh water and use a hair dryer to dry it off to prevent any trace of rust.

    I have done this over and over again without any problems or stress to the laser tube.

    (From: Sam.)

    CAUTION: If all you have removed using the hot water trick is one or both end-caps, DON'T attempt to run the tube until you are sure all moisture is gone from inside the head. Otherwise, there may be corona/arcing at various places which at the very least, will make it hard to start and may cause damage to the head and/or power supply.

    I have simplified this tube removal technique a bit if the end-caps have been taken off and don't need to cut the cylinder at all. Remove the six (6) nylon set-screws by first scribing with a sharp knife or Dremel cutoff wheel and turn COUNTERCLOCKWISE. Then carefully use a paper clip or knife blade to dig out the hot-melt glue in the other six (6) holes. (This helps to free up the attachment to the inner wall.) Put the head into hot water for a couple of minutes. Hot water from the tap is probably adequate and a bit of dishwashing liquid won't hurt to make it slide easier. The heat also expands the aluminum faster than the glass. Then, finger pressure alone on the metal cathode end-bell should be sufficient to break any remaining attachment of the hot-melt glue and slide the tube a fraction of an inch inside the cylinder. Then, just push it back out from the other end. It may take sevearl applications of hot sudsy water to loosen the tube but if the set-screws have been removed and the hot-melt glue holes cleared, it should work eventually. I've done this with several heads without damage to the tube inside.

    CAUTION: Since this is basically a fragile glass bottle you're trying to get out with some force (though hopefully not much), accidents can happen. Therefore, provide some protection between the tube and your fingers when pushing.

    I know this works with hot-melt glue-mounted tubes. This includes most or all newer Melles Griot tubes but some older Aerotech tubes use RTV which may not loosen up at all. If you're real lucky, your tube is just held in place with set-screws.

    But, the following would appear to be the definitive word on dealing with Melles Griot and other HeNe laser heads that use a non-RTV type rubber for mounting the tubes. Test a bit of the adhesive to determine if some heat will soften it - if so, your task is much easier.

    Replacing the HeNe Tube in a Melles Griot Laser Head

    So you found a new tube with 10 times the output power of that tired, bedraggled, worn old one taking up space inside a cylindrical laser head and it would be nice to replace it. This is possible as noted above but the following is perhaps the semi-official way of doing it and applies directly to most Melles Griot cylindrical laser heads but others may be similar.

    Note that some of the recommeded procedures will stink up the house so you may want to do this somewhere else like someone else's house. :)

    (From: Lynn Strickland (stricks760@earthlink.net).)

    The HeNe tube is usually mounted and aligned using nylon screws, then potted with RTV Silicone or hot-melt glue, and then the screws are cut off.

    1. Take off the end-caps (if it has any) by first clamping the head lightly in a vice. Heat the ends of the housing with a heat-gun, one at a time, and when the epoxy starts to smell, pull the end-caps off with a pair of locking pliers. Piece of cake, but don't burn yourself. Oh, some heads have pressed-in metal end-caps. If it does, you're on your own. (I have been known to use a pipe cutter though).

    2. Look at the head and find the potting holes. Half will probably be Nylon screws. Heat up an exacto blade and melt a small slot into the screws. Then unscrew them, and save them for later.

    3. If the alternate holes are hot-melt (and they probably are), put the entire head in the oven for 10 to 15 minutes to soften the hot-melt, then push the tube out of the housing. It kind of smells up the kitchen if you're not careful, and I take no responsibility if you burn down your house.

    4. Suspend the new tube inside the housing, using Nylon screws (you saved, them, correct?).

    5. Ideally, you would then re-tweak the mirror alignment once it's mounted inside the housing. A filed down hex wrench can be used on the locking collar at the cathode/output-end but it isn't easy and you'll probably be okay to skip it. A better method is to carefully drill access holes for a hex wrench opposite each of the three adjustment screws in the side of the cylinder. Tweaking the anode-end mirror without getting shocked is the real challenge though and I would definitely recommend skipping it!) Putting the end-caps back might help stabilize it a bit more, but you've won most of the battle already.

    (From: Daniel Matthews (daniel@wpmedia.com).)

    To disassemble, I first remove the screw in plugs by slicing into them with a hobby knife and then unscrew them. After that, I put on a pair of thick gloves and heat them in front of a ready heater until they're hot enough to push the tube out of the aluminum housing. Then, I clean the melted rubber off of the glass.

    I also have heads here that I reassembled. I put the centering plugs back in, screwed them all down flush leaving the tube snug and centered. Then, I inject black RTV Silicone into the other holes. After the RTV it cures then I trim the plug with a razor blade to leave a smooth fill level with the aluminum. Just looking at it, you can't tell they were ever disassembled.

    (From: Sam.)

    So with RTV, the next guy to attempt to disassemble the head will be using all the 4 letter words. :)

    Determining Electrical Characteristics of Unmarked HeNe Tube or Laser Head

    So, you found this fabulous HeNe laser in the dumpster and would like to power it. There are no markings of any kind - not even the manufacturer is known. If it is a laser head, the tube itself may be labeled - if you can get to it nondestructively. Where even the tube is unmarked, start by narrowing down the range of expected electrical characteristics: The rest is experimentation. You will need an HeNe laser power supply capable of handling a tube with the worst case voltage and current based on its size. Make sure you include a 75 K ohm ballast resistor of adequate wattage (10 W will be sufficient for anything up to 10 mA). A laser head will usually have an internal ballast resistor. Make sure the polarity is correct - see the section: Identifying Connections to Unmarked HeNe Tube or Laser Head. Once you get the tube to light, adjust the current for maximum beam intensity. Running at slightly higher than optimal current won't do any immediate damage but shouldn't be allowed to continue for too long. It's best to do this with a laser power meter but your standard complement of eyeballs will be close enough for most purposes. If using a meter (you probably won't notice the following effects visually), give the tube a few seconds to stabilize after a change in current - sometimes the power output may initially increase but then settle back to a lower level and you might as well operate the tube at the lowest current that results in maximum output. Then, label the HeNe tube or laser head with your findings so you will know how to deal with it the next time you pull it out of the cabinet. :-)

    Measuring the Negative Resistance of a HeNe Tube

    This is probably more of an academic exercise than anything else. However, a change in the tube's voltage versus current characteristics could indicate a problem with gas fill composition or pressure, or damaged or deteriorated electrode(s). A comparison between the suspect tube and an identical model that works could be revealing.

    For currents within and well beyond the normal operating range, a HeNe tube acts as a negative resistance - reducing the current results in an increase of tube voltage and vice-versa. Reducing current also results in an increase in the magnitude of the incremental negative resistance. Below 2 mA or so for a typical small HeNe tube, this magnitude rises so quickly that it is impossible to maintain a discharge even with very large values of ballast resistance. Going the other way, at some very large current (probably measured in amps), the incremental resistance turns positive (just before the tube melts or explodes!). For any given HeNe tube, power supply, and ballast resistor combination, there will be a range of current over which the discharge will remain stable. This is roughly the range over which the negative resistance of the tube plus the effective resistance of the ballast resistor, power supply, and regulator (if used) remains positive.

    Measuring resistance, negative or otherwise, is just a matter of determining the relationship of voltage to current for the device. It is trivial for common electronic components but more complicated for HeNe tubes due to the high voltage (particularly the starting voltage) produced by the power supply. (See the section: Making Measurements on HeNe Laser Power Supplies.) However, if you have a high impedance high voltage probe for you DMM or VOM, or a high voltage meter, it can be left attached even during starting without fear of a melt-down (though even its high resistance and small capacitance may alter tube behavior and/or prevent starting).

    One straightforward approach will require the following:

    A typical circuit is shown below:
    
                       Rb                     Rm
         HV+ o--------/\/\------+-------+----/\/\----+
                      75K       |Tube+  |    20M     |
                              .-|-.     |             / Close ONLY after
                              |   |     o         S1 |   tube has started!
                              |   |     +            o 
                          LT1 |   |     V            +
                              |   |     -           VOM (20M input, reads V/2)
                              ||_||     o            -
                              '-|-'     |            o
                      Rs        |Tube-  |            |
         HV- o---+---/\/\---+---+-------+------------+
                 |    1K    |
                 o -      + o
                 Current (I)
               1V/mA or direct
    
    
    Note: Where the VOM or DMM is connected after the starter (to the tube or head), a power supply with a high impedance parasitic voltage multiplier starting circuit is recommended to minimize the risk of damage to your meter should the tube drop out during the tests. The load of the meter will prevent such a circuit from developing significant damaging voltage. See the chapter: Complete HeNe Laser Power Supply Schematics for some suitable designs.

    To provide additional protection for your meter, consider putting a series stack of neon bulbs (NE2s, about 90 V each) across its input to bypass any voltage greater than the expected value while the tube is lit. For example, if the maximum range of your meter is 1 kV, use 11 or 12 NE2s.

    For the following, I assume the circuit above.

    I ran some tests on several small HeNe tubes using the following slightly modified circuit:
    
                       Rb                      Rm
         HV+ o--------/\/\------+-------+-----/\/\----+
                      75K       |Tube+  |     15M     |
                              .-|-.     |              / Close ONLY after
                              |   |     |          S1 |   tube has started!
        (From AT-PS1          |   |     o       +-----+
         or AT-PS2B           |   |     +       |     |
         depending        LT1 |   |     V    Rc |     o 
         on the tube)         |   |     -    2M /     +
                              |   |     o    +->\    DMM (10M input - Adjust Rc
                              ||_||     |    |  /     -    so that DMM reads
                              '-|-'     |    |  \     o    exactly V/10.)
                      Rs        |Tube-  |    |  |     |
         HV- o---+---/\/\----+--+-------+----+--+-----+
                 |    1K     |
                 | +-------+ |
                 +-| 10 mA |-+ M1 (Panel meter plugged into current sense test
                 - +-------+ +     points on AT-PS1 or AT-PS2B front panel)
    
    
    Depending on the voltage requirements of the tube, I used either Aerotech Model PS1 HeNe Laser Power Supply (AT-PS1) (tubes up to 1 mW) or Aerotech Model PS2B HeNe Laser Power Supply (AT-PS2B) (tubes above 1 mW). Current control was via the adjustable internal regulator when using AT-PS1 but with a Variac for AT-PS2B (its regulator is currently disabled). Both of these units have parasitic voltage multiplier starters and with the circuit wired as shown above, even if the tube cuts out, the maximum voltage doesn't go above about 2.5 or 4 kV for the AT-PS1 and AT-PS2B, respectively (maximum of 400 V at the DMM itself).

    And, yes, S1 is just a clip lead. :)

    The following chart summarizes the results (I was too lazy to graph these data or take measurements every .1 mA!):

              | Melles G.  Metrologic Spectra-P. Uniphase   Aerotech   Melles G.
              |  LHR-002     ?????       88         098       LT2R      LHR-080
      Current |  .5-1 mW     .8 mW     1.25 mW     1 mW       2 mW       2 mW
       I(n)   | V(n) R(n)  V(n) R(n)  V(n) R(n)  V(n) R(n)  V(n) R(n)  V(n) R(n)
     ---------+-------------------------------------------------------------------
      2.5 mA                          1135       1103
      3.0 mA    1141                  1095 -73K  1064 -70K
      3.5 mA    1110 -61K*  923       1062 -59K  1033 -57K  1667
      4.0 mA    1080 -58K   896 -46K* 1036 -46K  1007 -42K* 1631 -64K  1519
      4.5 mA    1052 -49K   877 -31K  1016 -31K*  991 -30K  1603 -51K  1480 -69K
      5.0 mA    1031 -37K   865 -23K  1005 -24K   977 -26K  1580 -47K  1450 -58K
      5.5 mA    1015        854 -21K   992 -23K   965 -22K  1556 -44K* 1422 -50K
      6.0 mA                844        982        955       1536 -40K  1400 -38K
      6.5 mA                                                1516 -36K  1383 -29K*
      7.0 mA                                                1500       1371
    
    The following are unusual or higher power HeNe tubes. The LHB-570 is actually a wide bore multimode one-Brewster HeNe tube so the 4 mW is actually only valid for a particular OC mirror. Note the low operating voltage and magnitude of of the negative resistance for this tube.
              | Melles G.   Melles G.
              |  LHB-570     LHR-050
      Current |   4 mW         5 mW
       I(n)   | V(n) R(n)   V(n) R(n)
     ---------+-------------------------------------------------------------------
      3.0 mA    1130
      3.5 mA    1100 -50K
      4.0 mA    1080 -40K
      4.5 mA    1060 -35K  2013
      5.0 mA    1045 -28K  1970 -73K
      5.5 mA    1032 -27K  1940 -50K
      6.0 mA    1018 -24K  1920 -35K
      6.5 mA    1008 -23K* 1905 -29K
      7.0 mA     995 -22K  1891 -24K
      8.0 mA     986       1881
      8.5 mA     980
    
    The '*' denotes the approximate recommended operating current for the tube (more or less guessed if the data wasn't available!). Below the lowest current listed for each tube, the magnitude of the (negative) resistance increased beyond the point where stability could be maintained with the 75K ballast resistor and the tube would not remain lit. It is interesting that the two lowest power tubes (both 12.5 cm long, bore approximately .5 mm) have their operating points close to the dropout current. Rb for these tubes is typically increased to 100K or more to assure stability.

    Due to the effects on the V-I characteristics with temperature, there was some drift in the readings. For example, going to the highest current listed above for a particular tube and then back to the lowest current resulted in perhaps a 1 to 2 percent change in voltage until the tube cooled down.

    More sophisticated analysis is left as an exercise for the student. :)

    Determining Output Power

    This would be easy with a laser power meter. However, most of us are not so fortunate as to own such an instrument. See the section: What Makes a Laser Power Meter So Expensive?. There are two aspects of this same problem: Note: There will often be a CDRH safety sticker (usually yellow or white) on the HeNe tube or laser head. The wattage listed on this sticker is NOT a reliable indication of output power. It is an upper bound and may be much higher than either the rated or actual output power. For example, a .5 mW laser will likely have a safety sticker value of 1 mW; a 1 or 2 mW laser will show 5 mW; and a 12 mW laser may show 15 or 25 mW. Some unscrupulous or careless HeNe laser or tube resellers will list this as the power output of the device - buyer beware! Few people can or will check this. If it sounds to good to be true, it probably is. :-(
    1. There are a few ways of determining the tube's specified output power:

      • Some manufacturers code the (usually minimum) output power into the model number. For example, Aerotech tubes and laser heads have a model number that is of the form: XYZ where X is the model designation, Y is the output power in mW (e.g., 2 = 2 mW, 05 = .5 mW, etc.) and Z is either R or P denoting a random or linearly polarized beam respectively.

      • Match the model number of the tube or laser head to the manufacturer's catalog listing. This may be easier said than done since many surplus tubes either don't have a model number printed on them or are old enough (but still perfectly good) so that the model is no longer listed in a current catalog. The manufacturer (if they still exist) will know and contacting them may be worth the effort. However, don't expect an overly enthusiastic response if you are asking about a 10 year old $20 HeNe tube! Any information so obtained may not be accurate either.

      • Attempt to compare the physical dimensions with those of tubes with known output power. This is not very reliable as the output power of a tube of identical diameter and length can easily vary by a factor of two or more by design or just due to sample-to-sample variations (at the time of manufacture, tubes are selected and sold based on their actual output power but they may appear to be physically identical).

      • Try to locate an indication on the tube itself of *measured* power output. Very often, the actual power output determined at the time of manufacture will be hand written or printed somewhere on the tube. This may be on the glass or metal shield (if used) or one of the ends. It may be on the outside of a laser head on the manufacturer's specification sticker (not the safety sticker, see above) or concealed inside. Examine every nook and cranny and the tube's secrets may very well be revealed!

      • Attempt to use the following equation to calculate expected output power:
                                              q * L
                           Po = T * A * I * (------ - 1)
                                              T + B
           
        Where:

        • T is the output coupler (OC) transmission in percent.
        • A is the cross sectional area of the beam.
        • I is a saturation parameter.
        • q is the small signal gain.
        • L is the gain length.
        • B is the sum of all cavity losses.

        For the typical internal mirror HeNe laser tube, q =.15/m and B will be close to 0 assuming there is no internal Brewster plate or etalon. A and L can be measured for your HeNe tube. Unfortunately, T and I are likely to be unknown but they can perhaps be estimated by comparison with another HeNe tube having a known power output. This would make an excellent exercise for the student! :-)

        However, what this equation does show is that all other factors being equal, when comfortably above the lasing threshold of (q * L)/(T + B) > 1, output power is proportional to bore length times its cross sectional area. But we already knew that!

        Of course, as noted above, the actual output power for any given sample tube of identical construction and dimensions can easily vary by a factor of two. The calculated value is at best the theoretical maximum - when the tube is new (or at its peak if initially overfilled with helium to compensate for loss over time), under ideal conditions, and possibly only on alternate Thursdays! :)

    2. It is much tougher to determine if the output of your HeNe tube is actually correct without a calibrated laser power meter. However, comparisons can be made.

      Maximum output power isn't achieved instantly for an HeNe laser when power is applied. Typically, it starts at 75 to 85 percent of its final value and reaches that only after a 10 to 20 minute warmup period. For long tubes or large frame lasers, an hour may be needed for the output power to stabilize. I've also noticed that power seems to peak and then decline slightly for many tubes during this warmup period. I don't know if this is an inherent properly due to the increasing temperature of the bore or just a matter of mirror adjustments not being optimal. Power also may take a few seconds or longer to stabilize after even a small change in operating current. Depending on where you are on the current versus output curve, it may go up and stay up, go down and stay down, or do one of these and then return to nearly its former value.

      In addition, for high power really long HeNe tubes (e.g., 15 mW or more) and/or unconventional HeNe tubes used in high quality lasers, there may be other physical factors affecting power output including mirror micro-adjustments, need for IR line suppressing or discharge stabilization magnets, rigid temperature and external force stabilized mounting, and even tube orientation (like: This Side Up!). In fact, where you have a weak beam or even no beam at all, gently pressing in the center of these long tubes (which bends them ever so slightly) can be a useful technique to determine which way the mirror alignment is off without actually touching the mirror mounts (though you will have to do this eventually to make the adjustments). In fact, just touching one side of the tube with your hand will cool it slightly and may result in a significant change in output power due to the change in mirror alignment due to thermal contraction and bending of the tube!

      For lasers with very long bores that are exposed (e.g., the SP-127), there may be one or more adjustments along the length of the bore to fine adjust its straightness. While slight misadjustment of these won't result in no beam, it could certainly greatly reduce power output.

      See the section: How Can I Tell if My Tube is Good?. However, none of these should be a major factor for small common inexpensive HeNe tubes (though there still may be some effects).

      • First, confirm that you are supplying the proper operating current. The output beam power will be maximum at the proper current - lower on either side. A power supply with a broken regulator could be producing greatly increased current which will result in much reduced output (and excessive heating, sputtering, and shortened tube life - and probably won't do the power supply much good either). At 2 to 3 times the rated current, there may be no beam at all!

      • If you have access to a working HeNe laser or a new HeNe tube with known output, this is best as the wavelength will be the same. But, keep in mind that a 4:1 ratio of beam intensity represents a perceived brightness ratio that is closer to 2:1.

      • If you have a (diode) laser pointer of known power and wavelength, it can be used. However, this gets to be complicated if the wavelength AND power differ (as is likely with many laser pointers (670 nm) at this time. In addition, power levels for laser pointers are maximums and the actual power is not generally known.

      Estimating relative power works better on your finger or palm (don't worry, you won't even be able to detect a 5 mW HeNe beam on your flesh from the any heating effect but don't do this with a 20 W argon laser!) in the raw beam than on a white card unless the beam is first spread out using a lens or equivalently and more easily accomplished, you view the spots through a lens to make them appear fuzzy. In either case, the amount of perceived beam spread depends on output power and the difference is much more apparent than just looking at a tiny bright dot.

      Both the perceived brightness AND the size of the spot will vary with HeNe beam power. After a little practice, estimating the output power will become second nature - sort of like recipe measurements: "just use a pinch of salt in the stew!". However, if you have a collection of neutral density filters, you can use these to match brightnesses which may be just a bit more precise! The laser power meter would be even better. :-)

      For relative power measurements, either of the simple laser diode based laser power meters described starting in the sections: Sam's Super Cheap and Dirty Laser Power Meter will actually work quite well. If you can calibrate one of these with a HeNe laser of known power output, better than 5 percent accuracy is easily achieved.

      Just give the laser enough warmup time to stabilize (10 minutes for a small HeNe tube, up to an hour for an 8 foot long SP-125!). See the section: Measuring HeNe Laser Output Power for additional tips.

      Measuring HeNe Laser Output Power

      Follow these steps to reliably measure the output power of your HeNe laser using a laser power meter (either homemade or a real one!):
      1. Set up the laser and power meter so that the entire beam falls on the sensor but covers as much of it as possible (or convenient). For a very small silicon photodiode, this may mean a combination of putting the sensor close to the laser (careful of the high voltage if a bare HeNe tube!) and using a focusing lens to reduce its size (but not so small as to concentrate it in a single dot!).

      2. Allow the laser to warm up. Initial power is generally much lower than after the tube has been on for 10 to 20 minutes (even more time may be needed for a long HeNe tube or large frame HeNe laser to stabilize). The average power may start out reduced by 15 to 25 percent or more and gradually creep up to its final value, often overshooting a bit before settling down. (I don't know whether the overshoot behavior is a fundamental characteristic of a HeNe laser or a symptom of less than perfect mirror alignment. However, I've seen it on too many tubes to be a coincidence.) In addition, for a normal (non-stabilized and non-precision) HeNe laser, expect a short term oscillation in power typically over a 1 to 5 percent range with a period of 10 to 30 seconds due to longitudinal mode cycling - just get used to it! (The amplitude for longer tubes may be lower since a larger number of longitudinal modes can be active simultaneously and their effects average out.) This may continue (though possibly at a reduced amplitude) even after infinite warmup time.

      3. Set the HeNe tube current to its recommended value or adjust it to maximize power output. I have found that the latter current is often lower than the manufacturer's current specifications. With the latter technique, allow time for the power to stabilize after a small change. For example, reducing the current by .5 mA may result in an immediate increase in output power of 5 percent, but this may settle back down and depending on where on the curve you are, may end up lower or higher than the original setting.
      Once the laser has warmed up, you are ready to take a reading. Where the power is varying due to mode cycling, unless you have a data acquisition system and data processing software, the best I can suggest is to eyeball the max, min, or average value of the readings as desired. This really isn't too difficult. :)

      A silicon photodiode or solar cell based power meter is quite linear with respect to laser beam power. For maximum accuracy, subtract or zero out the dark current (with the sensor covered) and locate the sensor far enough from the laser output aperture to minimize pickup of the glow of the discharge (though neither of these is a serious source of error unless you are measuring in the microwatt range).

      • If calibrated with an HeNe laser of known output power, absolute accuracy can easily be better than 5 percent even for a power meter using the photodiode from a discarded computer mouse or solar cell from Radio Shack! :)

      • If a calibration reference is not available, using a conversion factor of 0.42 mA/mW won't lead you too far astray. That value appears to be fairly accurate for the photodiode array from an old Mouse Systems optical mouse (the type with the red and IR LEDs shining out the bottom) and falls in between a couple of other detectors I've tried:

        • Photonics Detectors, Inc. part number PDB-V107 (3 mm x 6 mm): 0.41 mA/mW (about $2.00 from an electronics distributor like DigiKey).

        • Photodiode array from Mouse Systems optical mouse (1 mm x 3 mm): 0.42 mA/mW (free).

        • Photodiode from IBM barcode scanner (2 mm x 2 mm): 0.43 mA/mW (free).
        Other silicon sensors may have somewhat different sensitivities but they are still likely to fall between 0.40 and 0.45 mA/mW at 632.8 nm.
      For lasers of other wavelengths, sensitivity may be quite different. For example, based on the Typical Silicon Photodiode Spectral Response, it may be 50 percent or more lower at 488 nm from an argon ion laser. For near-IR, silicon photodiodes will probably have a somewhat higher sensitivity which then drops off but is still usable beyond 1,064 nm. However, the exact response curve is dependent on many factors so the power meter really needs to be calibrated for each wavelength.

      Testing of Non-Red HeNe Lasers

      HeNe lasers producing yellow (594.1 nm), orange (611.9 nm), or green (543.5 nm) beams are much more finicky with respect to everything! This is due to the fact that the gain for these lines is much lower than that at the common red (632.8 nm) wavelength. Power supply current, mirror alignment, and even the mounting of longer tubes, must be exactly right or performance may be affected dramatically.

      For visible non-red HeNe lasers:

      • Operating current - The range over which a non-red HeNe tube will output any sort of beam may be much narrower. Thus, an adjustable power supply (or adjustable ballast resistance where the power supply is not regulated) is recommended when setting up one of these. Just using any old power supply brick may result in poor performance or no output at all.

      • Mirror alignment - Drift in alignment which would only reduce the output of a red HeNe laser by 25 percent could conceivably result in nothing at all from a non-red HeNe tube. Thus, checking mirror alignment when a non-red HeNe laser head or tube is acquired is prudent. I routinely do this even for red HeNe lasers and have often found substantial errors even for supposedly properly adjusted lasers.

      • Mounting location - For longer tubes in particular, the points where they are attached to the laser head may affect power output due to distortion of the glass envelope and bore. The manufacturer usually recommends a two point mounting, typically near each end of the glass of the tube. This is probably where it was set up in the factory as well and is therefore likely best for actual use.
      HeNe lasers producing IR (1,152.3 nm, 1,523.1 nm, or 3,391.3 nm) shouldn't be nearly as critical, at least with respect to losing the beam entirely, as these have much higher gain than red tubes. However, power output and beam quality could still suffer where the conditions are not optimal.

      For more information, see the sections starting with: Problems with Mirror Alignment and the chapter: HeNe Laser Power Supplies.

      Identifying the Manufacturer of a HeNe Laser Tube

      For the most part, this is a curiosity thing. The future of the Universe won't be affected if you don't know who made your HeNe tube. However, it is sometimes useful to be able to identify the specific model and knowing the maker can help so the dimensions and style can be matched to a catalog or database entry.

      Some physical characteristics of HeNe tubes from various manufacturers are summarized below. Except for Melles Griot, this is from a rather limited sample so just use it as starting point. Unless otherwise noted, mirror mounts are the common 'you bend it' type. Some specific models - usually old, long, or other (than red) color tubes may have actual three-screw adjusters (not locking collars but permanently attached versions of the type shown in Typical HeNe Tube with Three-Screw Adjusters Added). Really old tubes will have mirrors Epoxied to fixed glass or metal mounts with no possibility of adjustment (though for those with exposed bores such as many Spectra-Physics models, very slight distortion of the glass will affect alignment though it's hard to devise a way of stabilizing any improvement.

      The tubes from the following 5 manufacturers are really very similar in terms of overall design (though I would assume that company proprietary details vary significantly). See Typical HeNe Laser Head for an example of this tube construction and its mounting in a cylindrical laser head. Tubes using the mirror mounts for both power supply connections are by far the most common for internal mirror HeNe lasers:

      • Melles Griot - For all but the smallest tubes (those less than 6 inches in lengths), the anode-end is rounded (most common) or tapered glass with a very small anode gas space inside the tube. The cathode-end is a slightly tapered metal plate. Most older non-barcode type tubes have three-screw locking collars installed on at least one end. These collars are also used for mirror alignment at the factory and the screws are then often sealed with red or gray Loctite(tm) or something similar, but they can usually be freed if field realignment is needed. Where locking collars can be installed, the gap in the restricted region of the mirror mount is quite wide - around 1/8th of an inch. As a cost reduction or perhaps as a result of years of experience showing that the mirrors tend to walk off on their own with locking collars, many if not all of Melles Griot's newer tubes have a narrow restricted region and don't accept the normal locking collars - maybe there is a special version with narrow tip set-screws. Most tubes include a ring shaped getter electrode with no visible getter spot. The metal cap at the cathode-end is butt joined for the glass-to-metal seal. The cathode can is only 1/2 or less of the length of the tube for all but the smallest barcode scanner tubes. See: Typical Small to Medium Size Melles Griot HeNe Laser Tubes, Typical HeNe Laser Head (Melles Griot), and Three-Screw Locking Collars on Melles Griot HeNe Laser Tubes.

        Aerotech (now a part of Melles Griot, these HeNe lasers are no longer being manufactured as far as I know) - Larger tubes have tapered glass anode-end and metal cap at cathode-end. Small to medium size Aerotech tubes have metal end-caps at both ends. Large cup-shaped anode gas space but otherwise quite similar to Melles Griot designs. See Typical HeNe Laser Tube Structure and Connections for a diagram that approximates an Aerotech mid-size tube (however, there may not be any getter electrode).

      • Siemens - For all but the smallest tubes, these have dome shaped metal caps at both ends with a flat very small anode gas space. The glass tube overlaps the edge of the end-caps in the glass-to-metal seal. One or more square textured flat getter electrodes (depending on tube length) with no visible getter spots. The cathode can is only 1/2 or less of the length of the tube. A typical example is shown in Siemens LGR-7641S HeNe Laser Tube.

      • Spectra-Physics - Heavy glass walled tube overlapping flat metal end-caps at both ends. Funnel-shaped anode gas space. Cathode can 1/2 or less of the length of the tube. Ring-shaped getter electrode and visible getter spot may be present. An older Spectra-Physics soft-seal HeNe tube is shown in Spectra-Physics Model 084-1 HeNe Laser Tube. Except for the unadjustable mirrors, this design is very similar to modern Spectra-Physics HeNe tubes.

      • JDS Uniphase - Long tubes tend to be narrower than other manufacturers. Funnel-shaped anode gas space with metal end-caps butt joined at both ends. A typical example is shown in Uniphase 098-1 HeNe Laser Tube.

      Linearly polarized versions of some models are available which add a plate at the Brewster angle inside the laser tube near the HR mirror. A few are also available with one or two Brewster windows or 0 degree (perpendicular) AR coated windows instead of mirrors.

      The smallest Melles Griot, Siemens, and Uniphase HeNe tubes are all similar with butt-joined end-caps and a cathode can which is nearly the entire length of the tube (I don't have any samples of really small Spectra-Physics tubes). Typical examples are shown in Small Melles Griot HeNe Tube and Uniphase HeNe Laser Tube with External Lens, the latter being a normal tube with a negative lens glued to its OC (removed by soaking the end of the tube in acetone overnight) to increase its divergence for the barcode scanner application (a second positive lens about 4 inches away was used to recollimate the beam.

      The following are based on a slightly different architecture:

      • Hughes (some models are now manufactured by Melles Griot) - Anode and cathode electrical connections are all made at one end of the tube. Except for the mirror mounts which are made of metal on newer tubes, the rest is of all glass construction. The cathode is via a separate terminal pin while the anode will either be the mirror mount at the same end of the tube (if metal) or to its own pin (if the mirror is soft-seal to glass as in older tubes). See the diagram in Hughes Style HeNe Laser Tube and photo in Hughes 3227-HPC HeNe Laser Tube. The capillary runs nearly the full distance between the mirror mounts (compared to the previous designs where it must remain at least 1 radius to 1 diameter away at the cathode-end). The cathode also usually runs most of the length of the tube. In the true Hughes design, there will also be a short tube surrounding the capillary near its end. A side-hole in this tube channels the cathode discharge. There may be a ring-shaped getter electrode with or without a visible getter spot. (Small tubes may only have a half-ring.) If installed in a laser head, the connections (and possibly the ballast resistor) will likely be covered with a molded in-place rubber insulator.

        A common mistake is to attach the PSU negative voltage to the mirror mount at the opposite end from the terminals instead of its terminal pin. The tube will lase but damage may result if left running this way due to heating and sputtering at the mirror mount attached to the negative lead of the PSU now being used as the cathode. Similarly, if the anode connection is attached to the cathode-end mirror mount, the discharge will bypass the bore, there will be no lasing, and the power supply or ballast resistor may be damaged.

        There are only two advantages to the Hughes style design that I can see: (1) The 'all connections at one end' construction may be required in certain retrofit or replacement situations and (2) since the actual length of the capillary can be somewhat longer for a given tube size, slightly higher power may be possible. However, Hughes style tubes would seem to be more complex and expensive to manufacture which is probably why you won't see many new instances of this construction.

        One (and probably two) Brewster tubes are also available. Linearly polarized Hughes HeNe tubes may actually be one-Brewster tubes with an external OC mirror fastened to the end of the tube.

      • NEC - Very similar to Hughes with all wiring at one end of tube as shown in NEC Style HeNe Laser Tube. Toshiba HeNe tubes are also built this way. Small Hitachi HeNe laser tubes are also similar.

      • PMS/REO - Similar to Hughes and NEC but their envelope is mostly of metal construction and also serves as the cathode internally. These are generally considered to be of very high quality but this is mostly due to the optics, though the metal envelope may help. The only tunable HeNe laser currently sold commercially uses a one-Brewster tube of this type. See: PMS One-Brewster HeNe Laser Tube. PMS/REO internal mirror HeNe laser tubes look similar but have an normal mirror mount instead of the Brewster window. Some older and/or "other color" tubes also include a heater coil glued to the mirror mount at the metal end of the tube presumably to reduce warmup time.

      Really old HeNe soft-seal tubes are often more along the lines of the Hughes style. Spectra-Physics (mostly older) HeNe tubes generally put the cathode in a side-arm with the bore exposed. This makes sense for laboratory lasers where magnets and such are required to be close to the discharge but is an awkward bulky fragile design for small tubes.

      Additional examples of all of these can be found in the Laser Equipment Gallery under "Photos of Assorted Helium-Neon Lasers".

      And, I'm sure there are all sorts of exceptions and a HeNe tube may appear in style to a particular manufacturer but could have some other origin (like a foreign clone). In other words, your mielage may vary. :)



    3. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

      Internal Mirror HeNe Tube Optics

      Cleaning HeNe Laser Optics

      Fortunately, this is almost a non-issue for internal mirror HeNe tubes as there is only one accessible surface that matters and it is only in the output beam - not part of the laser resonator. Thus, no amount of gunk or dirk on its surface can affect laser action in any detectable way. However, if it is not clean, the output beam may become diffused or distorted. And, eventually the Anti-Reflection (AR) coating and the surface of the glass itself may be etched permanently by finger oils which turn acidic and/or damaged by gritty dirt. Of course, there may be external optical components like lenses, mirrors, and prisms that need to be cleaned and could also be damaged from abuse or neglect.

      Those who maintain lasers professionally will insist on the use of laboratory (gas chromatograph or spectroscopic) grade methanol and acetone. For small internal mirror HeNe laser tubes and their optics, this really isn't necessary. The type of isopropyl alcohol sold in drug stores designated medicinal (91%) is quite acceptable but you will have to gently dry off the cleaned surface - the impurities will result in a cloudy film if just allowed to dry. Even rubbing alcohol (70 percent) will work in a pinch. However, if you are cleaning the mirrors of an external mirror laser, see the section: Cleaning of Laser Optics.

      The surfaces of Brewster windows are somewhat sturdier than mirror coatings but without knowing the precise material, assume they are still relatively soft. When cleaning a Brewster window with the tube powered and aligned (e.g., there is an intra-cavity beam), my criteria for 'clean' is when the scatter off the outside surface is less than or equal to the scatter off the inside (inaccessible) surface. (Scatter here means the fuzzy spot of light appearing on the surface, not the actual reflection.) Unless the tube is damaged or defective, the inside surface should be about as clean as possible!

      Lens tissue is best, Q-tips (cotton swabs) will work. They should be wet but not dripping. Be gentle - the glass and particularly the AR coating on the output mirror surface (and other optics) is soft. Wipe (don't press!) in one direction only - don't rub. Also, do not dip the tissue or swab back into the bottle of alcohol after cleaning the optics as this may contaminate it. The alcohol should be all you need in most cases but some materials will respond better to acetone or just plain water. Just blowing on the surface so it fogs and wiping very gently may help to rid it of the last traces of residue from the alcohol. (Unless you have spectroscopic grade solvents, this latter method is probably best for clearing the dust that invariably settles on the surfaces of glass optics and Brewster windows after a short time, even when exposed to a clean environment.)

      Note 1: The purity of medicinal and rubbing alcohol would appear to vary quite a bit. Some cheap brands are apparently only water and isopropyl alcohol while high priced ones may contain ingredients that will cloud your optics. You may have to try a few before finding one that is fairly pure - or just go for the real stuff. :)

      Note 2: The adhesive useed to attach the cotton to the Q-tip stick is probably soluble in acetone and perhaps alcohol. Some of it will then go into solution to collect on your optics. Thus, a Q-tip wet with solvent should be used quickly and only once before being discarded.

      For red (632.8 nm) HeNe lasers, the exterior AR coated OC mirror surface should generally be a uniform blue or purple color when clean. However, I have seen at least one that was greenish. The AR coating on lasers of other wavelengths will likely differ in color, but it may not be obvious, especially for IR (or UV) lasers. About the only thing that can be said for sure is that the color of the faint reflection from the AR coated surface shouldn't include much of the lasing color. And, high quality broad-band AR coatings may come very close to being invisible!

      CAUTION: Don't overdo it - optical components may be bonded or mounted using adhesives that are soluble in alcohol or acetone (but probably not water). Too much and the whole thing could become unglued. I still haven't found the itty-bitty collimating lens I lost in this manner. :-( In addition, any plastic optics may be totally ruined by even momentary contact with strong solvents.

      And, about keeping the inner surfaces of those mirrors clean. You say: "I can't even get to them, being sealed inside the tube. What are you talking about?". Well, while the environment inside the HeNe tube should free of contamination, there can always be little particles of unidentified 'stuff' left over from the manufacturing process. So, while there are generally no restrictions on the orientation of these tubes, it is probably not a bad idea for them to be stored and installed horizontally if possible so none of that 'stuff' can fall on the mirrors. This might be excessive caution but it is usually quite easy and painless.

      Why You Shouldn't Touch the Mirrors or Mirror Mounts

      The laser resonator for internal mirror plasma tubes is totally sealed safely inside. However, there are still some very good reasons to avoid touching the output coupler or pressing on the mirrors and their mounts. Some comments:
      • The outer surface of the Output Coupler (OC) uses an Anti-Reflection (AR) coated mirror and putting your grubby finger prints on it will definitely mess up the quality of the beam coming out. Eventually, the finger oils, acid, dirt, grit, and bicycle grease will etch or otherwise affect the optical quality of the surface and AR coating.

      • The outer surface of the High Reflector (HR) usually doesn't pass any beam that matters so it isn't as critical unless you are using the low power beam from that end for optical feedback or something like that - in which case it is quite critical due to the low power of the beam exiting from that end of the tube.
      If you accidentally touch either mirror, carefully clean it, preferably with lens tissue and alcohol. See the section: Cleaning HeNe Laser Optics
      • Excessive pressure on either mirror mount may result in problems with mirror alignment but for most tubes, you would really have to work at this to have any effect. And, where there are adjustment screws of any kind, don't be tempted to tighten them up. :) If they appear loose, just touching them could mess up alignment. See the section: Problems with Mirror Alignment.

      • Of course, if the tube is powered, the mirror mounts carry the high voltage and high voltage return.

        • If the cathode is earth grounded, it would be safe to touch - at least you won't get a shock.

        • You definitely don't want to touch the anode if the cathode is earth grounded. Even if the entire affair is floating or powered via an isolation transformer, it is likely that the tube will go out due to the momentary change in capacitance as you touch it and then will attempt to restart. You will then likely get zapped by the starting voltage to your body capacitance - no matter how non-magnetic your personality may be! :-)

        Interestingly (and I definitely DON'T recommend this), touching the anode of one of those little bar code scanner power supplies like the one described in the section: HeNe Inverter Power Supply Using PWM Controller IC (IC-HI1) (well actually, precisely that one), resulted in only a mild tingle - and the smell of burning flesh. :-( Its maximum current is only 3 or 4 mA which is unpleasant but not really likely to be particularly dangerous. Then again, your body may react in unpredictable ways like throwing the entire affair across the room!

      Damage to Mirror Coatings of Internal Mirror Laser Tubes

      There are two ways the inside surface of the mirrors can be damaged on internal mirror lasers: by having "stuff" land on top of them or by having the original dielectric coatings come off. (Of course, the AR coating on the outer surface of the OC mirror can be damaged by overzealous cleaning or other abuse.)

      Sputtering Overcoat or Debris on Mirrors

      The most likely cause of damage to inner surface of the HR or OC mirror on internal mirror HeNe lasers is due to sputtering as a result of running the tube with reverse polarity for an extended period of time. Where the anode mirror mount is used as the cathode, sputtering of the metal will actually deposit a nice metallic film on the mirror in close proximity to it. After only a few minutes, the power will drop and lasing will eventually cease entirely. After 30 minutes, the coating may be so thick that the mirror will appear opaque. Note that strictly speaking, the coatings aren't actually damaged, just covered. So, a suitable acid wash or something might restore them to pristine condition. Unfortunately, this is tough to do on a sealed tube but might be possible if you salvage the mirrors for some other laser project. :)

      Apparently, the careful use of reverse polarity may actually be used by some manufacturers to 'tune' the power output of a HeNe tube. This might be needed to reduce the gain of a 'hot' tube that is lasing on an adjacent spectral line in addition to the desired one. However, I can't imagine any hobbyist wanting to ruin a perfectly peculiar tube of this type or to want to reduce output power on any laser! :)

      There are two ways for reverse polarity to occur depending on the style of the HeNe tube. However, they are both due to carelessness or lack of knowledge:

      1. HeNe tubes where the cathode connection is to the cathode-end mirror mount (e.g, the most common types) - In this case, accidentally reversing the power supply leads will do it.

      2. HeNe tubes where all connections are to terminals at the anode-end (Hughes style) rather than to the mirror mounts - It is easy to not realize or forget that the cathode-end mirror mount isn't supposed to be connected at all since there is a separate terminal via a glass feed-through for the cathode connection at the anode-end of the tube. I've done this a couple of times but caught it before detectable damaged occurred.

        As noted elsewhere, the HeNe tube will appear to operate normally - perhaps it will be even easier to start - but degradation will happen in short order and at that point, your options are quite limited - as in there are none.

        Of course, running a tube on AC will do the same thing and an autopsy of one that had died in this manner showed a clear indication of a dark overcoat on the HR mirror, though it wasn't obvious from external examination.

        A drop in power even with correct polarity and current over the course of several hours may also be a result of sputtering but of the actual cathode electrode once it has lost its "pickling". See the section: HeNe Tube Seals and Lifetime. There is nothing that can be done for this either. However, check for other causes like mirror alignment and improper power supply current before giving up.

        A metallic coating on the inside of the glass anywhere in the tube except near the getter may be an indication that sputtering has occurred. For example, Melles Griot HeNe tube cathodes typically have several holes around their perimeter near the end cap/mirror mount. Metallic spots on the glass at these holes are a definitive confirmation of sputtering and likely means end-of-life.

        Running the tube with grossly excessive current (perhaps 2X optimal ormore) may also result in sputtering damage though other things will likely die first like the ballast resistor(s) or power supply.

        In rare cases, a bit of debris may find its way to a most inappropriate spot in the center of one of the mirrors. Despite clean-room assembly, foreign objects can find their way inside HeNe tubes! This is why I recommend storing and using laser tubes on their side, not vertically!). A speck of dust in exactly the wrong place can result in an interesting, though perhaps useless, multimode beam. :) Sometimes, careful tapping will remedy the situation. I don't know if other more drastic measures (like blasting with a YAG laser) have a reasonable chance of success

        Mirror Coating Vanishes

        This is a problem I guarantee you won't see everyday, at least not on internal mirror laser tubes. No, I'm not referring to sputtered electrode material or other debris on the mirror but something rather strange. :)

        I was sent a HeNe tube with a hole in the Output Coupler (OC) mirror. OK, it isn't quite a hole in the glass, but the dielectric coating on its inside surface is completely obliterated - as though someone had gone in there with abrasive and removed it - wiped it clean (a beautiful job, I might add!) - but only in the central area (slightly larger than the diameter of the actual bore, about equal to the diameter of the inside of the restricted area of the mirror mount - a coincidence?). And, the Anti-Reflection (AR) coating which is apparently placed under the mirror coating is totally intact (at least that's what it appears to be - there is about the same reflection from the inner surface and the AR coated outer surface).

        I have to say that this is the weirdest thing I've ever seen in some time. (Note that damage to external mirrors, even flaking, isn't particularly unusual depending on the storage conditions or prior cleaning attempts but such damage to internal mirrors is unusual. The second weirdest thing would be that HeNe tube where the discharge changes color from anode to cathode. See the section: HeNe Tube Lases but Color of Discharge Changes Along Length of Bore.) I can't imagine that this effect was a result of natural causes and consider any internal cause to be highly unlikely in any case. The discharge looks normal and the operating voltage is normal similar to that of other identical model tubes. The only conceivable explanation from within is that it was run with excessive current for an extended period of time somehow resulting in ion bombardment (inverse sputtering? - see below for some additional info) of the OC mirror which is at the cathode-end of the tube. I don't even know if this is theoretically possible. Since the HR mirror at the anode-end of the tube is in perfect condition, it isn't likely to be an internal optical effect either (too great a light flux in the resonator) since I would think that would do the same thing at both ends. The fact that the diameter of the clear area is significantly larger than the bore also precludes this possibility.

        Total reflection from the inner and outer surfaces of the OC in the area of the hole is about 2 percent which is too bad. I'd love to try to use this tube with an external OC mirror. However, the total single pass gain of a tube of this length is also only around 2 percent so there would probably be insufficient gain to sustain oscillations. At best, it would be marginal. I initially made a half-hearted attempt to get it to lase anyhow but nothing happened. Later, I did a more careful test with some success - see below.

        I've never ever seen a HeNe tube with any internal damage to either mirror before. Thus, I'm inclined to suspect an external cause. Maybe someone was using it to align a high power Nd:YAG resonator and forgot to remove the tube before firing up the big laser. POW! No more mirror. :) This, however, was denied by the former owner. Other possibilities are that the coating was of poor quality and flaked off on its own (though I could find no evidence of any debris) or that this tube was used as part of another high power and/or invisible laser for aiming purposes and the main beam accidentally made its way back to the mirror by reflection from the work-piece.

        I am attempting to find out more about the history of this tube. So far, what I do know is that it was originally part of a Postal scanner of some sort and was operational when removed from service. At some point between then and now, someone or something went in and did a thorough cleaning job. :)

        FLASH - Some new info: I just discovered that for at least the first 5 minutes of operation from a cold start, the negative discharge may decide to originate inside the mirror mount rather than where it belongs at the cathode. And, it may abruptly switch back and forth at random times. Whether this is due to a broken connection between the cathode and mirror mount (unlikely), depletion of the cathode 'pickling', or that the warranty has expired, I do not know. So, the inverse sputtering theory is back in the running even though it would seem more likely that this would more likely result in a metal overcoat than removal of the mirror coating!

        I have now taken some photos of this tube. See Melles Griot 05-LHP-120 HeNe Laser Tube with Missing OC Mirror Coating. The photo on the far left shows a normal 05-LHP-120 with the weird one sitting next to it. The middle shot is of the that one under power with the discharge to the cathode the way it is supposed to be. The photo on the far right shows the discharge taking place to the OC mirror mount instead - probably due to a bad connection between it and the aluminum cathode can.

        As promised, I did some more experiments in getting the tube to lase with an external mirror. It now produces up to about 0.3 mW acting as a two part resonator containing a low reflectance intermediate mirror. With the wiped-clean mirror properly aligned, the weak modes due to the slight reflection from it (in the original tube) and the extended resonator formed with the external mirror compete with one-another. As the tube heats and expands, the output comes and goes periodically. Pressing gently on the external mirror mount to adjust the length of the total cavity ever so slightly results in very distinct power cycles - the classic behavior of an interferometer. A very cool toy if nothing else. :) For more details on these interesting experiments see the section: External Mirror Laser Using HeNe Tube with Missing Mirror Coating.

        I have recently found a second tube with a similar electrical problem. The resulting sputtering has indeed overcoated the cathode-end mirror to the point that there is no longer any laser output but the coating hasn't fallen off yet. :) Unfortunately, the discharge doesn't remain inside the mirror mount long enough to try the obvious experiment to see if its coating will eventually flake off.



      3. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

        Problems with Mirror Alignment

        Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes

        Note: While written specifically with HeNe laser tubes in mind, the techniques described in the following sections apply equally well to HeNe, Ar/Kr ion, and other sealed laser tubes which have internal mirrors on compliant mounts (those attached to the end-caps via narrow sections of ductile metal tubing).

        Some (mostly older) HeNe and other internal mirrors tubes will actually have adjustment screws as part of the tube assembly. I'm not talking about the locking collars found on many Melles Griot and some other tubes to stabilize the mirrors. See Three-Screw Locking Collars on Melles Griot HeNe Laser Tubes. These may be used for adjustment but are not ideal for that purpose. Rather, some tubes have actual three-screw adjusters where the screws run parallel to the tube's axis and press against an adjoining disk. Selected models from Aerotech, Hughes, Melles Griot, Spectra-Physics, and others have been found to have these. Some like those on certain surplus (Xerox) Spectra-Physics laser heads are quite large with fine control of alignment. If your tube is one of these - and its gas fill is still good - the procedures below for mirror adjustment can be considerably simplified. No special tools will be needed and fine control of mirror angle should be easy to achieve with just a tiny (WELL INSULATED!!) hex wrench. This sort of adjuster can often be added to a modern tube as well. See Typical HeNe Tube with Three-Screw Adjusters Added for an example of one approach.

        Precise mirror alignment is critical to proper functioning of HeNe tubes and lasers in general. For a HeNe tube, the mirrors must be aligned (parallel to each other and perpendicular to the tube bore) to a pointing accuracy better than one part in 1/10th of the ratio of bore diameter to resonator length to achieve optimal performance.

        For a typical HeNe tube, this is one part in 2,500. If the alignment is off by one part in 1,000 (1 miiliradian or 1 mR), there will likely be no output at all. You won't fix this by trial and error! Spherical mirrors may have a somewhat wider range where a beam will be produced but still require precise alignment to achieve optimal performance. Alginment (and nearly everything else) is even more critical for HeNe tubes producing non-red (yellow, orange, and green) beams as these have much lower gain.

        I now routinely check mirror alignment on any HeNe laser heads or tubes I acquire by gently pressing sideways on the mirror mount at the cathode (grounded) end of the tube. I may also do the basic "walking the mirror" tests as described in the section: Walking the Mirrors in Internal Mirror Laser Tubes which will identify tubes where the alignment of both mirrors was never quite right (most likely when new from the factory). If I can increase power output by more than about 5 percent in either case after a 20 minute warmup, I will adjust alignment as described in subsequent sections, below.

        Where a HeNe tube produces a weak or low quality beam or doesn't lase at all and no other faults have been identified (such as improper operating current, or problems with the gas fill), mirror misalignment is quite possible. However, it does take effort to mess these up as the mirror mount tube(s) must actually be bent. Casual handling won't do it. It would have had to be dropped or used as a hammer! :)

        Other possible causes of less than perfect mirror alignment include the following:

        • Repeated thermal cycles may result in annealing of stresses left in the mounts when they were originally aligned during tube manufacture. Alignment of one or both mirrors may then drift slowly over the tube's lifetime.

        • If the central glass capillary - which is relatively heavy for an HeNe tube - is supported along its length by metal spacers or 'spiders', these can move, warp, deform, or loosen up over time. The result will be a slight change in axial bore position. While the mirrors will still be aligned with each-other, they won't be aligned with the bore. If the tube was dropped or whacked, the bore may end up loose or even warped due to movement of its supporting structure(s).

          I've seen one case where the bore was supported at the OC-end by a cup affair which had a set of fingers that looked sort of like the pedals of a tulip and these were actually loose around the bore (either the tube had been used to hammer nails, or the mirror mount next to the cathode can had been accidentally used as the cathode connection for this Hughes style HeNe tube where the cathode has its own separate terminal - thus overheating the cup), or it had overheated due to excessive current or some other cuase. Thus the bore was free to move laterally resulting in erratic behavior. Orientation and/or tapping on the tube would make the beam come and go. There is no way to tighten up such an assembly but if you can find an orientation where the end of the bore is actually resting on something solid (and not just floating), it should be possible to realign the mirrors for that bore position. (However, this particular tube must also have that dreaded warped bore as its behavior is, well, strange - adjustment of the mirrors alone isn't sufficient to achieve reasonable power output.) See the section: The Yellow HeNe Laser Tube with a Warped Bore.

          Some really long lasers with exposed bores (usually with external mirrors but not necessarily) have one or more lateral adjustments along the length of the bore to correct for unavoidable droop or warp in the glass work. Where these are misadjusted, the output power will be reduced and beam shape may suffer. One example of such a laser is the Spectra-Physics model 127 (and the similar 107 and 907) with the 0-82 plasma tube. It is unlikely that anything accidental that didn't smash the tube would result in enough misalignment of these to result in no beam at all, but the power and beam shape could definitely get messed up.

        • Where locking collars are used to stabilize the mirror alignment, it is possible for these to slip or change position ever so slightly after many thermal cycles. In fact, I've found many older laser heads that were way low power to only be suffering from locking collar creep disease. :) Collars are common on Melles Griot tubes though most other companies don't seem to believe in them (possibly for this reason! And, Melles Griot also appears to have abandoned them for some if not most or all of their HeNe tubes). Sometimes, just not quite so gently rocking the collars back and forth via the set screws (but not even turning them), or even just tapping on the mount will restore the mirrors to their original and proper alignment.

        • Another way for alignment could conceivably change on its own would be if the tube was tightly fastened to its housing by the mirror mounts (e.g., with cable clamps) and a bending stress was being constantly applied to them.

        • And, of course, alignment could have been less than perfect when shipped from the factory! Quality control isn't always so stellar and specifications allow for substantial variation in output power. As long as the tube met catalog specs, it may have been considered good enough! That's one reason why HeNe laser output power is almost always rated as 'minimum' - some tubes may produce or be capable of much more.

        Note: For really long high power HeNe tubes (e.g., above 15 mW or so), see the comments in the section: How Can I Tell if My Tube is Good?. Your tube may need to warm up for 1/2 hour or more, or it may require external adjusters permanently installed or you may have it mounted incorrectly. DO NOT attempt to remedy the mirror alignment problems by physically bending the mounts if gently rocking the mirrors (see below) results in any beam. Your likelihood of success is about the same as winning the State Lottery Super Seven. Adjustments may not be needed in any case as there may be nothing wrong with the tube!

        There are two types of situations:

        • The tube produces an output beam but its power is less than expected or seems to be distorted (not the nice circular TEM00 Gausiam beam that is produced by most HeNe tube).

        • The tube appears to work in all other respects but there is no output beam.

        The procedures described below are simplified versions of those that can be used for testing and adjusting of mirror alignment on many types of lasers (including HeNe and Ar/Kr ion lasers where one or both mirrors are external to the tube. See the sections: External Mirror Laser Cleaning and Alignment Techniques, Sam's Approach for Aligning an External Mirror Laser with the Mirrors in Place and Daniel's Method for Aligning External Mirror Lasers. The CORD "Laser/Electro-Optics Technology Series" also has a basic alignmnet procedure outlined in the chapter: "1-7 Optical Cavities and Modes of Oscillation".)

        These techniques are also ideal for use with internal mirror argon ion (blue/green) tubes because a readily available red HeNe laser can be used for testing and adjustment (having a different color laser for the alignment procedure simplies it considerably). Here, they have been adapted specifically for use with small internal mirror HeNe tubes.

        Note: It is assumed that your problem HeNe tube has each of its mirror mounts separated from the end-cap/electrode assembly by a restricted area that is not obstructed. If this is NOT the case (at one or both ends), there may already be a mirror adjusting device permanently attached to the tube and it will have to be used (unless it is removed) rather than the tools described below. In its favor, fine adjustment with such a device is more precise (though it will be less convenient for 'rocking the mirror') and alignment problems are less likely in the first place (unless someone was mucking with the screws!). Note that some older HeNe tubes have absolutely no means of adjusting the mirrors - they are bonded directly to the end-cap(s) or glass tube. In that case, best to move on with your life. :)

        Minor Problems with Mirror Alignment

        A beam which is much less intense than expected or distorted (not circular with a reasonably smooth gaussian profile) may be due to bad mirror alignment. Checking mirror alignment as long as the laser produces some sort of beam is easy and very low risk. Correcting it may be possible as well.

        If there is no beam at all at the nominal tube current but no evidence of bent mirror mounts or other visible damage, this technique may also be used with care to see if one of the mirrors is SLIGHTLY misaligned. However, if gentle rocking of the mirror mount does not result in a beam (see below), DO NOT attempt to actually bend the mount since there is no way of knowing in which direction the correction (if any) is needed. See the section: Major Problems with Mirror Alignment.

        Quick Check of Mirror Alignment

        Here is the instant version of this procedure for quickly checking to see if one mirror mount is slightly misadjusted: (The more complete version follows it.)
        1. Take a sheet of paper and roll it up into a tight tube that just fits over the mirror mount extensions. Wrap adhesive tape around the end you will be using on the tube to prevent it from loosening. This is your mirror rocking 'tool'. (You may need two such tools as some tubes have different diameter mounts at each end.) The nice thing about this tool is that it is virtually impossible to either permanently bend the mount accidentally or to damage it as long as you are reasonably gentle in what you do. For a paper tube of about 6 inches, a couple of pounds of side-ways force can be applied safely.

        2. Power up the tube and while holding it stead (careful: high voltage!), use your instant tool to gently rock the mirror mount at each end. If a tube with no output produces a beam for some particular amount of pressure and direction, you know it needs mirror alignment. Similarly, if doing this can increase the output power substantially, one or both mirrors also need alignment. With any luck, only one of the mirrors is very slightly misaligned. However, even if it appears to be the first one you tried, the other may actually be the problem. Therefore, don't jump to conclusions and attempt to correct it just yet.
        If the preceding tests show that alignment is needed, read the following sections for instructions on exactly what to do next.

        Minor Mirror Alignment Procedure

        Here is the more detailed procedure for checking and correcting minor mirror alignment problems:
        1. A (homemade) tool for rocking (and possibly adjusting) the mirror mount is needed. See the section: Means of Adjusting HeNe Tube Mirrors.

        2. Mount the HeNe tube in such a way that the ends are free and clear so the adjuster(s) can be used without interference.

          • For short HeNe tubes (perhaps 15 inches or less), a suitable support is a wooden V-block clamped to your workbench. Secure the tube with tape or Velcro straps.

          • For longer HeNe tubes, limit support to exactly two locations at or near the ends. This will permit you to press gently sideways in the center from any direction to very slightly bend the entire tube (GENTLY please!). There will very likely be a noticeable effect on output power especially for tubes over 24 inches in length (even to the point of restoring some sort of beam to a tube that wasn't lasing at all). By determining which direction results in the greatest effect can help to identify how the mirror(s) should be adjusted to restore power permanently.

        3. Start with the cathode end (arbitrary choice) and power it up the HeNe tube at the optimal operating current. If you are using a plate or tube type adjustment tool, take care to assure that it doesn't extend beyond the reduced diameter section of tubing and only applies force to the metal of the mirror mount, not the mirror or its frit seal!

        4. Allow the tube to warm up and stabilize a few minutes before checking or attempting any adjustment of mirror alignment. There is a chance that during this time, the output power will increase to a normal (expected) value. (And then you can see if it can actually be improved.) Where there was no beam at all initially, one may appear (weak or otherwise) at some point during the warmup period making the problem orders of magnitude easier! The longer the tube, the more time is needed for the output to stabilize. A rough guideline is 1 minute for each inch of tube length. I just pulled this 'guideline' out of thin air but it's better than nothing! :)

        5. GENTLY rock the tool back and forth as you watch the beam's reflection from a white surface. Do this in X (horizontally) and then in Y (vertically). Go easy! It doesn't take much force to change alignment through the entire range that matters - perhaps a few ounces at most. Even modest finger pressure will do it - but only try it this way at the cathode-end and only if the cathode is grounded! Don't get carried away and actually bend the mount at this time - or break the seal :-(.

          What you should see is the beam power (brightness) pass through a maximum and then diminish on either side of this point. Testing is best done with a laser power meter but one of your eyeballs (or both of them) will work well enough for most purposes.

          • If the maximum is at the relaxed position in both axes (you can try the 45 degree ones as well to be sure), mirror alignment is correct (or at least close enough that the chance of being able to improve it without using more sophisticated adjustment equipment and a laser power meter is vanishingly small.)

          • If the maximum (or any beam at all) is off to one side, you can VERY CAREFULLY try bending the mount permanently to attempt to correct it. First, determine the angle of the tool that results in the least force being needed to correct the alignment.

            CAUTION: The mirror mount is ultimately attached to the glass envelope of the tube. The glass-metal seal may not be that strong. Don't get to carried away! With care this adjustment should be possible - barely. :-)

            Note: Where the maximum intensity results with the mirror very slightly deflected, it is possible that the mirror alignment at the opposite end of the tube is actually to blame and you are simply compensating for its pointing error. Thus, it is better to check the mirrors at both ends of the tube before attempting to adjust either of them. However, the only way to be sure is to measure the maximum beam power AND and also examine the shape of the beam. It should have a circular cross-section, a Gaussian profile, and not have any off-axis arcs or other artifacts) when both mirrors are precisely parallel to each other and perpendicular to the bore of the tube. (Note: Don't confuse a weak spot or spots off to one side due to 'wedge' of the OC mirror with an alignment artifact.)

            • Increase the force gradually until you have a feel of how much it takes to actually deform the mirror mount. Even a significant pointing error will only require a nearly microscopic correction. The change in mirror mount angle that you need to achieve is likely to be a fraction of a mR - less than 1 part in 1,000! Not easy.

            • Approach the desired deflection in small increments and overshoot just enough so that the mirror mount springs back to the optimal position. Avoid repeatedly bending it back-and-forth or you will eventually be using the HeNe tube as a high-tech wall hanging :-(.

            • Once optical output is maximum and this point is *roughly* centered when testing by rocking the mirror mount, pat yourself on the back and consider it as good as it gets. Don't push your luck!

          • In the case where there was no beam at all, if GENTLE rocking doesn't result in a beam at any position, DON'T press your luck at this end! The misalignment may be too great for this approach or the problem may be with the mirror at the other end of the HeNe tube!

        6. Gently remove the tool (if relevant) without applying excessive force to the mirror mount.

        7. Repeat this procedure for the anode end of the tube - just be careful not to touch the high voltage!

        Alignment should now be the best that is possible by adjusting the mounts at each end independently. Confirm by rechecking it at both ends and making any very *slight* adjustments that may be needed. This is where the addition of permanently installed adjusters may be desirable. See the section: Home-Built Three-Screw Mirror Adjusters for Internal Mirror Tubes come in handy for tweaking but these may be overkill for inexpensive HeNe tubes.

        However, although the mirrors will be parallel to each other (ignoring the mirror curvature), their central axes may not be aligned with the bore. Thus, power output could still be low - possibly quite low. If only one mirror mount was messed up originally and that is the one you touched, the chance of there still being major problems is small but I've seen many supposedly healthy HeNe tubes where mirror alignment was far from optimal even from the factory!

        If you really want to fully optimize power, you will need to go through the procedure discussed in the section: Walking the Mirrors in Internal Mirror Laser Tubes. The use of the three-screw adjusters is definitely recommended if going beyond this point!

        PERFORM ANY ADJUSTMENTS ONLY AT YOUR OWN RISK! Checking the alignment by gently rocking the mirror(s) is safe and effective. However, actually bending the metal is much more difficult and likely to result in death to your HeNe tube. The required pointing accuracy of much less than 1 mR is not much to fool with! If the brightness change that is bothering you is just barely perceptible or you just *think* that it may not be perfectly centered, LEAVE THE MIRROR ALIGNMENT ALONE! Plexiglas or wood plates (even with any inserts) and plastic tubes are really too soft for precise control beyond the elastic limit (i.e., when actually bending the metal permanently). Your control will be poor and you will be much more likely to bend the mirror mount far off to one side never to work again or break it off completely. The lever type adjusters can be more precise but may result in excessive stress to the mounts if used to make more than very small adjustments since it applies an unbalanced force spreading the mirror mount and end-cap apart.

        Means of Adjusting HeNe Tube Mirrors

        Unlike the mirror mounts on high quality lab or industrial lasers, those on inexpensive sealed HeNe tubes are integral to the metal end-caps and not normally considered user adjustable. There is usually a restricted region separating the mirror mount itself from the end-cap and actual deformation or bending is required to alter mirror alignment. As expected, there is no easy way to do this without the assistance of some sort of (homemade) tool - and repeated flexing of the metal tubing beyond its elastic limit WILL result in eventual failure of the seal.

        You cannot just grab the mirror mount in your hand and deform them as though your are Superman (unless you are) since additional leverage and finer control is needed (not to mention the several kV that may be present at one end of the HeNe tube end at least!).

        • For testing that the mirrors are aligned optimally, these tools will enable you to 'rock the mirrors' while watching for maximum output beam brightness.

        • For actually adjusting the mirror alignment, these tools will enable the mirror mounts to be aligned with reasonable accuracy.

        Here are some suggestions for easily fabricated tools or adapters which will permit fairly precise movement of the mirror mounts. The "plate" and "tube" types are best for 'rocking the mirror' to check alignment without changing it. The "lever" type may be more precise for making initial adjustments since it applies force at the exact place that it is needed. The "three-screw" type is unsurpassed for making fine adjustments in alignment without any risk of permanently ruining the mirror mounts by bending them too much. The "collar" type (Three-Screw Locking Collars on Melles Griot HeNe Laser Tubes) is useful for stabilizing alignment but can be used for final tweaking as well.

        • Plate type - Obtain a plate of rigid nonconductive material such as Plexiglas about 4 inches square and 3/8ths of an inch thick (high quality hardwood plywood will also work but is not as sturdy). Use a micrometer or caliper to measure the diameter of the adjustable portion of mirror mount. Drill a hole of this diameter in the center of the plate (preferably using a drill press) with a sharp drill bit of the proper type for the material you are using. You want a snug fit but not one that is so tight that installing and removing the tool may deform the mount. If the exhaust tube interferes with the tool, drill a small clearance hole for it.

          You may find that for rocking the mirror mounts, a strip of plastic perhaps 1" x 6" x 1/4" with a suitable hole drilled near one end may be more convenient than a large plate since it won't get in the way of other things as much. However, this may not be sturdy enough for actually adjustments.

          A more robust enhancement for either one is to obtain or machine a metal sleeve that just fits over the mirror mount and glue this into a press-fit hole in the insulating board (rather than just using a bare hole).

        • Tube type - This adjuster can be constructed from a piece of rigid plastic tubing that just fits over the mirror mount. A six inch length will provide enough of a lever to easily 'rock' the mirror and even bend the mirror mount if needed. Even a piece of paper rolled up and taped so it just fits over the mirror mount will have enough stiffness to rock the mount for testing. Whether it can effectively be used for permanent adjustments will depend on its stiffness and the brand of laser. Melles Griot, Siemens, and Uniphase mounts are much more resistant to permanent deformation than those on Aerotech lasers for which a tool made from a rolled up piece of printer paper is quite adequate though I imagine that with enough bending back and forth, they will become work hardeded! Try not to find this out the hard way. :)

          It probably won't even be necessary to remove the HeNe tube from its case to use this tube type tool and it may be your only option if the HeNe tube is permanently glued inside a laser head barrel. But then, how could its mirror alignment have gotten messed up in the first place? Only the tube knows for sure and it's probably not telling. :)

          Note: If testing or adjusting at the output end of the HeNe tube, the visibility of the beam may be impaired by this type tool. In this case, you should either use the plate-type tool or watch the weak beam usually visible from the opposite end of the HeNe tube (remove any opaque coating that may be present).

          I have actually used a tool of this type (actually, a female Alden high voltage connector!) and succeeded in correcting the alignment of a small HeNe tube which had no output beam at all.

        • Lever type - Another way of adjusting the mirrors without constructing any fancy tools is to use a piece of metal in the narrowed region between the mirror mount and HeNe tube end-cap to VERY slightly spread them apart on one side. I have actually used a LARGE straight blade screwdriver for this purpose (but with great care so as not to go overboard since there is a lot of leverage and the required displacement is microscopic). If you have a sacrificial screwdriver, file a semicircular cutout in the tip to better fit the narrow area of the mirror mount - this will improve leverage and reduce the mangling of the edges that otherwise occurs. I constructed a special tool from a strip of mild steel by adding the semicircular cutout and bending the end around so it can be used inside of a cylindrical laser head. The Sam's Special Mirror Tweaker can be used both by rocking toward or away from the mirror or by twisting at right angles to this - useful when the exhaust tip-off would interfere with the tool for the desired direction of adjustment. Gentle rocking or twisting is used for testing mirror alignment. Actual adjustment is done with more of a "bouncing" motion - letting the inertia of the tool jog the mount in small increments. At some HeNe laser companies, mirror alignment on the manufacturing line may actually be done using a similar approach.

          • The advantage of this approach is that it is possible to achieve quite fine adjustments by a kind of gentle repetitive pressing while watching the beam because you are applying force at the precise location where the metal must deform.

          • The disadvantage of this approach is that you can only move in one direction without changing the position of the tool or rotating the HeNe tube. Thus, it is not as convenient as the plate or tube type tools for checking alignment by rocking the mirror mount.

          • I don't know how far you can push your luck with this technique but it is what I generally use for routine tuneup of newly acquired HeNe laser tubes and tubes inside heads (at least at the cathode-end)! I've used it on Aerotech, Melles Griot (new style without locking collars), Uniphase, and Spectra-Physics lasers so far including resurrecting some that didn't lase at all.

        • Three-screw type - It is also possible to construct mirror mount adjustment assemblies operated by thumbscrews or set screws to correct or optimize the alignment. For a small misalignment, this would avoid the risks of actually trying to bend the mounts since the range of motion would still be within the elastic limits of the metal. This type of adjuster is really best for fine tweaking where a beam of some sort is already being produced, not for initial alignment where the mount is bent at a visible angle!

          See the section: Home-Built Three-Screw Mirror Adjusters for Internal Mirror Tubes for details.

        • Collar type - These consist of a close fitting metal sleeve with three tapered point set-screws in tapped holes pressing against the edges of the mirror mount and tube end-cap at the restricted region thus applying force to very slightly spread them apart. Most or all higher quality Melles Griot laser tubes used to come with a locking collar permanently installed at one or both ends (barcode scanner tubes probably don't have any and newer Melles Griot tubes appear to have done away with them as well). See Three-Screw Locking Collars on Melles Griot HeNe Laser Tubes.

          This approach is really best for stabilizing alignment once it has been optimizing, not for twiddling. The control may be too coarse and the effects of adjusting any given screw may at times be counter-intuitive since it applies a rotating/side-ways torque to the mount. Adding a tiny drop of penetrating oil to each of the screws will minimize the tendency to of the screw to 'stick' thus easing adjustments. However, apparently, some major HeNe tube manufacturers (you can guess at least one of them) use this approach for all tweaking once the tube comes off the production line. I guess no coarse alignment is needed on a brand new tube. :)

          I have built my own from that piece inside Sears garbage disposals that locks the rotor to the cutting disk thing. :) (If you have ever disassembled an InSinkerator or Sears/Craftsman garbage disposal you will know what I'm talking about. If not, well....) Any thick steel or aluminum cylinder that fits over the mirror mount with a some clearance (at least .5 mm/.020 inches) can be converted into a locking collar with a bit of work. A drill press will be needed to make three holes around its circumference. Drill the holes as equally spaced and centered as possible. (A clearance hole or slot will be needed if the exhaust tube gets in the way.) Then tap the holes for a screw size slightly thicker than the space between the two sections of the mirror mount. File or grind down three suitable cap screws or set screws(Allen wrench type) to give them smooth tapered ends.

          CAUTION: Use a well insulated tool (hex wrench) for adjustment unless you are are sure the mount is directly grounded! Don't over tighten! The entire useful range is only a small fraction of a turn of each screw. Go overboard and you risk ripping the mirror mount off of the tube - which is not generally desirable. :( If your mirror mount is sitting at a 20 degree angle, see the information below on initial alignment - you will have to bend metal to get it close enough for the collar adjustments to be of any value. Also, the collars on some will have their screws quite tight. It is generally possible to apply a considerable amount of torque to the screws to loosed them if the mirror mount is attached to a large metal end-cap as it is on the cathode-end of Melles Griot (and many other) tubes. However, where the mirror mount is fused directly into the glass of the tube, it is quite possible to break the glass-to-metal seal with excessive force. One way around this is to carefully hold the collar itself and apply the torque so that the tube itself is free to move as it see fit.

          Note: It is virtually impossible to adjust these collars where the tube is still mounted inside a cylindrical laser head without providing access holes, especially at the anode-end where it is recessed more to provide space for the ballast resistor or where the tube is just much shorter than the head. Melles Griot has special tools for this. I filed down the short end of a hex wrench and mounted it in a plastic handle but this just barely deals with the cathode-end - for the anode-end, the tube most likely must be removed from the laser head. Or, several such modified wrenches with different angles on the hex end are needed to accommodate arbitrary orientations of the set-screws (not to mention the issue of high voltage insulation). See the section: Getting the HeNe Tube Out of a Laser Head Intact.

          However, if you're willing to modify the laser head very slightly, a simple alternative is to drill access holes for a hex wrench in the side of the cylinder opposite each of the adjustment screws. With care, this can be done on a drill press with little risk to the laser head. For the cathode-end, the holes just need to be large enough for the wrench (unless there is a ballast resistor for the cathode in which case they will need to be slightly larger so the wrench can be insulated). However, for the anode-end, the holes will definitely need to be made oversize to allow for the hex wrench to be wrapped in a most excellent insulator to deal with both the operating voltage, and starting voltage as the discharge is likely to drop out momentarily and restart due to the capacitance of the wrench when it contacts the anode. And, the wrench must also be provided with a most excellently insulated handle. I'm really surprised Melles Griot doesn't provide access holes as a standard feature. Nearly every laser head I've checked could have benefited from some tweaking. :) But note that peaking the output power may not result in the best overall stability in output power with laser head orientation (especially for long high power lasers). In any case, I would only recommend adjusting one of the mirrors, usually the output mirror - which is the cathode-end for most red (632.8 nm) lasers - but may not be for "other color" lasers. Messing too much with both mirrors (aside from the higher risk of losing lasing entirely!) may result in a change in beam pointing alignment with respect to the laser head.

        Of course, a nearly infinite number of variations on all of these schemes are possible. However, Vice-Grips(tm) (despite being suggested by a person who should have known better), wrecking bars, and 12 pound hammers are NOT appropriate tools for adjusting the mirrors on HeNe laser tubes (or any other lasers, for that matter)!

        CAUTION: For all of the tools, make sure that, pressure is ONLY applied to the tube of the mirror mount beyond the narrow section - not the part attached to the body of the HeNe tube, or the glass or frit seal of the mirror itself. And, don't go overboard - the amount of force needed isn't that great if applied at the appropriate place in the proper direction. Someone I know ("Dr. Destroyer of Lasers") ruined a possibly salvageable large green HeNe tube from overzealous attempts at alignment by cracking the cathode-end glass-to-metal seal. It is especially important to avoid applying any pressure to the mirror glass (which is quite soft) or the glass frit (glue, glass 'solder') holding the mirror in place which is even softer. On some HeNe tubes, there is just a thin ring of this material and it can be easily fractured. I've done it, hisssss. :-(

        CAUTION: DO NOT use a metal (conductive) material for the tool as the mirror mounts probably connect directly to the high voltage power supply!

        Providing two such tools - for both the cathode and anode ends of the HeNe tube, may simplify some of the alignment procedures. This will also be required if the diameters of the mirror mounts at each end of the tube are not the same.

        Alignment jigs may be used in the factory during tube manufacture but these are made from strong rigid components so that even the smallest adjustment of the thumbscrews actually gets transmitted precisely to the mirror mount. Anything as complex as this is overkill for checking mirror alignment but might be desirable to permit fine tuning while the laser is operating.

        Major Problems with Mirror Alignment

        Where the mirror mounts are obviously bent or damaged, or if the techniques described in the section: Minor Problems with Mirror Alignment don't result in any beam, the HeNe tube starts normally, the power supply is providing approximately the nominal current, and other problems have been ruled out, further testing must be done 'off-line' - not powered. This requires a second (working) laser or special optics. Without one of these, anything you do will be hit or miss (mostly miss) as the mirrors must be nearly perfectly aligned before there will be any output beam at all. What is more likely to happen is that you will end up breaking the seal from repeated bending of the mirror mount.

        If only one mirror is actually misaligned, you can use the procedures from the section: Minor Problems with Mirror Alignment to identify the error (by rocking the mirror and looking for a beam with power on) and then carefully tweaking its alignment. In any case, this should be attempted first (unless you are sure both ends or misaligned).

        Where the mirrors at both ends of the tube are messed up, the chances of ever getting a beam with any testing of this type is quite slim - especially for those high power expensive HeNe tubes. Getting close won't be good enough since rocking either mirror by itself will never result in any beam.

        Unless your baby is a high power and/or expensive HeNe tube, it may not be worth the effort to attempt the procedures described below. While testing and/or correcting major mirror alignment may represent an irresistible challenge, the cost in terms of time, materials, and frustration could prove to be substantial. And, as noted, those longer tubes are exponentially more difficult to align! For anything longer than 8 or 10 inches, your odds of success are probably better in your State's Lottery - and then, when you win, you could just buy a new tube! :)

        As if this isn't enough, if one (or both) of the mirrors on your HeNe tube are not planar (often concave at the high reflector end), or there is an internal Brewster plate or etalon, even more care will be required in equipment setup and subsequent steps may be complicated at that end at least.

        In addition, the output-end (output coupler or OC) mirrors on some lasers have faces which are ground with some wedge and thus their surfaces are NOT quite parallel. This eliminates all ghost beams that are parallel with the main beam (though there will be one or more weak ghost beams off to one side) and also minimizes reflections back into the resonator. Alignment is complicated for a mirror where wedge is present due to non-parallel reflections and slight refraction through the mirror. I don't know how likely wedge is with small internal mirror HeNe tubes but check for it in any case before considering attempting alignment of a non-lasing tube (see the section: Ghost Beams From HeNe Laser Tubes). Wedge is common in large frame HeNe lasers with external mirrors.

        The longer the HeNe tube, the worse it gets!

        I would suggest that if the tube is valuable enough to warrant the expense, see if one of the HeNe laser manufacturers or laser system refurbishers will perform the alignment for you. The ratio of their probability of success compared to your probability of success will approach infinity. OK, perhaps not quite infinity. It probably won't be significantly greater than the ratio of the mass of the Sun to that of a typical electron. :-) I have no idea if this is a viable option or what it might cost.

        Having said that, if you are still determined to proceed, alignment is best done with a working narrow beam laser (i.e., HeNe, argon ion, etc.).

        If you do not have a working laser to use for this purpose, various plans for construction of laser mirror aligners using simple optics and readily available materials are provided in: "Light and Its Uses" [5]). However, some of these are for wide bore tubes and may not work well with the 0.5 to 1.5 mm bores of typical modern HeNe tubes.

        If you have another functioning HeNe laser or tube (you can use the power supply for the one you will be adjusting since it will not be needed until the mirrors are roughly aligned), or possibly even a collimated diode laser or laser pointer) it may be possible to use it as an alignment laser to adjust the mirrors. A low power (i.e., .5 to 1 mW) laser is adequate and preferred since it will be safer as well.

        The general idea is shown in Principle of Mirror Alignment Using Reflected Beam. With the beam of a low power Alignment Laser (A-Laser) and the bore of the Tube Under Test (TUT) are lined up, mirror alignment will be perfect when the beam reflected from the inner (active) surface of the TUT mirror facing the A-Laser is centered in the aperture of the A-Laser (AL-Aperture) and/or the hole in the Bore Sight Card (BSC) next to the TUT. The diagram shows a TUT mirror mount that is bent at an angle much much greater than anything you should EVER encounter!

        Plan on spending a lot of time on this. Therefore, select a location to work where you can spread out and won't be disturbed for hours. The kitchen table is probably not appropriate!

        • Step 1 - Mount the Alignment Laser (A-Laser) and Tube-Under-Test (TUT): Provide a mounting so that both lasers can be arranged precisely in line with each other and separated by at least 1 tube length (the further apart they are, the more sensitive will be the test to pointing accuracy). This is likely harder done than said but is THE most important step as anything you do to the mirror mounts will depend on the absolute precision of this setup. Unfortunately, without an optical bench, this may be very difficult to achieve. However, a length of 2"x4" U or box (or larger) extruded aluminum stock should provide the necessary rigidity.

          • The TUT should be mounted on a pair of V-blocks. (Line the V-blocks with tape to protect the TUT from scratches if they are made out of metal). Provide some means of firmly but gently fastening it in place so it stays where you want it (e.g., elastic bands)!

          • If the non-output mirror of the TUT has an opaque coating, the paint or tape will need to be carefully removed.

          • The A-Laser (tube or laser head) needs to be mounted so that it can be positioned precisely on-axis with respect to the TUT. The best way to do this is with a pair of X-Y vernier positioners of the type used for this purpose in optics research labs. An alternative (which is what I have done) is to adapt some surplus microscope mechanical X-Y stage assemblies bolted to a rigid base provided with clamps to hold the alignment laser head. Make sure that the settings can be locked in position so they don't drift due to the weight of the A-Laser. You may need to be resourceful and improvise! See the section: Simple Adjustable Optics Platform for details of one very basic approach which is quite adequate for checking laser mirror alignment.

            The adjustable (dual X-Y) mount for the A-Laser and V-blocks for the TUT should be securely clamped or screwed to a rigid surface so that their relationship cannot accidentally shift by more than the diameter of a fat hydrogen atom. :-)

          • Drill or punch a clean circular hole in the center of a white piece of cardboard or other opaque material just larger than the diameter of your HeNe beam - usually about 1 mm. Fasten this card to the front of the A-Laser so that its beam passes through the center of the hole. Henceforth, I will call this the 'AL-Bezel'.

        • Step 2 - Fabricate a tool for adjusting the mirror mounts: See the section: Means of Adjusting HeNe Tube Mirrors.

          Note: If a mirror mount on the TUT is very visibly bent (and this is not just compensating for a mirror that was accidentally fritted in place at an angle), it should be straightened as best as possible (by eye) before the procedure below is attempted. Otherwise, initial alignment between the A-Laser and the TUT will have too much error or be impossible to achieve at all. To check for this damage, rotate the TUT on the V-blocks and watch the surface of each mirror. If *significant* wobble in its angle is evident, it should be corrected now by CAREFULLY bending the mount. At least, if you screw up and break the seal, at least you won't have wasted any additional time and effort :-(.

        The following three steps, (3) through (5), may need to be repeated for the High Reflector (HR - fully reflecting mirror) and Output Coupler (OC - beam output) ends of the TUT. If you find a problem at one end and think you fixed it, you can try powering up the tube to see if a miracle occurred before repeating the procedure for the other mirror. :) You can start with either end of the tube if you have no idea of which mirror might be messed up.

        If you are using a non-red laser (e.g., green argon ion) it may be possible to get a clean reflection all the way back in from the far mirror (the HR if the OC is facing the A-Laser). If so, everything should be done without changing TUT position. This is the preferred way of aligning any laser since correct allignment can pretty much be assured by getting the A-Laser beam to bounce up and back inside the tube just as the photons will do when the laser is operating normally. However, with the closer mirror in place, this can be very difficult, confusing, and time consuming unless everything is bolted down rigidly. And, even then, may be virtually impossible due to the many confusing reflections. It is trivial (well, almost trivial!) for lasers with removable mirrors but you don't have that luxury. :)

        So, even if you are using different colored lasers, since you really can't remove - and shouldn't really even move the mirror facing the A-Laser, the reflected strong spots from its surfaces will likely totally obscure the much weaker return from the far end - even if it was aligned perfectly. It might be possible to just deflect it slightly - just enough to move the obscuring spots out of the way. This is easy and safe to to do with those tubes having built-in three-screw adjusters or three-screw locking collars but should probably be avoided where it is necessary to bend the mount unless you can provide a jig (like an adjuster or collar) to just deflect it slightly and temporarily.

        See the section: External Mirror Laser Cleaning and Alignment Techniques for more information - at least to get the general idea. Some changes and simplifications will be required. Also see the section: Daniel's Method for Aligning External Mirror Lasers since this was written specifically for HeNe lasers.

        If what you have is a tube with an internal HR mirror but external adjustable OC, that procedure (also with slight modifications) will be more appropriate.

        However, when using a red laser to align a red HeNe laser (or any time the A-Laser and TUT are similar color lasers) not enough light can pass through the mirrors to get a return spot - it is too small by a factor of 10,000 or so! (In addition, even if you are using different colored lasers, since you really can't remove - and shouldn't really even move the mirror facing the A-Laser, the reflected strong spots from its surfaces will likely totally obscure the much weaker return from the far end - even if it was aligned perfectly.) Assuming this is what you are doing, the procedure will have to be repeated after reversing the TUT end-for-end. This is what is addressed in the remainder of this procedure.

        • Step 3 - Alignment of the A-Laser and TUT: This must be absolutely precise. A fraction of a mm is significant. Take your time. You have all week. Here are two ways of achieving this:

          • If the wavelengths of the two lasers differ significantly, there will be enough transmitted light so that the beam from the A-Laser is able to pass through both mirrors of the TUT and project a bright spot on a screen placed beyond the TUT. An ideal situation exists where a red (632.8 nm) HeNe laser is being used to test a green (543.5 nm) HeNe tube or vice-versa.

            The reason for this behavior is that the dielectric mirrors used in these HeNe tubes have a reflectivity which peaks at the laser wavelength. As the wavelength moves away from this, they transmit more and more light. For example, if you sight down an unpowered red HeNe tube, it will appear blue-green and quite transparent indicating that blue-green light is passed with little attenuation but red light is being reflected or blocked. (Actually, orange and possibly yellow light is also reflected well by these mirrors as shown by their typical goldish appearance.)

            However, this approach cannot be used if the wavelengths of the two lasers are the same or even fairly close since the reflectivity of the two mirrors will be a maximum and very little light will be transmitted. This will be the case when attempting to check one red (632.8 nm) HeNe laser with another (which is probably what you are doing, right?) or even with a 670 nm diode laser pointer.

            Proceed as follows:

            • Place the TUT in position on the V-blocks. Fasten it down so it cannot move - even if you should need to apply force to adjust the mirror.

            • Position a white card to act as a screen just beyond the far end of the TUT.

            • Power up the A-Laser. Go back and forth between the X-Y adjustments at each end of the A-Laser mount to get the beam from the A-Laser to pass as cleanly as possible through the bore of the TUT. Watch for where the spot falls on the front of the TUT's mirror and for a clean spot exiting the other end of the TUT that is projected onto your screen.

            • Alignment of the A-Laser with respect to the TUT is optimal when the brightness of the light of the A-Laser exiting the other end of the TUT is maximum and the presense of off-axis circles or arcs are minimized (the pattern is symmetric).

              Note: Except for a very short TUT, it is likely that the A-Laser's beam would be wider than the bore of the TUT at the far end at least. Make sure you are optimizing the central peak of the beam of the A-Laser by checking on all sides to make sure. Just getting a beam out the other end is not enough.

              For long tubes with exposed bores (or long external mirror lasers with exposed bores), any warp of the capillary may prevent the passage of a clean beam (as well as mess up the output beam when lasing). Sometimes there are adjustments to maintain bore straightness. For internal mirror lasers, there may be a "This Side Up" label indicating an orientation that minimized bore warp.

          • Another approach is to construct an external bore sight - a sort of laser capillary simulator - to align the two tubes. With care, this can be at least as accurate as the previous method and will work equally well regardless of the relative wavelengths of the A-Laser and TUT. From my experience, the bore sight method may be superior especially for long HeNe tubes - even if a non-red A-Laser is available! See Bore Site Method of Internal Mirror Laser Tube Alignment while reading the procedure, below:

            • Place the TUT in position on the V-blocks. You will need to remove and replace the TUT a couple of times so don't fasten it down yet.

            • Put matching marks on the TUT and the V-blocks so that the TUT can be easily removed and replaced without changing its position or rotational orientation. (Even very slight unavoidable manufacturing errors in the centering of the capillary in the TUT will be enough to cause problems if the orientation can not be exactly duplicated.)

            • Fabricate a pair of Bore Sight Mounts (BSMs) - wood blocks or metal angle brackets each having a 1/2" diameter hole approximately centered on the TUT axis.

            • Fabricate a pair of Bore Sight Cards (BSCs) - just some 1" x 1" pieces of thin cardboard (more white cards!) with a center hole approximately the diameter of the TUT's bore that can be fastened onto the BSMs with adhesive tape.

            • Position the BSMs so that the BSCs will be almost touching the mirrors at each end of the TUT (but take care not to scratch them - a couple of rubber covers with their centers removed would be useful). Fasten the BSMs to the base of the V-block assembly with adhesive tape.

            • For each end of the TUT, place a BSC against the BSM and fine tune its position so that its hole is precisely centered on the TUT's bore. Do this (using a dental mirror if necessary) by looking through the hole in the BSC and down the length of the TUT's capillary. Fasten the BSC in position with adhesive tape and then double check that it is still perfectly centered - an error of .1 mm matters! (Refer to the top diagram in Bore Site Method of Internal Mirror Laser Tube Alignment.)

              Note: For HeNe tubes with an internal angled Brewster plate or etalon, there will be a slight shift in the apparent position of the bore at that end due to refraction. However, the hole must be lined up with the physical location of the bore, not its (shifted) image.

            • Temporarily remove the TUT from the V-blocks without disturbing the BSMs and their associated BSCs.

            • The holes in the two BSCs now form a bore sight assembly which exactly matches the position of the ends of the TUT's actual bore. Use the A-Laser's X-Y adjusters to precisely center its beam on the holes of BOTH BSCs. If the A-Laser's beam is narrower than the hole(s) in the BSCs, hold a translucent screen against the BSC so you can see exactly where the beam falls. Once the centering at both ends is perfect, lock the X-Y adjusters in position and then double check that nothing has moved. (Refer to the middle diagram in Bore Site Method of Internal Mirror Laser Tube Alignment.)

            • Replace the TUT in the proper orientation on the V-blocks. Fasten it down so it cannot move - even if you should need to apply force to adjust the mirror. At some point, it may be desirable to remove the BSM nearest the A-Laser. However, it does serve as a second screen to view the reflected spot - with double the sensitivity to mirror deflection as the AL-Bezel on the A-Laser (because the beam from the A-Laser first bounces off the TUT's mirror, then the OC mirror of the A-Laser, and back to the BSC on the front of the TUT). (Refer to the bottom diagram in Bore Site Method of Internal Mirror Laser Tube Alignment and Principle of Mirror Alignment Using Reflected Beam.)

            Alternatives to the pair of BSCs include a certifiably dead HeNe tube of the same diameter as the TUT with its mirrors removed (so red light can pass easily) or some other substitute that would sit on the V-blocks with tiny holes at each end to align the A-Laser's beam. If you do opt for the dead tube approach, first make sure you have a valid death certificate for it - see the section: How Can I Tell if My Tube is Good? and then make sure to offer the appropriate ritual prayers and sacrifices to the "god of dead lasers" before dismembering the tube! :-) In either case, make sure your substitute actually provides equivalent alignment to the TUT - as noted above, manufacturing tolerances may result in the bore being noticeably off center even in a healthy tube.

        • Step 4 - Checking and correcting the alignment: With everything in position and the A-Laser powered up, there should be a reflection of the A-Laser's beam back onto the AL-Bezel from the mirror of the TUT. If the alignment of mirror facing the A-Laser is perfect, these reflections, it be entirely within the hole in the AL-Bezel with just a symmetric halo (with some dancing interference fringes showing) due to multiple reflections between the A-Laser and TUT's mirrors. This assume that the mirror is planar and has no wedge. A grossly misaligned mirror is shown in Principle of Mirror Alignment Using Reflected Beam. Where you are using the bore sight method, the hole in the BSC may be used instead of the AL-Bezel (which is then removed as shown in the bottom diagram). This doubles the sensitivity to alignment error.

          There will actually be two sets of reflections from the two surfaces of the mirror glass of the TUT. The one from the inner surface - which is probably much stronger, especially for the OC which is Anti-Reflection (AR) coated) - is the relevant one but both should coincide when alignment is correct (assuming no wedge). This is shown in HeNe Laser with Reflected Dot.

          In the case of a curved mirror, one of the spots will be somewhat spread out and if the centering of your A-Laser isn't absolutely perfect, it will be offset to one side even if the mirror alignment is perfect (but I already warned you about dealing with tubes having curved mirrors). Go bad and double check the setup - if it is possible to center this reflection with the A-Laser beam still passing cleanly through the bore, alignment of this mirror is probably fine. The reflection from the curved inner surface can be identified by moving the A-Laser from side-to-side: It will move by a greater distance than the reflection from the flat outer surface.

          • CAUTION: If the alignment looks good, DON'T TOUCH IT! Maybe the real problem is at the other end of the TUT or something else!

            If the reflections are off to one side, FIRST CHECK THAT YOUR SETUP HAS NOT SHIFTED POSITION. GO BACK AND DOUBLE CHECK YOUR A-LASER and TUT ALIGNMENT! For slight errors, problems with the setup are more likely than problems with the TUT's mirror alignment.

          • If you are positively sure beyond any shadow of a doubt that imperfect TUT mirror alignment is the cause of your not-centered reflections, you can attempt to use your tool to CAREFULLY adjust the mirror mount until the reflections are perfectly centered.

            Again, double check that the critical alignment of the two lasers hasn't shifted before messing with the mirrors!

            CAUTION: The mirror mount is ultimately attached to the glass envelope of the tube. The glass-metal seal may not be that strong. Don't get to carried away! With care this adjustment should be possible - barely. :-)

            • Increase the force gradually until you have a feel of how much it takes to actually deform the mirror mount. Even a significant pointing error will only require a small correction. And, the final accuracy needs to be a fraction of a mR - much less than 1 part in 1,000! Not easy.

            • Approach the desired deflection in small increments and overshoot just enough so that the mirror mount springs back to the optimal position. Avoid repeatedly bending it back-and-forth or you will eventually be using the TUT as a high-tech wall hanging :-(.

            • As alignment for the mirror you are adjusting approaches perfection, you will see multiple spots from multiple reflections between the output mirror of the alignment laser and the mirror of the TUT as the light bounces back and forth between them. When perfect, there will likely be a halo of dancing interference fringes surrounding the hole as well.

        • Step 5 - Double check the (new) alignment: If you adjusted the mirror, go back and check the alignment of the A-Laser and TUT to assure that it is still perfect. A significant change in the angle of the mirror could affect the apparent location of the bore of the TUT used for alignment in step (3). In this case you will need to correct the alignment and then repeat steps (4) and (5). Relax - this process will converge so you won't be stuck in an infinite loop forever!

        • Step 6 - Flip the tube: Turn the TUT around and repeat steps (3) to (5) for its other mirror.

        If mirror alignment was your problem (and for larger tubes, if you believe in minor miracles!), the TUT should hopefully now produce at least some output beam when powered up.

        • If it produces even a very weak beam, all you need to do is to perform final mirror alignment. There still may be a lot of work ahead of you but with care, the rest is easy! Just make sure you never make an adjustment such that you lose the beam entirely.

        • If there is no evidence of a beam at all, one or both mirrors may still be too far out of alignment. Go back and repeat steps (4) to (6) for both ends of the TUT. If there is still no output, with luck, only one mirror is now at fault and can be identified and corrected with the TUT powered up.

        In either case, see the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes.

        Walking the Mirrors in Internal Mirror Laser Tubes

        If your laser tube produces any sort of beam and the alignment of both mirrors independently is optimal (either after testing and/or after correcting it as described in the sections starting with: Problems with Mirror Alignment, it's time to either be happy with what you have, or attempt to optimize the output power and beam quality by adjustments to both mirrors.

        Indications for the need of further alignment include:

        • Output power with proper excitation (e.g., the optimal tube current) is significantly less than expected based on tube specifications and there is no evidence of other physical problems (like contamination of the gas fill or damaged mirror coatings).

        • The beam profile is not perfectly circular and Gaussian (assuming it is supposed to be TEM00).

        • There are beam artifacts like off-axis arcs, rings, spots, or fuzzy areas. (However, a single weak spot or row of spots is probably just due to the OC having some 'wedge'. See the section: Ghost Beams From HeNe Laser Tubes.)

        See Effects of Walking the Mirrors for an exaggerated (hopefully!) illustration of why this happens. As can be seen, although the mirrors may be perfectly parallel to each other and there is still some output, by not being aligned with the bore/capillary, portions of the beam are cut off, less than the full amount of gain medium participates in the lasing process, and there can be reflections from the walls and other structures in the tube to create artifacts.

        For external mirror lasers with fine adjustment screws on the mirror mounts, the "Walking the Mirrors" procedure isn't really at all difficult: Both mirrors are moved in the same small increments using the micrometer screws (so they remain parallel), first in X until power is maximized, then in Y, and then back and forth optimizing each direction until no further improvement is detected. This aligns the mirrors so they are precisely perpendicular to the bore. Your typical obsessive-compulsive laser physicist type spends his/her life playing with these knobs. :) See the section: Walking the Mirrors in External Mirror Lasers for more info.

        For an internal mirror laser tube without screw adjusters, a modified approach must be used. I will tell you up front that this is a royal pain and is most easily done if you have three hands (or at least a rigid means of mounting the laser tube and the proper tools). But it can be done and for some cases - most commonly where a tube is marginal to begin with due to age or use, or where someone else, (of course)! has played with the alignment - the improvement in performance (power output and beam quality) over adjusting the mirrors independently may be quite dramatic.

        For all measurements of output power, a laser power meter is highly desirable. It doesn't need to be fancy since maximizing power is what's important, not an accurate value. Anything that will convert photons to a meter reading will be fine including the absolutely trivial ones described in the sections starting with: Sam's Super Cheap and Dirty Laser Power Meter. It's just that your basic allotment of eyeballs isn't very good at detecting small changes in intensity! :) Note that mode cycling of your HeNe tube will result in small variations in output power - these can be annoying but need to be mentally discounted in determining the maximum power output readings.

        • The best way to walk the mirrors is to fabricate and install the adjusters described in the section: Home-Built Three-Screw Mirror Adjusters for Internal Mirror Tubes if your tube doesn't already have them. Then, the same basic procedure as used with external mirror lasers can be used for your internal mirror tube except that there will be three (not quite independent) axes to deal with. However, this is really no problem and the same rules apply: Optimize each axis before proceeding to the next. Go around a couple of times and you are done. :)

        • Where your tube doesn't have screw adjusters and you are too lazy to build them, the procedure is awkward but can still be done. In some ways it is actually simpler as only one axis will need to be optimized - the trick is to locate it. However, I expect that you will wish you had constructed the three-screw adjusters before you are done with this effort:

          • Fasten the tube down securely to a rigid support so that both mirror mounts are accessible for whatever means of mirror adjustment you have available. (See the section: Means of Adjusting HeNe Tube Mirrors.) It will be necessary to be able to rock BOTH mirrors at the same time in a given axis. Thus, there must be two sets of adjusters AND at least the one for the anode-end of the tube must be well insulated! To enable multiple axes/directions to be tried, it is probably best to be able to rotate the tube. Mark the top of the tube so you can keep track of its orientation (if rotating it).

          • Power up the tube and set up your laser power meter to be monitoring its output. If you are just using your eyes, project the beam through a lens (positive or negative) so that it spreads into a large spot on a white card. This will help you to detect small changes in brightness as well as to show beam artifacts which also are an indication of alignment problems. As you approach alignment perfection, the beam will look more and more like that ideal circular Gaussian profile. When alignment is off, it can be elliptical, have fuzzy edges, sometimes extra off axis rings or spots (but don't confuse these with wedge in the OC), etc.

          Now you are all set. The following assumes you can only deflect the left-hand mount one way (specifically, downwards, as would be the case if you were using a big screwdriver as a lever type adjuster). If you can go both ways and/or don't need to rotate the tube to check different directions, the following procedure to determine misalignment direction and magnitude will go a lot quicker.

          • Using your mirror adjustment tools (insulated if necessary!), gently deflect the left-hand mirror mount downward just enough to cause the power reading to be reduced slightly (say, 10 to 25 percent). At the same time, gently deflect the right-hand mirror mount upward and rock it up and down attempting to locate the point of maximum power. (Left and right, and up and down are arbitrary - interchange if more convenient as long as you are consistent.) The objective is to locate the point of maximum power while maintaining the mirrors parallel but changing their pointing angle by equal amounts.

          • Note the maximum power reading on your meter or eyeball(s). :)

            • If the new maximum power reading is greater than the previous one, repeat the above test after shifting the deflection direction by about 45 degrees (e.g., by rotating the tube by 45 degrees).

            • If the new maximum power reading is less than or equal to the previous one, go back and attempt to locate the direction resulting in the maximum reading. Then, narrow down the direction by rotating the tube in smaller increments.

          In English, what we are attempting to do is find the direction and amount to adjust the mirror mounts to line up the mirrors with the bore. The proper direction will result in the most dramatic power increase with both mounts deflected. The opposite direction will result in absolutely no power increase - power will always decrease no matter how much either mount is deflected. In fact, this is a good test to determine if your adjustment direction is correct: Rotate the tube 180 degrees and confirm that power always descreases, even for very slight deflections of the left-hand mount.

          Once the direction and magnitude of the error has been determined, it is time to actually adjust the mounts.

          • While still being gentle, vary the deflection of the left-hand mount (in the direction determined above) in small increments while rocking the right-hand mount in the opposite direction to maximize the peak power output. This will identify the magnitude of the required alignment change.

          • Once the power output has been maximized, release the right-hand mount and note the (now reduced) power reading.

          • Use your mirror adjuster to now bend the left-hand mount permanently to obtain the same power reading as above.

          • Finally, bend the right-hand mirror mount to restore and maximize power output.

          Mirror alignment should now be absolutely positively optimal and perfect. :)

          As mentioned numerous times, DON'T attempt this unless you are determined to do something to help your tube. One slip of the adjuster and you will be worse off than before and may need to go back to square one: acquiring a new tube or at least restoring basic alignment. If the tube's deficiency is small, leave it alone! Or, install the three-screw adjusters which are a lot less likely to kill a tube than a big screwdriver!

        Tubes that are marginal due to age or use and output a very weak beam seem to benefit the most - a 200 percent or more boost in power is quite possible (though it will still likely be less than their ratings when new). This is probably because the gain is lower and therefore mirror alignment becomes even more critical. The same alignment errors might only result in a 10 or 20 percent reduction in power for a tube in good condition. And the misalignment might have always been present - factory quality control isn't perfect and tubes would be considered good enough as long as their catalog ratings are met or exceeded when new.

        I have improved the performance of several internal mirror HeNe tubes using these techniques. One of these is discussed in the section: Strengthening a Weak Siemens HeNe Tube. Another was a cute little Melles Griot 5" HeNe tube which was only putting out .1 mW. It's output was boosted to about .3 mW by walking the mirrors (still less than the .5 to 1 mW for other similar size tubes). Some additional improvement might be possible with more work.

        Simple Adjustable Optics Platform

        For simple alignment checks, an adjustable platform can be constructed in about 10 minutes from common materials. This isn't quite as precise as an expensive lab jack or the all-metal version shown in: Alignment Laser Three-Screw Platform. However, for many purposes, it may be good enough. Of course, there's no law against making the design below out of metal! The same basic approach can be used for either the alignment laser (as shown in the photo) or the mounting for the Tube Under Test (TUT). From my experience and that of others, the latter arrangement is more intuitive for aligning the TUT's bore to the A-Laser's beam. However, it may be difficult to prevent the platform's position from shifting while checking or adjusting the mirrors compared to a mounting with a fixed base that can be clamped to the workbench. And, easily accommodating multiple size TUTs would be more of a challenge.

        Also see the section: Sam's Eazalign(tm) Internal Mirror Laser Tube Alignment Platform since your internal mirror laser tubes may deserve only the finest in alignment equipment! :)

        You will need:

        • A piece of Plexiglas, hardwood plywood, or even high density particle board, about 6" x 12" x 3/4" (or whatever size your laser or other optical system will fit on with room to spare to spare for the adjustment screws).

        • Three, tapped (totally through) metal spacers about 3/4" long. A thread size of 8-32 or 10-32 is adequate though more threads/inch would be even better.

        • Three, 2" (or longer) machine screws to fit the tapped spacers.

          The following is best done using a drill press but it is not essential:

        • Drill holes so the tapped spacers will be a snug fit in the board 1/2" from the corners at one end of the board and at the center of the other end, 1/2" from the edge.

        • Press the spacers into the holes. If they are too loose, secure them with 5-minute Epoxy (how else to guarantee a total assembly time of less than 10 minutes!).

        • Round off the ends of the machine screws using a file or grinding wheel so they will each contact the surface upon which this assembly will rest at a single point.

        • Install the machine screws into the tapped spacers so they poke out of the bottom of the platform.
        There you have it! This will permit a laser or other optical system secured to this platform to be adjusted easily for height, pitch, and yaw. If you need more of a height adjustment range, sit the platform on something else.

        While this isn't quite as precise as one milled out of a solid block of high strength (aircraft quality) aluminum alloy using anti-backlash spring-loaded micrometer adjustment screws, it will suffice for many purposes and costs next to nothing!

        Sam's Eazalign(tm) Internal Mirror Laser Tube Alignment Platform

        Note: It is recommended that the sections starting with: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes be read first so that the various terms, alignment techniques, and risks are understood.

        OK folks, this is what you have all been waiting for. :) The ULTIMATE in precision and convenience. Well, sort of, at least if you construct and use it with reasonable care. Depending on the length of the platform, almost any size tube can be checked for alignment or realigned quickly and easily. Does this sound like a sales pitch yet?? :) The Eazalign platform combines the Primary Alignment Laser (PA-Laser), adjustable Tube Under Test (TUT) mount (ATM), optional Bore Sight Mounts (BSMs), and Far Reflector (FR) mirror or Secondary Alignment Laser (SA-Laser) into one handy (but not so compact) package. ;-)

        This approach should also do a decent job with those annoying curved mirrors since the same reference is used at both ends of the TUT without the need to remove and replace it.

        In addition to supporting the various alignment techniques discussed previously, the Eazalign platform adds a way of providing a return beam so that both mirrors can be checked and aligned in place (without turning the tube end-for-end). For small to medium size HeNe tubes (up to 10 inches or so) using a different color A-Laser (e.g., green HeNe to align a red HeNe, this can be accomplished without the need for a second A-Laser by using a flat first surface mirror (the Far Reflector or FR) on an adjustable mount set up to return the A-Laser beam precisely back to its output aperture

        However, for longer tubes or where the PA-Laser is the same color as the TUT and it isn't possible (at least in finite time) to get a clean beam down the bore, the use of SA-Laser will be needed. Where the Bore Sight Method is used to align the TUT to the A-Lasers, the A-Laser colors won't matter.

        The basic setup is depicted in Eazalign Internal Mirror HeNe Laser Tube Mirror Alignment Platform and consists of the following components:

        • Main Mounting Rail (MMR) - Ideally this is milled out of a solid block of stainless. :) Just kidding. A length of 2" x 6" extruded aluminum U or box stock is nearly as good, though somewhat expensive. But, a well seasoned length of a pine 2" x 6" should suffice for occasional use. However, if you are going into the HeNe laser repair business, invest in the aluminum.

          The length will be determined by the maximum size of the TUT that needs to be accommodated and the size and number of A-Lasers. Figure 3 to 4 TUT lengths plus space for the A-Laser(s) or FR mirror.

        • Primary A-Laser (PA-Laser) - All implementations will have one of these. Any 1 to 5 mW HeNe laser producing a narrow well collimated beam can be used. However, when aligning the TUT to the PA-Laser by passing its beam down the TUT's bore, the PA-Laser must be of a sufficiently different wavelength so that enough of its gets through the dielectric mirrors. This usually would mean a green HeNe laser to align a red one or vice-versa. (A green or blue argon ion laser could be used but I personally dislike running a power hog for this purpose.) However, note that while a green beam will get through a red tube quite well, all sorts of confusing reflections may be present as well which can be quite confusing.

          The PA-Laser must be rigidly fastened in position and centered radially aimed precisely down the axis of the Mounting Rail. Its height will depend on the design of the Adjustable TUT Mount (below) to enable TUTs of various diameters to be accommodated. An easy way to mount the PA-Laser is to attach it to a metal plate or piece of wood and then fasten this to the main platform using three screws with a combination of a flat washer, one or more split, Bellview (cupped), or rubber washers, and another flat washer. The compressible washers will provide enough range of adjustment to line up the PA-Laser's beam. Stiff springs could also be used.

        • Adjustable TUT Mount (ATM) - This must be able to move the front and back ends of the TUT vertically and horizontally more or less independently. Using the basic arrangement of the Three-Screw Optics Platform with the addition of V-blocks for the TUT and a pair of spring loaded screws pushing it side-ways will suffice. From my experience and that of others, it is easier to adjust the TUT's mount than the A-Laser(s) to align its beam to the TUT or Bore Sight. Furthermore, for this technique, the A-Laser(s) should only need to set set once and then left alone.

          The ATM should be located along the MMR such that the distance between the front mirror of the longest TUT to be accommodated is at least one of these TUT lengths from the PA-Laser's output aperture.

          The adjustments closest to the PA-Laser should be located approximately at the same axial position as the front mirror of the TUT. This will make its settings mostly independent of the other set of adjustments. Obviously, those would ideally be located near the rear mirror of the TUT but this would only be possible for a single size TUT!

          It is critical that these adjustments be quite precise and have a provision to be locked in place once they are set. Thus, fabricating the ATM out of aluminum or steel with micrometer screws would be best but wood will work here as well unless you are going into production alignment. :)

        • Bore Sight Mounts and Bore Sight Cards - These will be needed where the Bore Sight method is used to align the A-Laser(s) to the TUT.

        • Far Reflector (FR, if used) - Where the PA-Laser's beam is still quite narrow after a distance of about 4 maximum TUT lengths (say 2 mm diameter) and the PA-Laser's beam is to be passed through the TUT's bore, an SA-Laser is not essential. A planar (or slightly concave) first surface mirror on an adjustable mount can be used to return the SA-Laser's beam back to the TUT's mirror.

          The mount doesn't have to be anything special - I used one from a barcode scanner. It is basically stamped sheet metal with two adjustment screws but has adequate precision and works quite well. After it is adjusted, fasten a white card with a hole the size of the PA-Laser's beam at the FR to it to act as an output aperture for the virtual SA-Laser.

        • Secondary Alignment Laser (SA-Laser, if used) - The use of the SA-Laser is the preferred method where the platform is long enough that by the time the A-Laser's beam has passed through the TUT and reflected from the FR mirror, it is excessively wide to provide adequate accuracy in alignment. This is typically on the order of 1 meter. It is actually better in all cases but might not be justified for short tubes or occasional use. However, if you are aligning same color tubes, passing the SA-Laser beam through the TUT is not an option.

          Like the PA-Laser this can be any 1 to 5 mW HeNe laser with a narrow well collimated beam. Its color doesn't matter since there is no need to pass it through the TUT's bore. Mounting should be similar to the PA-Laser with its output aperture about 1 TUT length beyond the TUT's far mirror (assuming the longest TUT to be aligned).

        The PA-Laser and SA-Laser are set up to their beams are precisely aligned with each-other. In other words, the beam from the PA-Laser is centered on the SA-Laser's output aperture and vice versa. If adjustable mounts are not used for both lasers, this can be done with shims. Where the FR mirror is used in place of the SA-Laser, it is adjusted so that the return beam is precisely centered on the PA-Laser's output aperture.

        To use this system:

        • The alignment of the PA-Laser and SA-Laser or FR mirror is confirmed to be correct.

        • The TUT is installed and locked in place on the ATM and it is adjusted for a clean PA-Laser beam through the TUT's bore. Or, if using the Bore Sight method, the BSMs and BSCs are installed and checked for proper alignment with the PA-Laser beam and then the TUT is attached to the ATM and adjusted to line up with the holes in the BSCs.

        • The TUT's mirror closest to the PA-Laser is checked and adjusted if necessary to place center its reflection in the output aperture of the PA-Laser. If it is quite close to start, double check the position and orientation of the ATM - with a curved mirror, slight radial position errors will result in a shift in post angle. There may be nothing wrong with the alignment. If tweaking the ATM can center the spot and still result in a clean beam through the bore, alignment is fine at that end.

        • The TUT's mirror furthest from the PA-Laser is then checked and adjusted to center the reflections on the FR or SA-Laser aperture.
        That's it! Since only one reference is used (the precisely centered beams of the PA-Laser and FR mirror reflection or SA-Laser) and the TUT is never moved once it is installed on the ATM, this should result in very precise alignment in a short amount of time.

        Initial Alignment and Tweaking of Three-Screw Adjusters

        These are the type found on some HeNe tubes which are way too small to really provide fine control. See Three-Screw Locking Collars on Melles Griot HeNe Laser Tubes. However, they do work well enough with a bit of care and a lot of patience! :) Home-built adjusters can be constructed somewhat larger so the adjustment range will be greater (see the section below).

        Realize that the 3 adjustments are not really independent since each uses the other two as the pivot. However, the net effect is fairly predictable.

        CAUTION: Don't get carried away while turning these screws - it is possible to rip the mirror mount off the tube! The entire adjustment range is less than 1 turn of each screw once they are snug.

        Any of the procedures and setups described above can be used to determine when the mirror is properly aligned. For longer tubes, I recommend the Instalign technique be attempted first. With care (and a bit of luck), this will get you to a lasing state without the requirement for fancy alignment platforms and jigs.

        The following assumes that only one end of the tube is misaligned. Where both ends of the tube are messed up or in an unknown state, your task is just that much more challenging! :)

        • Apply a tiny drop of lubricating oil or graphite to the threads and tip of each screw to minimize sticking.

        • Start with all 3 screws loosened at least 1 turn before contact.

        • Determine which direction the mount needs to be deflected and carefully turn the screw or screws opposite that direction to bring the reflected beam back towards the center. For the Instalign procedure, this would be 1/2 of the total beam displacement.

        • Turning each screw somewhat beyond the optimal setting will result in actual bending of the mount. This is only a fraction of a turn so go slow. Move it a bit, back off, and see where it remains. This is best done where the other 2 screws are loose but the options on movement angle are, of course, limited (as in, only 3). But, with care, iterating among the 3 screws will get you where you want to be eventually. The objective should be to adjust the mount so that with all three screws loose, it is close to optimal alignment. Then, only very slight movement will be needed to tweak it.

        • Where possible, it's better to loosen the screw or screws on the opposite side than to further tighten a screw if it is already more than snug.

        • Once the alignment is as good as you can make it from the reflected spot, apply power to the tube. If there is a beam, alternately adjust each screw to peak the power.

        • Pressing on the mount (with an insulated stick if it is the anode) will help determine which screw to turn and whether to loosen or tighten it.

        • There will always be a certain amount of backlash in the screw adjustments so it will probably be necessary to back off, then approach from the loosened state if you overshoot. For a long tube (e.g., 15 inches), the entire adjustment range for lasing will be a small fraction of a turn of each screw. If the beam disappears entirely while adjusting one screw, restore it using that same screw! Never attempt to adjust another one if there is no beam - you will have to go back to the initial alignment to get it back!

        • Check for mirror alignment at the other end of the tube using the mirror walking procedures and correct it if necessary.

        • With the power at its maximum, adjust all three screws equally (while making sure the beam doesn't disappear!) so that the loosest of them is still quite snug (this will mean tightening or loosening the screws depending on where they were set). Then retweak the output power after adequate warmup. This will assure that the screws won't change position with thermal cycles.

        Home-Built Three-Screw Mirror Adjusters for Internal Mirror Tubes

        It is possible to construct mirror mount adjustment assemblies operated by thumbscrews or set screws to correct or optimize the alignment of HeNe and other internal mirrors laser tubes with the common mirror mounts consisting of a pair of tubes separated by a compliant narrow section.

        For a small misalignment, this would avoid the risks of actually trying to bend the mounts since the range of motion would still be within the elastic limits of the metal. This type of adjuster is really best for fine tweaking where a beam of some sort is already being produced, not for initial alignment where the mount is bent at a visible angle! And, as noted in the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes, some (mostly older) HeNe tubes have these built-in for both mirrors or possibly just the OC.

        See Typical HeNe Tube with Three-Screw Adjusters Added for an example of one approach. With even very basic machining skills and a little scrap metal, a set of these should be very easy to fabricate.

        I would recommend 1/8" to 3/16" brass or mild steel for the plates. (In some cases, the inner plate can just be replaced with a protective metal or plastic washer bearing against the metal end cap of the tube.) Aluminum would probably be acceptable as well but might deform or wear too easily if the adjustments get much of a workout. :) (Even acrylic plastic (Plexiglas) or other non-metal material might be adequate for minor corrections and in addition to easier machining, have the benefit of being an insulator!) Drill the holes in the center of the plates (preferably reamed to the correct size) to just fit over the two sections of the mirror mount(s) and use Epoxy or another adhesive to secure them in place. A clamp arrangement would also work (and permit easy removal of the adjusters in the future) as long as it is designed so that there is no tendency to deform the tubes of the mount and not tightened excessively - which could ruin your whole day by cracking the glass-to-metal seal(s). Use fine-thread set-screws with rounded tips.

        The adjusters should be firmly attached (glued with Epoxy or carefully clamped as noted above) to the HeNe tube end-caps or bolted to a rigid baseplate. (However, in the latter case, expansion of the HeNe tube as it warms up will complicate matters.) They could be left permanently in place applying the proper force to the mirror mounts to maintain mirror alignment and always providing the option of making fine adjustments at any time if needed. with these built-in for one or both mirrors.

        When actually adjusting the mount, no single screw should be so excessively tight that there is a chance of liberating the mirror mount from the rest of the tube! The entire useful range is only a fraction of a turn of each screw. If you are headed that way (or the mirror mount is sitting at a 20 degree angle), it will need to be moved into a initial position (using one of the other tools described above) before the three-screw adjuster can be safely used. Adding a tiny drop of penetrating oil to each of the screws and its contact point will minimize the tendency to of the screw to 'stick' thus easing adjustments. Once you have found the best setting, incrementally snug up each of the screws so they are all applying at least a little pressure to the mirror plate. This will maximize long term stability.

        CAUTION: Use a well insulated tool (hex wrench) to avoid a shocking experience!!

        However, the use of such drastic measures may be gross overkill for use with these small inexpensive HeNe tubes unless you have a machinist sitting around with nothing to do. :)

        Sam's Instalign(tm) Procedure for Internal Mirror Tube Mirror Alignment

        The following is a quick way of checking and correcting the alignment of internal mirror HeNe and Ar/Kr ion laser tubes if the bore is precisely centered with respect to the envelope. Centering is usually pretty good on newer tubes but I've seen a some older ones where it was off by a good fraction of a millimeter.

        The idea is to roughly center the beam of an Alignment Laser (A-Laser) in the bore and then rotate the Tube Under Test (TUT) to check for wobble in the reflection back to the A-Laser's aperture. Where the bore is centered, any shift in the position of the reflection will be due to the misalignment of the mirror.

        • Set up your TUT on a pair or V-blocks about one tube length from the A-Laser, which can be almost any type as long as it is reasonably well collimated - HeNe, diode, argon ion, etc.

        • Align the TUT and A-Laser so that the A-Laser's beam is centered on the TUT-s mirror and reflects back to its aperture centered or just off of center.

        • Without moving the TUT lengthwise, rotate it on the V-blocks and watch the reflected spot on the A-Laser's aperture.

          • On a perfectly aligned mirror, the spot will remain in one position as you rotate the TUT.

          • Where there is misalignment, the spot will wobble back and forth as the TUT is rotated.

        • Where wobble of more than say, 1/2 mm is detected, first confirm that it isn't due to some other cause like a label on the TUT which shifts its position as it is rotated or its bore not being centered (and the original mirrors having been aligned to the bore). If you are sure the offset is not due to one of these causes, use one of the mirror adjustment tools (or tweak the set screws if your laser has them) to reduce the wobble to zero. In other words, rotate the TUT so the beam is at its maximum offset and bend the mirror to return it to halfway between the max and min. Repeat the procedure zero in on the optimal setting.

          With a bit of luck this will be sufficient to now allow gentle rocking of the mirror to result in a beam. With more luck, you will have a beam the instant power is applied! :)

        I've used this approach successfully on HeNe tubes up to about 20" in length (so far). It is particularly effective where only one mirror is misaligned since the result will be close enough to use minimal force in rocking the mirrors to find the precise setting. Add-on adjusters or locking collars will then be sufficient to fine tune it.

        Sam's Approach for Aligning an External Mirror Laser with the Mirrors in Place

        Here is a quick procedure for aligning an external mirror laser where the area between the mirrors and Brewster windows is at least a couple of inches long and accessible.

        As with most of the other techniques, this one requires an Alignment Laser, preferably a well collimated HeNe laser of a different wavelength than the laser being aligned but a same color laser can be used though the transmitted beam will be much weaker.

        • Rigidly mount and align the A-Laser to the bore of the Laser Under Test (LUT) with the LUT's OC-end facing the A-Laser (the OC will pass more light where the two lasers are the same wavelength and is thus makes the procedure easier).

        • Adjust the OC mirror so that the reflection of the A-Laser's beam is centered in the A-Laser's output aperture.

        • Position an opaque white card with a hole the size of the A-Laser's beam next to the HR-Brewster window so that the A-Laser's beam passes cleanly through the hole. Use a stable third hand. :)

        • Adjust the HR-Mirror so its reflection of the A-Laser beam also passes cleanly through the hole. The scatter from these two beams should also appear on the Brewster window and should coincide. Fine tune the HR to make it so.

        • Move the card and third hand (or use a second set) to a position next to the OC-Brewster window, again with the A-Laser's beam centered in the hole.. There should be some evidence of the reflection from the HR mirror on the card. Fine tune the HR mirror so it is centered. Similarly, fine tune the HR mirror so that the scatter of the two beams on the OC Brewster window coincide.

        • When alignment approaches perfection (adusting either mirror), you should see an increase in brightness of the scatter off the Brewster windows indicating that the beam (even if it is quite weak) is bouncing back and forth a few times at least. If the LUT is powered up, there should be flashes at this point as well and further alignment can be done by optimizing output power by walking the mirrors.
        Using this approach, I can replace and align either mirror in a low gain HeNe laser with a 15" resonator (see HeNe Laser Tube with Two Brewster Windows Mounted in Home-Built Resonator) in under 5 minutes assuming the A-Laser is still in place and aligned with the LUT's bore.

        Quick Course in Fine Tuning a Large Frame HeNe Laser

        This is an abbreviated version of the procedures discussed in the section: External Mirror Laser Cleaning and Alignment Techniques. It applies to external mirror HeNe, Ar/Kr ion, and other similar gas lasers.

        (From: Steve Roberts (osteven@akrobiz.com).)

        Get an analog power meter in front of the laser before doing this. You must be able to see the changing trends in the power output. This assumes clean optics and a good tube at proper current levels.

        WARNING: This procedure is not for the timid, easily distracted, or faint of heart!

        When tuning a laser, you work with either the verticals or the horizontals, but never both at the same time. Failure to do this makes it easy for you to misalign yourself into non lasing in a fraction of turns on the adjustments.

        Start with the verticals, pick a direction for the front screw to turn, either left or right, detune the laser power by about 30% , then go to the rear VERTICAL screw and peak the power, Leaving the HORIZONTAL screw UNTOUCHED. If the power is greater after peaking then before, keep going in the same direction till it falls off then go back to the peak, and then keep going the other direction, doing the same simple process of slightly detuning, peaking and measuring. If you write down your power meter readings, you will get the idea and will be able to find the vertical sweet spot. You are scanning the cavity lasing path across the bore.

        Then do the horizontals, same procedure, pick a direction, slightly detune, peak with the other mirror, etc.

        Note: this is the short version of this. On most larger lasers, you would move the front and rear mirrors the same direction by about the same amount. However since this is a large frame HeNe, different rules apply, It usually takes a large frame HeNe about 2 to 3 minutes to settle down after a adjustment is changed, keep this in mind and go slow.

        This is a iterative process, you have to repeat the steps many times on both the horizontal and vertical axis till you have the exact peak, if you have one of the many lasers (e.g., argon lasers like the Lexel, Ionics, 60X) that stresses the Brewster stems when adjusting the mirrors, make sure the Brewster covers are off or relaxed.

        Digital power meters take too long to update when tuning a laser, making it easy to scan past the exact peak. and you can't see which way you are going, it is very important to use a wide scale analog meter.

        Daniel's Methods for Internal Mirror HeNe Tube Mirror Alignment

        (From: Daniel Ames (dlames3@msn.com).)

        The following notes are what I observed and used successfully for aligning a 5 to 7 mW red HeNe laser with another red HeNe laser. (In addition to the procedure that follows, there is a simpler one using three HeNe tubes - (1) a laser to produce a beam for alignment, (2) the tube needing alignment, and (3) an identical tube which is used for setting things up. See: Now for the Quick and Easier Shortcut.

        The Basic Procedure

        Last night I was trying to fine tune the HR mirror on a 16 inch Melles Griot HeNe internal mirror laser tube when suddenly, no more beam (ooops). So now I was starting to panic, because I needed to ship this laser tube the next day, plus at the moment, the only working lasers that I have are other red HeNes.

        For the procedure below, I used a 2 mW HeNe for the reference laser (R-Laser). Thanks to the sections starting with: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes I found and read up on how to realign a red HeNe with no lasing occurring, with another red HeNe. Well I didn't have the time to construct a bore sight and mounting block to (hopefully) get the R-Laser's beam positioned so that when the TNA (Tube Needing Alignment) was installed into the bore sight's mounting blocks, the reference beam would shine directly on center and parallel to the axis of the TNA's bore.

        So I decided to improvise, using a HeNe tube I had laying around which has Brewster windows for use with external mirrors to substitute for the TNA and mounted it on my previously made alignment platform. This tube is virtually the same diameter as the TNA and could thus be swapped for it with its bore in the same location. With no mirrors, the red beam easily pass through it permit accurate alignment.

        Note: (If you have a dead HeNe tube of the same diameter. you could pull the mirrors off and use it for a bore sight, (but first, read the section: How Can I Tell if My Tube is Good?). And, be advised that the "god of dead lasers" could become very upset with you, plus Sam and myself both agree that this would be sacrilegious) so you will have to make some type of atonement with the laser gods. Maybe, resurrect two other helpless laser tubes for each one used for this purpose. :-)

        The TNA and this special HeNe that I used, are the same diameter (+ or - a few .001"s). This allowed me to align the mirrorless tube (MT) with the reference beam right down the center of its bore (with the two Brewster windows facing upwards), with a very clean red spot exiting the rear Brewster window.

        Next, I carefully removed the MT and without moving the alignment platform, installed the TNA. Of course, the centering of the reference beam was off axis slightly (on the vertical plane) due to the MT's Brewster window's index of refraction. But this didn't matter in this case, even with the the curvature of the internal reflecting surfaces of both lasers' OCs, because the outer reflecting surfaces of both OCs were flat and parallel. So I used the small reflected spot for reflection alignment, not the larger one caused by the curved internal reflecting surfaces of the OCs.

        I chose to use the OC mirror-end on the TNA for reflection alignment back to the R-Laser's OC for two reasons:

        1. When you get the TNA OC aligned so that it's reflected spot is centered in the R-Laser's beam and producing interference patterns dancing around in the spot, you are then ready to power up the TNA to see if it will actually produce a beam. If it does, (good job!) now you can use the TNA's beam to align it with the R-Laser's beam (providing you have two power supplies). (This took me approximately 20 minutes at the most for the initial alignment of the TNA's OC once the R-Laser's beam was aligned down the center of the TNA's bore, and yes, it did actually produce a beam.

          But even if you only have one power supply, I found it a good idea to use the TNA's OC for reflection alignment instead of the HR mirror because if during the fine tuning of the TNA, it happens to get so far off alignment to where there is no longer a beam and you haven't yet moved either the reference beam or the alignment platform, than you can save a lot of time by not having to set everything up all over again!

        2. The OC has a higher (%) of transmission and by aligning the reference laser's beam right down the center of the TNA I found using a white or fluorescent orange sticker behind the rear HR mirror that there was still a small amount of the R-Laser's beam getting through the OC and a weak smaller diameter spot to exit the rear HR mirror of the TNA.

          Although it shouldn't make any difference as to the amount of light from the R-Laser actually getting through the HR and OC mirrors on the TNA tube reguardless of which end of the tube the beam enters, what I discovered is that by shining the R-Laser beam into the OC-end of the TNA, you will see a faint "halo" around the faint dot of the R-Laser's beam. This was very helpful in centering the two axis of the cavities bores. (This probably has to do with the curvature or the OC spreading the beam slightly inside the bore. --- Sam.)

          When shining the R-Laser beam into the HR of the TNA, there was no detectable halo even at only 1" behind the OC mirror. Plus, as Steve Roberts mentioned, by using a bright orange (or red) fluorescent sticker to view the beam (but in this case, the one exiting the TNA), you can see the faint patterns or optical deviations during alignment much more easily. This is very helpful in this procedure as the amount of light from the R-Laser beam actually getting through the TNA's optics is very small. A darkened, but safe place to work, is advisable for this method.

          Note: I used a 13" long Melles Griot tube that was already aligned and lasing well just for testing this procedure. When I got the TNA tube aligned and centered with the reference beam, I could actually see the tiny dot of light that exited the TNA and dancing with its optical interference. You want to get more than just any dot exiting the HR mirror of the TNA, what you want to do, is keep fine tuning the alignment of the reference beam with the TNA, until you see the brightest dot & centered in the halo.

        Now for the Quick and Easier Short Cut

        As long as you have two identical tubes (diameter wise) and one is already lasing well, and providing that the OCs do not have a "wedge mirror" (see section: Ghost Beams From HeNe Laser Tubes, then you actually could save a lot of time by not having to align the reference beam exactly down the TNA's bore.

        What I did: I used the The Lasing Tube (TLT) in the alignment jig as the substitute for the TNA. Then, I just aligned it with the R-Laser's beam until I got the reflected and "round" spot centered and with the reference beam. I was viewing the reflected larger spot on the TNA substitute tube. If the larger reflected spot is oval shaped the you need to re-center the reference beam with the center of the TNA's OC mirror. Once this is done, you could be home free for the initial dual alingment.

        Then, replace the TLT with the TNA and don't move the alignment. With the OCs facing each other, if the reflected spots are not centered, just dial in the reflected spot from the R-Laser to center itself with it's larger and small reflections. There should be a lot of optical interference now - dancing or flickering. If the spots are centered and round, then there is a good chance that this mirror on the TNA does not need adjusting at this point, so turn the TNA around and repeat the above procedure, then power it up. If all went well, it will be lasing somewhat. Then, only fine tuning will be required. See the section: Minor Problems with Mirror Alignment. Now you can fine tune the alignment of whichever mirror needed original adjustments.

        Notes on Viewing the Spot

        One method that I have not seen anywhere on the Web, for fine tuning any laser's beam without the professional and high dollar alignment equipment is the following (I tried it and found it very helpful):

        Just getting a laser to produce a beam, after realigning the optics as you know, is really only step one, but getting the optics (fine tuned) is another step. Sure you could just try to adjust by the brightness, but who wants to stare at a bright laser spot continuously. And, if one doesn't have a power meter, here is what I did.

        I used a clean lens {concave/concave, at a distance greater than the lens's reflected focal length, to eliminate reflected interference patterns on the OC and back through the lens}, in front of the OC with the beam centered in the lens and with a white piece of paper at a close enough distance from the laser for visual clarity, (my setup was 4 feet).

        By using a lens with an appropriate focal length, the beam's spot was spread to about 3" in diameter which made it much easier to see variations in the beam and it's patterns as the optics were adjusted, or even just with a slight amount of pressure on them. Of course, a laser power meter would be a good substitute and helpful, but not everyone has one. The lens's focal length and the distance between the laser's OC & the viewing paper are variable.

        I have fixed (realigned) many HeNe tubes that were not lasing at all to start with. But now, I definitely find it much easier to fine tune them using the lens to widen the spot. Then I watch the outside edges for rings and off center irregularities. Argon ion laser tubes are also very difficult if they are not lasing at all, but at least on many of them, their mirror mounting plates are easier to apply pressure to at the X-X, Y-Y, and Z-Z axis and all combinations, to see if you're not to far off and which way. This would apply, for example, to NEC tubes though Cyonics/Uniphase types use HeNe style mirror mounts.

        Notes on the Alignment Jig

        I found that with my homemade alignment jig, similar to the one discussed in the section: Simple Adjustable Optics Platform but much heavier and with two horizontal adjustment screws, it was actually easier to mount the TNA on it and leave the R-Laser stationary. I don't really know if the leverage of the geometry works out any different, but it appeared to be much easier to make the vertical and horizontal adjustment. But of course this will depend on your alignment jig's design plus, how stationary it is and your preferences.

        Daniel's Method for Aligning External Mirror Lasers

        This was written for HeNe lasers using a dual Brewster window plasma tube and external mirrors. Some slight modification will be needed for single Brewstered tubes or other gas lasers.

        The following assumed that you have a setup consisting of a Reference Laser (R-Laser (a HeNe laser is assumed since for aligning a HeNe laser, that is the toughest as the wavelengths are the same. The laser being aligned consists of the Tube Under Test (TUT) and the resonator with the external adjustable mirrors (front and rear). The pinholes are aids to alignment.

        
           +---------+    |                     |                       |
           | R-Laser |==> :            ||       :     /===========\     :       )|
           +---------+    |                     |          TUT          |
                       R-Laser       Front    Front     (Removed)     Rear     Rear
                       Pinhole       Mirror  Pinhole                 Pinhole  Mirror
        
        Where the R-Laser and TUT are of difference wavelengths, overall difficulty will be somewhat reduced as more light will get through the front mirror. For more on the alignment jig and related topics, see the sections starting with: Daniel's Methods for Internal Mirror HeNe Tube Mirror Alignment since much of the setup is similar.

        Note: One assumption made here for the procedure below to work properly is that the Brewster windows on the TUT are of high quality plane parallel optical glass or quartz and are set at equal and opposite angles (/=====\ NOT \=====\). This should be the case with most commercial tubes. However, if these conditions aren't met, there can be a slight shift in position and/or angle of the beam passing through the tube's bore which would mean that aligning the resonator mirrors with the tube removed would NOT result in exactly the same adjustment settings once the tube is replaced.

        (From: Daniel Ames (dlames3@msn.com).)

        Note: since not all HeNe laser tubes follow the same design, ideally the way you want to orient the resonator's optics is to have the curved mirror for the TUT furthest away from the R-Laser. If this turns out to be the TUT's OC mirror that's all the better, since we will be looking for the reference beam's reflection from this rear mirror to align it back through the pin holes within the cavity. Either mirror as the rear mirror will work for this method, but I prefer to use the curved mirror since it will counteract the (+) divergence of the reference beam.

        First determine which mirror is the [curved] one, simply by observing the reflection of the R-Laser's beam from the two surfaces of each individual mirror. On the curved mirror end of the TUT's resonator, place a white or fluorescent orange card between the Brewster window and it's related optic, then take your strongest Hene and shine it through (from the outside) of this mirror, then dim the lights and see if you can see the beam on the target paper. Chances are, you will, faintly. If so, then you can use a HeNe to align a HeNe. Sounds impossible, but it is NOT as I have actually done it and succeeded with this using only a 5 mW HeNe laser - HeNe to HeNe.

        Note: Some pinholes are helpful. Three (3) pinholes, just barely larger in diameter than the R-Laser's beam would be ideal, unless one has a very symmetrical eye. But at least one pinhole is a must for within the resonator's cavity.

        • One pinhole would be placed in at the center of the TUT's first mirror on the (inside) of the resonator since we are using such a reduced power reference beam. This is called the 'Front Pinhole'.

        • The second is placed about an inch away from the TUT's rear mirror, again, on the inside of the cavity and about an inch away from the mirror. This is called the 'Rear Pinhole'.

        • The third (optional, but very helpful pinhole) would be placed in front of the R-Laser's OC and centered with the beam. This is called the 'R-Laser Pinhole'.

        Remove the TUT from the resonator and align the R-Laser's beam right down the optical center of the mirrors starting at the curved mirror, (hopefully it's the HR).

        Now, we are ready for the actual alignment. This part is even easier if the mirrors are easily removable.

        • Alignment Step #1: Adjust the first mirror of the TUT's resonator, (the one closest to the R-Laser), until it's second (inside) surface reflection is centered inside the R-Laser's pinhole and producing interference patterns (resulting from light bouncing back and forth between the R-Laser's OC and the front mirror of the resonator).

          Also look at the pinhole just inside the TUT's first optic, the reflected beam from the R-Laser's OC should be fine tuned to be concentric with the first pinhole inside the TUT resonator.

        • Alignment Step #2: Check the original reference beam's position with respect to the resonator's rear inside pinhole. If nothing has moved, then it should still be passing right through the pin hole. It is? Good job!

        • Alignment Step #3: Ignore the R-Laser's secondary reflected beam and use the R-Laser's original beam to align the two optics within the resonator by first adjusting the rear optic's alignment until it's reflected beam is concentrically passing through the rear internal pin hole, then fine tune this rear adjustment until it also passes cleanly through the front internal pin hole.

        • Alignment Step #4: Look at the first surface of the first internal pin hole to see if the reflected spot is visible, or passing again right through this pinhole. If it is to the side of this pin hole, I would recommend (only) adjusting (fine tuning) the rear mirror's position, not the first mirror.

        • Alignment Step #5: Finally, now that the two optics are aligned, install the TUT and without touching the mirror adjustments, just position the tube so that its cavity allows the R-Laser's original beam to pass cleanly through it. If your brave enough to power up the tube, while adjusting it's position, then when you get it concentrically aligned with the optical axis of the (previously adjusted mirrors) it should lase and save the planet (at the very least), all at the same time! :)

        Flavio's Comments on HeNe Tube Mirror Alignment

        (Portions from: Flavio Spedalieri (flavios@ihug.com.au).)

        Below is a simple diagram that shows the end configuration of a typical internal mirror laser tube:

               \
        	 \ __   __
        	--|  |_|  |-
                  |  |_|  | |====> Laser Beam
        	--|__| |__|-
        	 / ^    ^
               /   |    |
                   |    +--- Adjustable part of mirror mount
                   +--- Fixed part of mirror mount
        

        The end of the mount is divided in two with a gap between the first and second sections. At the time of manufacture, HeNe laser tubes are aligned for optimum power output.

        On some HeNe tubes (as well as internal mirror argon ion laser tubes), this gap may me covered by a ring with three (3) adjustment 'grub' screws as shown below:

               \     ___
        	 \ _|   |_
        	--| |   | |-
                  | |(X)| | |====> Laser Beam
        	--|_|   |_|-
        	 / ^|___|^   (X) denotes adjustment 'grub' screw (1 of 3 shown).
               /   |  ^  |
                   |  |  +--- Adjustable part of mirror mount
                   |  +--- Ring with three (3) grub screws
                   +--- Fixed part of mirror mount
        

        If the tube has the metal ring with the grub screws, some people have been tempted to re-adjust this - very BIG mistake.. and the reason is this: With the ring in place and the screws tight and sealed from the factory, the whole assembly is very solid. Now, if you try to re-adjust the grub screws, trying to extract more power, more than likely you will throw out the entire tube out of alignment. The screws are so tight, that very slight, and gentle and precise alignment is very difficult to achieve.

        For tubes that DO NOT have this assembly, once the mirrors are out of alignment, it is extremely difficult to re-align the tube. Been there, done that. :(

        Now, the reason that there are troubles with realigning a tube so it is stable are two-fold:

        1. Mirror mounts are very difficult to physically change position (very slight movements) and maintain that position. You are physically bending metal so it is easy to overshoot the desired position. In addition, the mount will not relax to its final position and stay there - there may be some drift or creep over time especially after multiple thermal cycles.

        2. Heat - even if the tube is only on for a few seconds, the very slight temperature differences can be enough to change the mirror alignment. At the factory, the alignment is optimized after a warmup of 30 minutes or more and minimum output power is usually specified after a similar period of time.
        In some older HeNe tubes, I have seen the affects of thermal expansion, the beam will drift in and out of alignment, and this start to occur only after about 5 to 10 sec after power-up.

        WARNING: All the adjustments that you do on the tube, unfortunately have to be done while the tube is powered up - so you have at least one end of the tube (usually the anode) floating at 2 kV or more once the tube is running (and even after power down due to tube and power supply capacitance). If during your adjustments, the tube decides to drop out, and re-start, you will have the 8 to 15 kV starting voltage - so please be very careful!

        As the tube is powered, try and push the mirror mount, and watch the beam, once you get a nice bright output, try and hold that position, and see if it will hold the output as you support that position - Note in which direction / movement you used to achieve this.

        If you have the ring/grub screw assembly, moving one of the screws will not necessarily adjust the mirror in the direction that you want, so you may have to use different adjustment/pressures on all three screws.

        (From: Sam.)

        If the alignment is nearly correct - gentle force or just touching the mirror mount results in full power - I would suggest as an alternative: Instead of actually attempting to bend the mount, add an external 3 screw adjuster to the problem mirror mount. This will operate within the elastic limit of the mounts so the risk of breaking them off from repeated unsuccessful attempts at bending them back and forth is eliminated. Let the tube warm up for at least 30 minutes, then gently adjust the screws to optimize power output.

        Rich's Procedure for External Mirror HeNe Laser Alignment

        As written, this would appear to be apply more to determining if a combination of HeNe tube and mirrors will lase. Modify as appropriate where you are doing this with an existing laser.

        (From: Richard Alexander (pooua@aol.com).)

        1. Use a good optic axis (very important). A good rail is worth the money.

        2. Use a second Hene laser. I pity those who lack this option.

        3. Use good adjustable mounts.

        4. Mount everything except the mirrors onto your optic axis.

        5. Once you have the beam of the 2nd HeNe shining down through the tube of your 1st HeNe (and out the other end), mount the far mirror on the optic axis.

        6. Adjust the mirror mount so that the beam of the 2nd HeNe reflects back through the tube of the 1st HeNe and strikes the 2nd HeNe next to the Output Coupler of the 2nd HeNe.

        7. Put the other mirror on the optic axis.

        8. If your tube is functional, you could apply power to it, and then fiddle with the last mirror until you get a beam.
        With practice, this method can be completed in less than an hour, though 4 or 5 hours is not unusual, either. If you have a 3rd HeNe, or better equipment than we had in tech school, you might get done much faster. Rich's Procedure for External Mirror HeNe Laser Alignment As written, this would appear to be apply more to determining if a combination of HeNe tube and mirrors will lase. Modify as appropriate where you are doing this with an existing laser.

        (From: Richard Alexander (pooua@aol.com).)

        1. Use a good optic axis (very important). A good rail is worth the money.

        2. Use a second Hene laser. I pity those who lack this option.

        3. Use good adjustable mounts.

        4. Mount everything except the mirrors onto your optic axis.

        5. Once you have the beam of the 2nd HeNe shining down through the tube of your 1st HeNe (and out the other end), mount the far mirror on the optic axis.

        6. Adjust the mirror mount so that the beam of the 2nd HeNe reflects back through the tube of the 1st HeNe and strikes the 2nd HeNe next to the Output Coupler of the 2nd HeNe.

        7. Put the other mirror on the optic axis.

        8. If your tube is functional, you could apply power to it, and then fiddle with the last mirror until you get a beam.
        With practice, this method can be completed in less than an hour, though 4 or 5 hours is not unusual either. If you have a 3rd HeNe, or better equipment than we had in tech school, you might get done much faster.

        Dave's Quickie External Mirror Alignment Technique

        The following works for the Spectra-Physics model 120 and other lasers with spherical OCs where the optics and machining are most excellent. Interchange OC and HR in the procedure below if your laser only has a spherical HR. I doubt it would work reliably depending only on close tolerances for a planar mirror.

        (From: Dave (Ws407c@aol.com).)

        As far as the terrible 3 point mirror mounts on the SP-120, I have developed a way to get the mirrors aligned without any cards or another laser. Just my two hands and a hex wrench. Within 5 minutes I get it every time. :-) I have also been applying this technique to the longer lasers with some good results.

        As you know, if one mirror is aligned correctly, the other is a cinch. I tighten down the OC and then back off each screw 3/4th of a turn. Then I loosen up the HR so it has a lot of play. I put my finger over the HR and wiggle in a repeating all over the place while hunting for a flash out of the OC. When I get a repeatable flash on the OC that's it, no problem to tweak in the HR. Works every time on the SP-120. :-)

        Mirror Alignment with just an Optical Power Meter

        Here's a way of aligning mirrors very quickly on small to medium length external mirror (one and two-Brewster) lasers. With care, it may work on large frame lasers as well. This approach takes advantage of the fact that the far mirror will be aligned when maximum bore light is returned to the front of the tube. Unlike the laser wavelength, most bore light passes through the mirror and there is ample power to monitor. It works great on one-Brewster HeNe lasers as well as the very difficult to align PMS LSTP-0010 and LSTP-1010 tunable HeNe lasers. I would recommend it only for lasers with screw adjustable mounts, not for HeNe laser tubes with three-screw locking collars or less.

        All that is needed is an optical power meter (laser, photographic, etc.) with enough sensitivity to respond well to the bore light. One with a "suppression range" feature is best but this is not essential. (The suppression range enables the constant light to be cancelled out so that the sensitivity to changes can be increased.)

        To align one mirror, place the sensor of the optical power meter at the other end of the laser, located to pick up the bore light. Set up the meter on a range that allows the maximum deflection of the meter while keeping it on scale, and/or set the suppression range to cancel out most of the constant bore light.

        Now all that's required is to twiddle (technical term!) the far mirror to maximize the power reading. With kinematic or gimbal mounts, this will actually be quite easy. The peak is broad so each axis will have an effect even if the other axis is way off. As the alignment approaches optimal, the reading will increase and with a bit of luck, will then spike as lasing occurs (assuming the other mirror was already aligned).

        For a laser with two adjustable mirrors, just repeat the procedure for the other mirror.

        It takes literally only a couple of minutes to do this for a PMS tunable laser (which uses a 1-B tube with permanently adjusted internal OC), which with its narrow bore is very difficult to align with any of the other techniques.



      4. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

        Collimation of HeNe Lasers

        Reasons for Poor Collimation

        The output of most HeNe lasers is a very well collimated beam - approaching the theoretical diffraction limited optimum possible for a given bore diameter. It isn't expected to look like that of a flashlight! However, a number of factors can affect this performance - and some are by design:
        • Optics in the output beam path. This is the most obvious but often overlooked possibility. Even if there are no external optical components, if the Output Coupler (OC) mirror of the laser is curved (concave) with no corresponding curve on its (non-active) outer surface, this forms a negative (diverging) lens. Beam divergence in this case can be several times what would be expected from diffraction limited calculations based on the tube's bore diameter. The raw beam is of no lower or higher quality. However, if what you expected was a diffraction limited beam divergence directly from the tube, you're out of luck,

          For any application requiring additional optics (like a beam expander with spatial filter), this doesn't matter as they can easily provide the corrections and will be essentially the same in either case. In fact, where just a wider parallel beam is desired (without a spatial filter), the external optics can now even be made simpler - just a single positive lens at the appropriate distance from the beam exit.

          To test for a diverging OC, observe your reflection from the output mirror and see if it is smaller than from a plane surface. Alternatively, reflect the beam from a well collimated HeNe laser off of the OC to a card or screen. If it spreads more quickly after reflection, your OC acting as a diverging lens. The outer surface will reflect weakly since it is AR coated - don't confuse this with the reflection from the actual OC. If the weak reflection does not spread as quickly, you have a negative lens as described above.

        • Dirty optics. A fingerprint or coating of dirt or condensed tobacco smoke residue or cooking grease can result in effects similar to poor collimation. See the section: Cleaning HeNe Optics.

        • Problems with mirror alignment. While this will likely result in a weakened and distorted beam (or no beam at all), excessive divergence might also be possible. Gently pressing on the mirror mounts (or turning the adjustment screws a tiny amount) in any direction should result in an (approximately) equal reduction in output if the alignment is optimal. It shouldn't result in a drastic change in beam shape. If it does, see the section: Problems with Mirror Alignment.

        Improving the Collimation of a HeNe Laser with a Beam Expander

        The following applies to any laser which outputs a substantially parallel beam but is written specifically for HeNe lasers. Collimation of laser diodes require a slightly different approach - see the section: Beam Characteristics of Laser Diodes.

        Although the divergence of a HeNe laser is already pretty good without any additional optics, the rather narrow beam as it exits from the tube does result in a typical divergence between 1 to 2.5 mR (half of total angle of beam). 1 mR is equivalent to an increase in beam diameter of 2 mm per meter.

        As noted in the section: HeNe Laser Tubes and Laser Heads, beam divergence is inversely proportional to the beam diameter. Thus, it can be reduced even further by passing the beam through beam expander consisting of a pair of positive lenses - one to focus the beam to a point and the second to collimate the resulting diverging beam. Though the beam will start out wider, it will diverge at a proportionally reduced rate.

        A small telescope can be used in reverse to implement a beam expander to collimate a laser beam and will be much easier to deal with than individual lenses. (This is how laser beams are bounced off the moon but the telescopes aren't so small.) Using a telescope is by far the easiest approach in terms of mounting - you only need to worry about position and alignment of two components - the laser tube and telescope. The ratio of original to expanded beam will be equal to the magnifying power of the telescope. Even a cheap 6X spotting scope will reduce divergence six-fold.

        If you want to use discrete optics:

        • The focusing lens should have a short focal length (F1) such as a microscope or telescope eyepiece (e.g., F1 of 10 mm) or low power microscope objective (e.g., 10X). Note: the objective lens from a dead CD player has an ideal focal length - about 4 mm - but is aspheric and would probably not be that great but give it a try!

          This will focus the laser beam to a (diffraction limited) point F1 in front of the lens from which it will then diverge.

        • The collimating lens should be a large diameter medium focal length (e.g., 15 mm D2, 100 mm F2) lens placed F2 from the focus of the first lens.
        For optimal results, the ratio of collimating lens diameter to focal length (D2/F2) should greater than or equal the ratio of HeNe beam diameter to focusing lens focal length (D1/F1). This will ensure that all the light is captured by the collimating lens.

        The beam will be wider initially but will retain its diameter over much longer distances. For the example, above, the exit beam diameter will be about 10 mm resulting in nearly a 10 fold reduction in divergence.

        Adjust the lens spacing to obtain best collimation. A resulting divergence of less than 1 mm per 10 meters or more should be possible with decent quality lenses - not old Coke bottle bottoms or plastic eyeglasses that have been used for skate boards. :-)

        Note that some HeNe tubes have wide divergence by design using an external negative lens glued to the OC. For these, removing this lens with a suitable solvent may be all that is needed to produce the divergence you want. See the section: HeNe Laser Beam Characteristics.



      5. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

        Beam Polarization of HeNe Lasers

        Typical Polarization Characteristics and Problems

        Sealed HeNe tubes with internal mirrors which are linearly polarized usually incorporate an internal Brewster plate to suppress the unwanted polarization orientation - and these HeNe tubes are more expensive. Where mirrors are external, the Brewster windows on the plasma tube accomplish this function and the output of these is always linearly polarized.

        Common inexpensive internal mirror HeNe tubes produce a beam that is either randomly polarized or slowly changing in polarization (as the tube heats) - possibly with a combinations of polarization states present simultaneously. Placing a polarizing filter in the beam of one of these lasers results in a variation in brightness, usually taking place over a few seconds possibly with sudden shifts as various modes compete for attention inside the resonator. The presence of any of these characteristics makes such a laser unsuitable for many experiments and applications. These tubes are normally designated as 'random polarized' (with an 'R' somewhere in the model number) which translates as: "The manufacturer has no idea of what the polarization characteristics will be at any given time". :)

        If the polarization were truly random, meaning all polarization states are present simultaneously (or on a short enough time scale that it doesn't matter), a simple polarizing filter in the beam path will produce a linearly polarized beam at the expense of at least one half the output power (that which is blocked because its polarization orientation is wrong and because of losses in the filter). However, where the polarization orientation of the laser is slowly changing, this approach will result in unacceptable varying output intensity from the polarizing filter. Additional optics including polarizing beam splitters, mirrors, and combiners can in principle, at least, produce a stable polarized beam but these are complex and expensive.

        Determining if a Laser Tube is Linearly Polarized

        There are several ways to determine if a HeNe (or other internal mirror) laser tube or head has random or linear polarization (all external mirror laser with Brewster angle windows are linearly polarized):
        • See if there is a 'P' in the model number. Those like Melles Griot use designations of 'LHR' for random polarized red HeNe tubes and 'LHP' for linearly polarized tubes. Many other manufacturers do something similar.

        • Look up the model number in a catalog or database. Optlectra's Product Search has specifications for several hundred HeNe tubes and heads.

        • Look through each of the mirrors into the unpowered tube and see if there is an internal angled Brewster plate near one end of the tube. A randomly polarized tube will have no additional optical components between the mirrors. CAUTION: Disconnect power and discharge the tube capacitance first!

        • Pass the beam through a polarizing filter to see if it is polarized. :) If it is polarized, there will be an orientation of the tube with respect to the filter at which virtually no light gets through (but see below).

        • Reflect the beam off of a microscope slide or other transparent surface at its Brewster angle (typically around 57 degrees - see the section: What is a Brewster Window?). If the beam is polarized, you should be able to rotate the tube or head orientation until virtually no light is reflected (but see below).
        Note: Random polarized lasers are generally not truly random but will tend to (longitudinal) mode cycle and some modes will tend to favor a particular polarization orientation. This will result in the polarization changing gradually or suddenly (especially as the tube is heating up) so at any given time, they may emit a polarized or partially polarized beam but its orientation will not be predictable. Thus, to confirm that your tube is polarized requires that the polarization remains constant - not just for an instant (30 seconds to 1 minute should be enough time to wait to know for sure). If the beam is reflected off of a non-metallic reflective surface (which acts somewhat as a polarizer), you may see a large variation in brightness due to the polarization changing especially if it is a low gain or short tube where fewer modes are active simultaneously.

        Even a polarized tube may show a small amount of variability of the low intensity beam passed by a polarizing filter or reflected from a Brewster angle plate - this is normal and one reason why the specifications only say 500:1 or 1000:1 and not infinity:1. The reason is that the tube's linear polarization results from the cavity gain being maximized by the internal Brewster plate at the polarization angle. However, gain function with respect to angle is not a singularity - there is still enough gain for a few degrees on either side to maintain oscillation. And, some samples are better than others. Also see the section: Typical Polarization Characteristics and Problems.

        Unrandomizing the Polarization of a Randomly Polarized HeNe Tube

        The best option where a polarized beam is required is to start with a HeNe laser that produces a polarized beam! However, for the experimenter, there is at least one alternative - magnets to the rescue!

        I have found that placing powerful magnets alongside a random polarized tube will result in a highly linearly polarized beam. While this may be common knowledge at the Afternoon Teas attended by laser physicists (assuming they drink tea), it certainly isn't something found in popular books on lasers.

        A type of magnet that works quite well has a strength of several thousand gauss. The ones I used came from the voice coil positioner of a moderate size hard disk drive. They are rare earth magnets with dimensions of about 1.25" x 2.5" x .375" with the broad faces being the N and S poles. The amount of polarization is most pronounced by placing one of the broad faces of the magnet against the tube near its mid-point. Some adjustment may be needed to optimize the effect. I do not know how much magnetic field strength is needed but even moving this magnet 1/4" away from the tube surface greatly reduced the ratio of light intensity in the two orthogonal polarization axes.

        CAUTION: These types of magnets are very powerful. In addition to erasing your credit cards and other magnetic media, they will tend to crush, smash, or shatter anything (including flesh or your HeNe tube) between them and/or between them and a ferrous metal. Some portions of a HeNe tube or laser head may contain parts made from iron or steel. These rare earth magnets also tend to be quite brittle. In addition, the violent uncontrolled movement may place you and a HV terminal in the same space at the same time as well! Take care.

        With the magnet's N or S pole placed on the side of the tube, the result was a vertically polarized beam. By rotating a polarizing filter in the beam path, beam intensity could be varied from nearly totally blocked to nearly totally transmitted and the polarization orientation followed the magnet as it was rotated around the tube.

        The control wasn't perfect - a small amount of light with a slowly varying polarization did sneak through. However, it was significantly less than 1 percent of total beam power for these particular tube and magnet combinations (I have tried this with 2 different tubes with similar results). The constant portion of the residual beam may have just been a result of the imperfect nature of the polarizing filter.

        By using two similar magnets - one on either side of the tube with N and S poles facing each other (mounted on an aluminum U-channel for support and so they would not crush the tube), the variation in residual beam intensity was virtually eliminated. I do not know if this effect was due to the increased magnetic field or its more homogeneous and symmetric nature. This was also used successfully with an enclosed HeNe laser head:

                                __S__
                               |_____| Rare earth magnet
              ____________________N_______________________
             |                                            |
             |             HeNe laser head                |=====> Polarized HeNe beam
             |____________________________________________|
                                __S__
                               |_____| Rare earth magnet
                                  N
        

        Use of Magnets to Generate Polarized HeNe Laser Beam shows acceptable locations for one pair of magnets along side a typical 1 mW HeNe tube. This placement was found to be effective but possibly not totally optimal - experimentation may be required.

        As far as I could tell, with this dual magnet configuration, the output beam characteristics were similar to those of a polarized HeNe tube. However, additional and/or more powerful magnets might be necessary with other tubes.

        Output power did not appear to be affected - in fact, it may have increased slightly (or perhaps it was my imagination but see the section: Magnets in High Power or Precision HeNe Laser Heads). A polarizing filter would nearly totally block the beam at one orientation and have minimal effect 90 degrees away from this.

        I do not know about the stability or reliability of this scheme but the only other effects seem to be to increase the required input starting/operating voltage and/or magnitude of the negative resistance of the tube slightly (current dropped by about 10 percent with the magnets using an unregulated power supply) and possibly to shift to point of maximum beam power to a higher tube current (5 mA instead of 4 mA for one tube - but this could have just been my imagination as well).

        Where the capillary of the plasma tube is exposed as with many older lasers, and the magnets can be placed in close proximity to the bore, their strength can be much lower. Some commercial lasers (like the Spectra-Physics model 132) offered a polarization option which adds a magnet assembly alongside the tube. However, I doubt that this is done commercially with any modern HeNe tubes with coaxial gas reservoirs.

        Since it is possible to control the polarization orientation with permanent magnets, the next step would be do this with electromagnets. This would permit polarization to be dynamically controlled. Adding a fixed polarizer would provide intensity modulation without any connection to the power supply or expensive electro-optic devices. Hopefully, by using multiple sets of coils distributed along the side of the HeNe tube, a lower field strength would be adequate. Liquid helium cooled superconducting electromagnets would definitely add to the cost of the project. :-) Perhaps, someday, I will try this out.

        Magnet Tests with a Polarized HeNe Laser Tube

        In an effort to determine how strong the effect of a magnetic field actually is on the polarization, I later tried the same scheme as described above but with a small polarized tube - the type with an internal Brewster plate. Not surprisingly, the effect of the Brewster plate is much more pronounced than that of even very powerful magnets. Here is the output power after warmup with different magnetic fields:

        1. No magnetic field: 0.75 mW.
        2. Strong transverse magnetic field (aligned with polarization): 0.70 mW.
        3. Strong transverse magnetic field (orthogonal to polarization): 0.50 mW.
        4. Weak magnetic field (almost any orientation): 0.80 mW.

        In all cases, the polarization was unchanged and output power was at least as stable as without any magnetic field. Thus, even the strong magnetic field was insufficient to overcome the losses of the Brewster plate at the (wrong) orthogonal polarization orientation but did reduce the gain at the (correct) aligned polarization orientation enough to cut output power by 33%. (For this short tube, lasing would probably have been killed entirely if forced to have its polarization orthogonal to the correct orientation.) These results are not unexpected - except perhaps for (4) - I do not know if the increase in power was simply a result of the usual Zeeman splitting effect suppressing the IR wavelengths or something else. A noticeable increase in output power due to Zeeman splitting is usually associated with long high power HeNe tubes, not the 0.5 mW tube used for these tests.



      6. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

        Repairing Leaky or Broken HeNe Tubes

        Gas Fill Problems with HeNe Tubes

        HeNe tubes can fail due to slow leakage through soft (Epoxy) seals (not found on modern tubes), or actual damage or defective manufacturing leading to a crack or break. Helium loss through diffusion is a special case and may be remedied easily without major effort or investment. See the section: Rejuvenating HeNe Tubes. Other gas fill problems will require a very non-trivial amount of work and access to sophisticated. Minor structural damage may be repaired but then a refill will be needed. See the section: Repairing and Refilling a HeNe Tube at Home? However, if your tube is broken in half, you should probably just salvage the mirrors (for possible future use) and move on to other things!

        The major HeNe laser manufacturers and laser repair companies may offer regassing services for larger more expensive HeNe tubes (high power internal mirror tubes or those with Brewster windows designed to operate within an external resonator). Figure on $500 or more to regas an HeNe tube, and more still if there is physical damage (assuming they will bother with it at all).

        Whether the cost of such an operation can be justified is another matter. For a high quality research laser it probably makes sense as the tube alone may cost several thousand dollars or more - if a replacement can be obtained at all. Even a basic HeNe tube with Brewster windows may cost over $600 (being much less common and thus much more expensive than the internal mirror variety). However, for small sealed internal mirror HeNe tubes, low cost replacements are readily available at perhaps 1/10th to 1/4th the cost of a regassing service (even cheaper if you are willing to use a surplus tube).

        However, where the tube has high mileage and died from use and age, it may not simply be a matter of regassing. The following is from the Melles Griot FAQ Page:

        "While re-gassing can provide some extension of the output performance in some gas lasers like the CO2, argon and the higher powered side arm HeNes (which have external optics), it is not recommended or provided for smaller internal mirror coaxial tubes. Typical end-of-life failure for a HeNe tube is cathode sputtering. This occurs when the protective oxide layer on the cathode is expended through continuous bombardment by the laser discharge. There is no cost effective way of regenerating this layer. When the oxide layer is expended, the discharge itself vaporizes the "raw" aluminum and deposits this material, in its vapor state, on other surfaces such as the optics and the bore."
        So, while refilling may help some, the sputtered aluminum coating will remain on critical surfaces. A careful visual inspection of the bore and mirrors may reveal whether a suspect tube is worth saving - a black or metallic film could indicate that serious sputtering has taken place. However, I've also seen tubes where discoloration in the bore, at least, had no noticeable effect on performance.

        Rejuvenating HeNe Tubes

        These techniques may be used on HeNe tubes that are marginal due to loss of helium though diffusion or a slight amount of contamination from air leakage due to age. Where the tube lases weakly, a helium soak or reactivation of the getter may help. If there is no lasing at all, many other causes are possible.

        The best way to determine if loss of helium or slight contamination is your problem is to check the spectrum of the discharge. See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge.

        • Loss of helium will show up as a significant reduction in the intensity of the helium lines in the spectra. The overall color will be more orange.

        • Contamination will show up as many additional spectral lines due to oxygen, nitrogen, and any number of other molecules that may have invaded your tube. With any significant leakage, these will quickly dominate the spectrum.

        Helium Soaking

        Where just the helium (remember how slippery those He atoms can be!) has leaked out, there may be an alternative to the dumpster or a major refurb. HeNe tubes which do not lase well or at all due to loss of helium can sometimes be rejuvenated by soaking them in helium at normal atmospheric pressure for a few days or weeks. You don't need a pressure chamber or any other fancy equipment - a few helium filled party balloons and a garbage bag will do just fine.

        However, there could be other causes like misaligned mirrors or excessive tube current (due to a defective power supply). Check for these possibilities first and confirm loss of helium with a spectrometer capable of actually measuring the relative intensity of the spectral lines if possible. From my experience, just viewing the discharge with a diffraction grating will not reveal a low helium condition unless it is extremely severe - as in almost none remaining. (I've yet to actually see this. If anyone has a HeNe tube with certifiably low helium, please send me mail via the Sci.Electronics.Repair FAQ Email Links Page. I'd be interested in testing it.)

        The point to realize is that it is the partial pressure of each gas inside and out that matters. Neon is a relatively large atom and does not diffuse through the tube at any rate that matters. However, helium is able to excape even when the pressure difference is small. For a typical HeNe tube at only 2 Torr (1/380th of normal atmospheric pressure), the partial pressure of helium in the tube is still much much greater than its partial pressure in the normal atmosphere. So, helium leaks out even though the total pressure outside is several hundred times greater. Conversely, soaking a HeNe tube in helium at 1 atmosphere will allow helium to diffuse into the tube at several hundred times the rate at which it had been leaking out. Thus, only a few days of this treatment may be needed if the problem is low helium pressure. Assuming that the desired partial pressure of He is 2 Torr, the ratio of age:soak-time will be about 380:1 or pretty close to 1 day of soak per year of the tube's age.

        Helium loss is most likely with soft-seal tubes - those with an Epoxy-type adhesive holding the mirrors or Brewster windows in place. However, it is also possible for hard-seal tubes using frit seals or optical contacting to lose helium though probably at a slower rate and rejuvenation will also take proportionally more time. Checking the intensity of the He lines with a spectroscope is really the only way to know for sure if He loss is the problem and to also monitor the soaking process.

        Almost any sort of helium supply will work for atmospheric pressure diffusion including welding supply grade and even the stuff sold for filling party balloons. (Note, however, that these sources are mostly the common isotope of helium, He4, not the light isotope, He3 which may be what was originally in your tube - see the additional comments below.) A party tank of helium may be as little as $15 or $20 or just buy a few prefilled balloons and empty their contents into an air-tight plastic bag containing the HeNe tube. However, make sure what you are getting is really helium and NOT hydrogen!! In addition to the flammability issues, any significant H2 that makes its way into your HeNe tube will make the situation worse - probably terminal. Also note that as much as 50 percent of what is in those party tanks may actually be air, nitrogen, and/or some other unidentified gas, so the process may take somewhat longer (approximately by 100 divided by the percent of actual helium) though most of these contaminants won't hurt the tube.

        The required amount of effort hardly seems worthwhile for a $15 1 mW HeNe tube but it is something to keep in mind for other more substantial and expensive types.

        (From: Mark W. Lund (lundm@xray.byu.edu).)

        I have rejuvenated HeNe laser tubes with low helium pressure. Since the partial pressure of 1 atmosphere helium is much higher than inside the tube you don't really need to use high pressure, or even increased temperature. I just put them in a garbage bag and blasted some helium into it from time to time. The length of time necessary in my case was a few days, but depending on the glass type, thickness, and sealing method this may vary. It would be good to test the power every couple of days so you don't overshoot too much.

        One warning: Helium has a lower dielectric strength than air, so don't try to operate the laser in helium as it may arc over.

        (From: Philip Ciddor (pec@dap.csiro.au).)

        My information is very old, but may be helpful. Early 2 mW red tubes had about 2 torr of He, so soaking in 760 torr (1 atmosphere) of He for 1 day per year of life roughly restored the initial He pressure, since diffusion rate is proportional to pressure difference. I have no data on the gas mix in current green or IR tubes, but if you can find it, similar scaling may be feasible.

        (From: Sam.)

        Gas fill probably isn't all that different for non-red HeNe tubes so the same general recommendations should apply. However, since their gain is lower, nearly everything about near-IR (1,523.1 nm and 1,152.3 nm), orange (611.9 nm), yellow (594.1 nm), and particularly green (543.5 nm) HeNe tubes is more critical including power supply current and mirror alignment. So, it is important to eliminate other possible explanations for low or no output or other problems before blaming loss of helium.

        I cannot overemphasize the importance of carefully monitoring the amount of helium that has diffused back into the HeNe tube (by removing it from the bag of He and testing with a spectroscope periodically and for a laser beam) - once helium pressure goes too high, the only (non-invasive) way of lowering it is to wait a few years or decades. :-) If power is just low and you are trying this, put the tube in the helium soak for a couple of days and then check power output again. If it has increased, repeat this procedure a couple days at a time until power levels off or starts to decrease. If power decreases after the first soak, helium loss isn't your problem!

        If it's possible to wrap the tube such that only the seals are inside the helium and not the electrode connections (the glass envelope shouldn't leak at any rate that matters), monitoring of power can be done without having to remove the tube from the helium container or whatever.

        CAUTION: Apparently, most modern HeNe tubes are actually filled with the light isotope of helium, He3, rather than He4 which for all intents and purposes, is the one found in nature (99.9998%). He3 has a higher energy state which may be better for exciting certain transitions. Thus, helium soaking with common He4 could result in problems including reduced maximum power, greater frequency spread, reduced stability, or something else. As noted above, once the HeNe tube has been helium soaked, the effects are irreversible without waiting many years. The only practical way to determine what isotope(s) of helium your tube used is probably to ask the manufacturer - even a high resolution spectrometer won't help if the helium has escaped. For a common red HeNe tube, there is little to lose by using common He4 though results may not be optimal. However, if the tube is from a specialized research laser, it would probably be best to have a professional laser refurb company or the original manufacturer deal with it. You could make matters worse.

        WARNING: In addition to not attempting to operate the HeNe tube itself in a helium atmosphere due the lower breakdown voltage, there may even be problems with He diffusing into power supply components or ballast resistors and lingering there. So, if possible, remove the HeNe tube from its laser head or system enclosure for the helium soak. Or else, wait awhile (your guess is as good as mine) after dumping the helium before applying power.

        Reactivating the Getter

        Where some air has entered due to age (not an actual leak, that you can forget about unless you want to try refilling the tube at home - see below), it may be possible to reactivate the getter and absorb/combine/react with the unwanted molecules. However, don't expect miracles.

        Note that not all HeNe tubes have getters. For some that do, the getter may never have been activated in the first place (if the gas fill was already deemed pure enough after pinch-off). See the section: Gas Fill and Getter for info on the getter in a HeNe laser tube. And, if the getter was activated, the source of the active material (in the getter electrode) may have been totally depleted during manufacture so there may be no more remaining.

        This only has a chance of working if the gas pressure is nearly correct - not if it has changed by a factor of 100. The closest example I have of the effect of the getter on tube vacuum is for a typical TV or monitor CRT:

        (From an engineer at Philips)

        "A regular CRT-type getter can reduce gas pressure from about 10-6 Torr to its final value of 10-9 Torr IFF the gasses can be gettered at all. H2, O2, N2, CO, and CO2 can be gettered. CH4 (Methane) can not be gettered but by heating, it can fall apart into C (a solid) and H2 that can be gettered. Noble gasses can not be gettered either, so their gas pressure will determine the final gas pressure in a picture tube."

        Of course, for a HeNe or Ar/Kr ion laser, those inert gas molecules ARE the desired result! :) Unfortunately, since the typical gas laser operates at a pressure 1,000,000 times higher than a CRT (a few Torr), any effect of the getter on detectable contamination is likely to be minimal. How to tell? If the color of the discharge is more towards white or pink than it should be and there is still at least some evidence of lasing, the getter has a good chance of returning it to normal assuming all its active material isn't already used up. If the color is too orange, then the helium loss may be indicated and a helium soak may be all the tube needs. See the section: Helium Soaking.

        However, there is probably nothing to lose if the tube is unusable and you won't be going the entire route of refilling it. Heating the getter can be achieved in a variety of ways including (depending on design and what you have available): DC current, glow discharge, Sunlight and Fresnel lens, RF, and induction heating, even a microwave oven. See the sections starting with: Methods to Activate Neon Sign Electrodes and Getters. The Solar heater approach is low tech and known to work where there is no visible 'white cloud of death' (heating the white stuff (which is probably unavoidable with the Sun's rays) seems to release previously trapped stuff making the situation much worse). See the section: Simple Solar Heater.

        The idea is to drive off some of the material remaining in the getter electrode onto the walls of the tube. If nothing appears or it turns milky immediately, the getter probably isn't capable of helping much - though even in this case, try out the tube again - it may have helped just enough. Lack of results could also mean that the getter electrode hasn't been made hot enough or the material it contained had already been fully used up.

        Note: If you expect to try your hand at actually refilling a leaky tube, DON'T attempt to reactivate the getter - you may need it later!

        The same approach can be used with ion laser tubes if they are made of glass and you can locate the getter. Those that are of all ceramic construction may still have a getter, but it may need to be heated by a precisely controlled current flow between the cathode end-bell and filament or something equally obscure like that - not easily guessed! Also, since these tubes are generally much more expensive than HeNe tubes, it may pay to have it professionally refurbed.

        Once the tube has been revived (or perhaps even before you make the attempt), adding an additional layer of Epoxy/TorrSeal at the tip of the exhaust tube, mirror(s), and any other possible areas of leakage would be a good idea. This is particularly relevant for modern hard-seal tubes since they shouldn't really leak at all (at least on time-scales that humans can understand). Thus, any contamination generally means an actual defect at the frit seals or exhaust tube (tip-off). Soft-seal tubes leak by design :) but adding an additional layer of sealant at the mirrors, end-caps, tip-off, and other suspect locations can reduce this somewhat. At least it won't hurt - unless you accidentally glop it on the OC mirror! :(

        I've successfully revived a couple of Melles Griot HeNe laser tubes which had getter electrodes but no visible getter spots (which means the material is transparent). One was a hard-seal tube that must have been contaminated in some way since after treatment, it has worked essentially unchanged for over a year. The other was a green HeNe laser tube that had an Epoxy seal at one end. However, all attempts to revive Spectra-Physics HeNe lasers have failed miserably and generally made matters worse. Heating the "white cloud of death" material (including what's no doubt inside the getter ring) must release whatever it previously trapped.

        Repairing and Refilling a HeNe Tube at Home?

        If you are really really ambitious, have lots of time on your hands, have access to lab supplies, laser grade He:Ne gas mixture, and a high vacuum system in good condition, you, too, can refill an HeNe tube - at least as an experiment. Whether it could be sealed off and then expect to have a long life is another matter.

        First, any physical damage would have to be repaired. For example, if an overzealous attempt at mirror alignment resulted in a mirror breaking off at the frit seal, it would have to be reattached - in as precisely the same position as possible using new glass frit or Epoxy (though that will leak over time). If someone yanked on the anode wire on a large HeNe tube broke the metal-to-glass seal, that would have to be repaired - again with Epoxy or by actually heating the glass to fuse it together. However, the latter risks shattering the entire tube if you aren't experienced in glass working. If you don't know where the leak is, then you need to find it first. :)

        Once the HeNe tube is known to be gas-tight, the seal is cracked at the exhaust tube, it is put on a high vacuum system to pump it down and backfilled with pure He:Ne gas mix several times while baking out impurities.are very finicky about gas purity.

        For more information on this sort of endeavor, see the chapter: Amateur Laser Construction, the section: Home-Built Helium-Neon (HeNe) Laser, and the introductory chapter: Home-Built Laser Types, Information, and Links for relevant information. Good luck! :-)

        Regrinding or Otherwise Compensating for a Chipped Mirror

        So that wonderful HeNe tube you had fallen in love with (we all have our peculiarities!) got knocked loose from its mounting and fell face down on the granite countertop. Exactly what were you doing with your laser? :) Now, there is a chip in the OC mirror which extends into the area of the beam and the beam is but a shadow (well not quite but you get the picture!) of its former self. OK, it ouptus a unique pattern but not quite what you had in mind! (Note that for the most part, a similar accident with the HR mirror at the other end of the tube wouldn't affect anything but its external appearance.) Is there any hope?

        Well, assuming the chip isn't too deep, it is possible to grind it out and then polish the resulting surface to optical quality. To do this properly will require a means of holding the tube just slightly off of perpendicular (to add some wedge - see below) to a rotating platform on which various grades of wet grinding compound can be introduced starting with something coarse like 400 grit and going up in stages to 1,200 grit or more, following by lapping with optical rouge for the final polish. That should get you a reasonably decent result after considerable effort and cost. But don't expect it to be to 1/10th lambda!

        One thing you won't be able to reproduce is the anti-reflective (AR) coating present on most HeNe OCs. (Well, not unless you have access to some vacuum coating equipment!) That is the reason I suggest grinding it on a slight angle - the resulting wedge will divert the reflected beam away from the axis of the cavity and minimize instability and interference.

        I was given a cute little HeNe tube with such a chip in the OC mirror. Now, this certainly wasn't worth spending much of anything to repair (it was only a .8 mW, barcode type HeNe laser after all!). So, I decided to experiment using the minimalist approach: emery paper. I started with 400 grit to remove the chip and then 1,200 grit. I also deliberately attempted to grind the surface parallel to the actual mirror rather than with wedge to see what would happen. All this just by hand so the result is also somewhat convex rather than perfectly flat. I need to find some rouge to attempt the final polishing if I ever bother.

        Even without fine polishing, the beam was much much cleaner than it used to be (formerly being spread out off to one side in random directions!). Just for grins and giggles, I went back to 600 grit to see what effect an even more random ground surface would have on the beam. The interference patterns are really quite interesting - sort of like a stellar globular cluster - so I may just leave it the way it this way. :)

        Another alternative where the area of the beam just touches the chip might be to push the mirror mount side-ways beyond the restricted area. With care, it may be possible to shift it by as much as .5 mm which could be enough.

        Or, use some optical cement to glue a flat piece of glass to the mirror filling the voids. With the proper material that closely matches the index of refraction of the mirror glass, such an approach may result in a beam that isn't too terrible. :)

        Using a Microwave Oven to Evaluate and Revive HeNe Laser Tubes

        WARNING: These are dangerous procedures, at least for your laser tube! Attempt at your own risk. There is a good chance that the tube will be ruined totally as a result of the glass or glass-to-metal seals cracking. There is also a chance that the procedures will make the situation worse or that any apparent improvement will be temporary.

        Note that using a microwave oven is safe for just checking to see if the tube is gas-intact and has approximately the correct discharge color. In this case, only a second or two is needed so heating is minimal. See the section: How Can I Tell if My Tube is Good?.

        The whacko procedures below may be used to provide an idea of what is wrong with a HeNe tube as well as to at least partially revive them in some cases. The difference between evaluation and revival is basically in cooking time and how many times the procedure is repeated.

        Alternative sources of RF energy can be used in place of the kitchen microwave but may not be quite as convenient or as readily available. :)

        I have had some modest success in at least partially reviving some old soft-seal HeNe laser tubes with the power output from 4 of 6 weak tubes being improved significantly, though not to anywhere near the rated specifications. However, one tube was destroyed due to the glass cracking (the first one I tried, not having a feel for the safe cook time), and on another, the power went down slightly. To what extent these results are due to getter reactivation or other phenomena is not currently known. The effects of the microwaves (whether it be from the discharge or just due to heating) would also appear to be useful as a diagnostic tool for evaluating HeNe tube condition.

        Since the entire tube or whatever has to be inside the oven (don't even think about drilling holes in the side or door!), this stunt probably only applies to smaller helium-neon laser tubes and maybe the getters in receiving tubes if you remember what they are. :) Here goes:

        • A microwave oven without a turntable but with a microwave stirrer (above the oven cavity) is preferred simply because it is more convenient. It is probably a good idea to use the turntable if that's all you have so that there are no hot spots.

        • Put a cup of water (or your chicken soup) in with the tube to act as a load and buffer - this will limit the peak field intensity at any point in the tube and protect the oven should the tube be up to air (or go up to air) and thus reflect all the energy back to the magnetron.

        • You will have to experiment with the location in your oven for best heating. On the one I used, it seemed to be somewhat off center - this is dependent on the particular model. For the advanced course, covering portions of the HeNe tube or using other means to concentrate the microwave energy where you want it may be useful. Removing the turntable or disabling the stirrer and then identifying the area(s) of peak microwave intensity may also permit more selective treatments. In either case, - take care you don't create excessive hot spots on or in the HeNe tube!

        • Limit your treatments to 5 seconds on high (once the microwaves start - about 2 to 3 seconds after pressing START for the filament of the magnetron to heat) and then check to see how hot all parts of the tube have become. Let the tube cool a bit before repeating. I was using a mid-size oven for my experiments - I don't know how much the strength of the microwave field will be affected by oven size. It may also be possible to run the oven on the LOW or DEFROST setting for a minute or two which should activate the microwaves for a few seconds, every 15 to 30 seconds (pulse width modulation is how power is controlled in most microwave ovens).

        • Watch the light show inside the HeNe tube. If it was originally gassy with an incorrect discharge color, the color will probably start out the same way but if the procedure is working, quickly transition to the proper pink-ish orange color. The nice thing about this technique (assuming you don't totally ruin the tube) is that you can watch the progress! A healthy tube should produce a very bright pink-ish orange display (mostly orange with a dash of pink). :)

        • After a couple of treatments, power up the tube in the normal way and see what, if any, progress (or the opposite) has been made. Then repeat the procedure until there is no further improvement (or immediately if the power has gone down).
        However, what you may find is that the power increases immediately after treatment but then decays back to its original value or below over the span of a day or so, or faster if the tube is powered. But, this may be repeatable, so if you just need a temporary boost, go for it! :)

        I would appear that the microwave treatment may do any or all of the following:

        • Release trapped helium and neon from the walls of the tube. This may be due strictly to the heating effects of the microwaves or the discharge taking place where it normally isn't located (outside the bore).

          Result: Temporary increase in output power (for a few minutes to a few days depending on subsequent use) - most dramatic where gas pressure was low originally as in a high mileage HeNe tube.

        • Increase gas pressure in tube due to heating of tube structure.

          Result: Temporary increase in output power (for a few minutes).

        • Activate getter due to heating of the getter electrode and/or glow discharge affecting material in it directly.

          Result: Removal of non-noble gas molecules, restoration of discharge color to normal, and permanent (as these things go) boost in power output if contamination wasn't too serious and there was still some active getter material available. More limited or no effect if supply of active getter material is inadequate or already exhausted totally.

        • Release old trapped contamination from getter spot due to microwave heating.

          Result: Increase in unwanted gasses and reduction in output power (possibly to 0.0 mW) or even total inability to sustain a discharge or start at all. It may be possible to reverse this and at least get back to where you started by selectively heating just the getter (possibly by some other means).

        I expect that none of these phenomena will be result in a substantial change in behavior for a healthy tube. Thus, microwave (or other RF) excitation/heating may represent a viable diagnostic tool for evaluating HeNe tube condition.

        See the section: Attempting to Revive Some Soft-Seal HeNe Tubes for some not terribly conclusive results from using this technique, additional discussion of some of the peculiar effects, and some tests with a more modest RF exciter.



      7. Back to HeNe Laser Testing, Adjustment, Repair Sub-Table of Contents.

        Reports from Sam's HeNe Laser Hospital

        Sam Succeeds in Aligning a LONG HeNe Tube

        Just to show that the alignment techniques in the sections starting with Problems with Mirror Alignment aren't some textbook exercises dreamed up by theorists, long HeNe tubes can be aligned from scratch using a minimalist approach. I did it once. (Well, actually twice but the other tube was pretty short so it doesn't really count.) How many data points do you need to prove something? :)
        • Patient: Aerotech style 30" long HeNe laser tube.
        • Symptoms: Starts but no output beam.
        • Vital Signs:
          • Output power: 0.0 mW.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial Tests: Checked alignment at both ends, no change. See additional comments below.

        I had a 30" HeNe tube sitting in my attic for about 2 years. It would start but not lase. (To power it, I am using an SP-255 exciter set at its minimum current of 7 mA with an 80K ballast resistance.) The lack of lasing is almost certainly due at least in part to mirror alignment problems. In fact, originally, one of the mirrors was obviously bent at a visible angle! I had tried to straighten them both the best I could when I acquired the tube but was unsuccessful at that time. I had used the basic reflection technique for mirror alignment but wasn't able to configure the setup stably enough to work on such a long tube.

        A few days ago, I decided what the heck, no darn HeNe tube is going to get the better of me! First, I tried using the beam from an argon ion laser (it's blue so would pass down the bore and hopefully could be centered). No dice. The beam diverged too quickly for the long bore and it was impossible to figure out exactly what 'centered' meant - there was no single easily identified best position and orientation. (I assume that when laser companies do this, they have additional optics to produce beam of optimal size and minimal divergence as well as a spatial filter to clean it up. I wasn't quite willing to go to that amount of effort!)

        I then contemplated building a light bulb and telescope rig as described in conjunction with the home-built lasers in Scientific American but concluded that such an approach wouldn't have any chance of working with a long narrow bore tube. I also attempted the method whereby the reflection of the discharge from the far mirror results in a slightly brighter spot exiting the near mirror but not knowing how far off the mirror alignment actually was, this proved impossible and even Sam's Super Cheap and Dirty Laser Power Meter) with its sensitivity boosted by using a 5 uA panel meter for the readout (about 2 uW full scale) could detect absolutely no change when tweaking the mirrors. Bummer. :(

        So, I decided to use the "Bore Sight" method described in the section: Major Problems with Mirror Alignment. Please refer to Bore Site Method of Internal Mirror Laser Tube Alignment for what should be fairly self explanatory diagrams of this technique if you don't want to read the feature length version. :) The Bore Sight Cards (BSCs) were screwed to the ends of my wooden "Big HeNe Tube Cradle" (a pair of V-blocks attached to a 1x4) and their 1/16" holes carefully lined up with the bore of the 30" Tube Under Test (TUT). With the TUT removed, the Alignment Laser (A-Laser, a 1.5 mW HeNe head) was placed on the platform described in the section: Simple Adjustable Optics Platform with its aperture about 2-1/2 feet from the nearer BSC and aimed squarely down the center of the two bore sights.

        The TUT was then placed back in the cradle in exactly the same orientation as before, first with the OC facing the A-Laser. A lever adjuster (read: big flat blade WELL INSULATED screwdriver) was used to tweak the mount at the OC end to center the doubly reflected spot precisely into the bore sight aperture. Note: Two reflections - First from the TUT mirror and second off of the OC of A-Laser - this actually increases the sensitivity to alignment error). Then, I turned the TUT end-for-end to do the same with its HR mirror.

        A weak beam appeared after the first attempt! I practically fainted. :) Then, I worked at boosting the power by additional mirror adjustment.

        If the tube dropped on the floor or blew up, I'd be disappointed, but I accomplished what I really believed would be impossible without a much more sophisticated alignment technique! This was TOO easy! :-)

        OK, it isn't perfect - At first I was only getting a maximum of 3 to 4 mW from this 30 inch tube (which should probably be producing 15 to 20 mW) and the power is constanting changing - going as low as 1 mW over a 10 minute or so period. The beam is pretty clean, just weak and variable. Even very slight finger pressure on the mirror mounts intensity or disappear entirely. Gentle pressure on the center of the tube, or the tube's orientation ("This Side Up") also affects it noticeably. And, "walking the mirrors" by applying equal pressure in opposite directions at both ends doesn't seem to help much if at all and these effects are inconsistent. In fact, at various times, the same amount and direction of mirror mount deflection may increase or decrease the output! The behavior has some similarity to normal mode cycling but where a HeNe tube is operating with insufficient gain and/or a limited number of available longitudinal modes. Thus, I conclude that at this point, the alignment is close enough that any further mirror tweaking, if needed, will be done with the tube mounted three-screw adjusters described in the section: Means of Adjusting HeNe Tube Mirrors.

        I acquired this HeNe tube along with a couple of other long tubes of pretty much unknown pedigree. They all appear to behave in a somewhat similar manner (but the alignment of the others was fine). Possible causes include any or all of the following (I welcome any additional suggestions):

        1. Need for magnets to suppress strong IR lines as mentioned above. Long HeNe tubes often need a series of magnets to reduce the gain of the very strong IR lines, particularly 3.391 nm, through the process of Zeeman splitting (see the section: Magnets in High Power or Precision HeNe Laser Heads). However, I don't know precisely when a tube is long enough for this to be a problem or under what conditions the use of magnets can be avoided. A shorter but otherwise similar 19", 12.5 mW tube operates just fine without any magnets. I also don't know how much of a boost magnets can provide when all is said and one (or I exhaust the World's supply of magnets!).

        2. Loss of helium as a result of diffusion through the glass isn't out of the question (though a rough check of the spectrum doesn't show anything amiss). Given the age of these tubes (probably pre-1990), this is a possibility but I kind of doubt it to be a problem since that 19", 12.5 mW tube dates from the same era and if anything the effects of helium diffusion should be lower for a longer tube (about the same surface area to volume ratio, but a lower seal area to volume ratio).

        3. Poor or failed design. It's quite possible that these tubes were never quite right and were sitting in the back of the 'dead inventory' storeroom until discarded. One was definitely much older than the others, so they all weren't from the same batch, but perhaps the same recipe. The erratic behavior (especially) could be explained by mirrors that were too long a radius or flat (it isn't easy to tell for such a long tube). Incorrect OC reflectivity would result in a reduced maximum output.

        4. Or, just the fact that I neglected to issue the special HeNe laser long tube chants and incantations (which I have unfortunately lost). Now, this is something I definitely need to consider, though I don't know what options are still available at this point! :)

        My guess at the moment is that assuming this (and the other long tubes) aren't simply defective, is that they need a wad of IR suppression magnets in strategic locations to boost the output power and mirror micro-adjusters to stabilize the output power. It better not be (4). Then there may be no hope at all!

        This tube looks exactly like any normal coaxial style HeNe tube, just a bit longer than most. I have a dead SP-124 laser (which is of similar length but with a side-arm tube and external mirrors) so I know what it does for magnets (See the section: Description of the SP-124 Laser Head) but with that design, the magnets can be placed next to the bore. With a coaxial tube, there is at least a 3/4" minimum separation meaning that the magnets would have to be much more powerful to result in an equivalent strength magnetic field inside the bore. And as far as I know, big cylindrical laser heads aren't any different than small cylindrical laser heads - no magnets. But perhaps this is incorrect. However, Melles Griot lists several 25 to 35 mW cylindrical laser heads in their catalog that are only 2 inches in diameter - leaving little room for powerful magnets!

        I did do some experimenting a bit later and found that a pair of really powerful rare-earth disk drive positioner magnets seemed to help a bit with maximum power now about 7 mW, but did little to reduce the fluctuations in power over time - up and down. However, a series of weaker ceramic magnets along the side of the tube didn't do anything good or bad. I then tried a series of 8 toroidal ceramic magnetron magnets with alternating N and S poles sitting under the tube and this boosted maximum power to a bit over 8 mW with just the right finger pressure on one of the mirror mounts. I expect that another bunch of these magnets above the tube would add another mW or so but kind of doubt this as a cure. I can't imagine that the laser heads these things were designed for required a couple dozen or more super strong magnets to function properly. Or, maybe there are very special locations for each magnet (part of the secret formula) allowing for fewer and/or weaker magnets to suffice. The use of the magnets did boost maximum power by 60 to 100 percent but getting another 200 percent boost in this manner seems unlikely!

        I do believe that the addition of the three-screw mirror adjusters will be enough to reduce the variations in power not due to mode cycling. With the tube in supported inside the aluminum cylinder from a dead 24" HeNe laser head (another of those 19" tubes, but this one was up to air), power starts off low (below 1 mW) when cold but peaks above 7 mW and remains above 6 mW without touching anything. Since slight finger pressure on either mirror mount will achieve 7 to 8 mW at any time, this suggests that it is indeed a matter of the pointing accuracy of the mirrors changing due to thermal effects.

        Repairing the Northern Lights Tube

        • Patient: Melles Griot style 14" HeNe tube named "Northern Lights" due to its discharge color.
        • Symptoms: Works but at reduced power (should be 5 to 6 mW). Discharge color strange. :)
        • Vital Signs:
          • Output power: 1.2 mW
          • Optimal current: 8 mA
          • Beam quality: Good.
        • Initial Tests: This tube came in with a power output of only about 1.2 mW and the peculiar discharge characteristics described in the section: HeNe Tube Lases but Color of Discharge Changes Along Length of Bore. There was no visible damage. Mirror alignment at both ends was adjusted which increased the power output to 2 mW, still way below what should be possible for a tube of this size. The conclusion was that there was internal contamination due to unknown causes and getter (re)activation would be required.

        Thanks to my Solar powered getter heater, the power came up to 4.6 mW (from 2 mW). I used a $1, 7" x 10" plastic Fresnel lens reading magnifier focusing Sunlight on both the front (the actually chemical) and the back of the steel or whatever U-channel getter loop. After a couple of these treatments, the discharge was almost uniform and the correct color. Only knowing that there was a problem would anyone notice the slight change along the bore. The power at this point peaked at 3.25 mW. Unfortunately, the Sun moved away from my HeNe tube reprocessing area (i.e., back yard) so further progress had to wait until it returned.

        Knowing that this lens would be of high enough quality and of adequate size, I built an adjustable mount for it so that the getter can be positioned reliably at its focus. Not that I had too many doubts - it was quite effective at instantly vaporizing leaves and the occasional unfortunate bug. :) See the section: Simple Solar Heater for details. The next day, with my fabulous contraption in-hand, I gave the tube a few more treatments of several minutes each, focused on the inside (active area) of the getter. After the third or forth of these, the maximum power leveled off at 4.6 mW which leads me to believe that the contamination has been eliminated. The discharge color is now perfectly normal and uniform over the length of the bore. It turns out that the operating voltage has increased by about 100 to 200 V (estimated) between the contaminated and present state. In addition, the output now peaks at just about the correct 6.5 mA rather than 8 mA as it did before.

        A summary of discharge color versus power output for this tube is given below. I assume behavior will be similar for other tubes though the power outputs will differ in both absolute and relative terms.

        • Color normal throughout: 4.6 mW.
        • Color normal at anode, barely detectably pink at cathode: 3.25 mW.
        • Color normal at anode, noticeably blue-ish white or pink at cathode: 2 mW.
        • Blue-ish white or pink throughout: 0 to 1 mW depending on severity (expected).

        Thus, even a very slight anomoly in discharge color can indicate that output power is likely to be much less than might be possible with a little 'cleanup'.

        Interestingly, there is still absolutely no evidence of a getter spot so I assume my procedure doesn't actually result in a significant amount of material being ejected from the getter. Other possibilities are that the active chemical is perfectly clear in both its original and 'used up' state or that it is designed to be retained within the getter structure.

        Some final mirror adjustments at both ends (together using the 'walking the mirrors' technique - see the section: Walking the Mirrors in Internal Mirror Laser Tubes) and the tube is now producing a very respectable 5.25 mW. I pronounce it cured. :)

        Followup: I retested this HeNe tube after a rest of several months. It appears to be unchanged or perhaps even improved a bit - output quickly climbed to 5.25 mW and was still increasing when I powered down. So I wonder its problems were not due to an air leak or residual air but to some sort of internal contamination. Another indication of this is that the discharge color variation was opposite of what I have seen with soft-seal HeNe tubes. It was correct at the anode but tended toward pink/blue at the cathode.

        Additional followup: After more than a year, I can detect no loss in power. Thus, an air leak is unlikely as the original cause of the malady. I can only conclude that it was from a manufacturing goof.

        Strengthening a Weak Siemens HeNe Tube

        • Patient: Siemens LGK-7639 HeNe laser head.
        • Symptoms: Works but reduced power (should be 2 to 3 mW). Discharge color possibly slightly pink.
        • Vital signs:
          • Output power: .35 mW
          • Optimal current: 6.5 mA
          • Beam quality: Fair (TEM00 but slightly elliptical).
        • Initial tests: Checked mirror alignment at cathode-end. It was almost optimal, performed some slight adjustments with minimal improvement.

        Further treatment required removing this tube from its cylindrical laser head. This wasn't that difficult as the the end-caps came off reasonably easily due to the brittle glue and after drilling out a pair of pop-rivets. The 12 RTV Silicone blobs were readily accessible and succumbed to my roofing flashing aluminum blade. Checking the alignment at the anode-end showed that it was also optimal in relation to the current cathode-end alignment. I thought that the discharge color might have been a bit on the pink side so I performed several Solar heating treatments on the getter but with absolutely no reaction of any kind.

        I was running out of ideas. Normally, I would not expect the alignment at both ends to have changed but after a comment from Daniel Ames (dlames3@msn.com) I decided to do some more fiddling with the mirrors at both ends of the tube. While applying pressure to the anode-end mirror mount with a piece of wood (dry and well insulated!) I pushed on the cathode-end mirror mount in the opposite direction (this retains parallelism and is equivalent to 'walking the mirrors' for an external mirror laser). Guess what? I found that there was an orientation where this would result in significantly increased power. So I took a chance and bent the anode-end mirror mount by carefully calculated amount. In other words, at random. :) Well, actually by an amount that was approximately sufficient to result in the decrease in power when pushing on the it previously. Then, I adjusted the cathode-end mirror mount for maximum power.

        I am now getting about 1 mW (compared to .35 mW when the patient arrived) without any of the special Siemens chants (those should help, right?). However, I don't think mirror alignment will go much beyond the 1 mW barrier. I suspect that the gain of the tube is still somewhat low and that the slight misalignment at both ends resulted in a much more dramatic drop in power than it would have when the tube was new. I doubt that the alignment changed much by itself (the tube was inside a sealed laser head so I know that it hadn't been touched by anyone else).

        Attempting to Revive Some Soft-Seal HeNe Tubes

        • Patients: 2-1/2 dozen Spectra-Physics model 084-1 HeNe tubes.
        • Symptoms: Varied from no or low power to perfect health.
        • Vital signs:
          • Output power: 0 to 3 mW.
          • Optimal current: 4 to 6 mW.
          • Beam quality: TEM00 (if present).
        • Initial tests: Checked all tubes for functionality and output power (when cold).

        I recently acquired about 2-1/2 dozen soft-seal HeNe tubes in varying stages of decay. Specifically, these are the Spectra-Physics Model 084-1 HeNe Laser Tube, a type commonly used in early barcode scanners. These use soft (Epoxy) seals for the fixed (totally non-adjustable) mirrors bonded to the tube end-plates. Most of the glass part of the tube is wrapped in thick aluminum foil (probably for thermal stabilization - this is common with even newer Spectra-Physics HeNe tubes such as their models 88 and 98), has an attached 100K ohm ballast resistor stack in heat shrink tubing, and rubber end-caps to more or less protect against shock and damage. (More details can be found in the section: An Older HeNe Laser Tube.)

        I performed an evaluation on each one just long enough to determine functionality and initial power output, if any. The 30 some odd tubes came through as follows:

        • Dead (4) - Tubes don't start at all - up to air, one visibly cracked.

        • Weak or no beam (7) - Tubes start but produce either a beam of less than 1 mW or no beam at all. Some had noticeably off-color discharges. One had the infamous discharge that changes color along length of bore.

        • Good (20) - Tubes appear to be perfectly normal with an output power of at least 1.5 mW (most were between 2 and 2.5 mW). (I am offering these to good homes essentially for the cost of packing and shipping. See the section: Sam's Stuff for Sale or Trade and Items Wanted for details.)

        These HeNe tubes have getter electrodes and associated getter spots. All the weak or non-lasing tubes showed noticeable deterioration of the getter spots with varying degrees of white or brown deposits. (In fact, 1 of the dead tubes was missing its OC mirror totally and 2 of the others were cracked with interiors that looked as though they had been stored in salt water or something else that resulted in crusty deposits and actual etching of the glass, cause unknown.) The good tubes have a spot which has mostly the normal metallic black appearance.

        I decided to try my solar heater getter reactivator first. This proved to be a big mistake. :( Since there is no way to aim the solar beam to the getter electrode without passing through the powdery stuff, it gets heated the most and apparently releases all the old trapped gasses that it had been accumulating over the years (probably 20 or so). Tube #1 (below) went from a pink discharge and no output (but probably very near threshold) to not being able to start at all. :(

        My next thought was to get back to my getter heater project and finish the coupling coil - but that sounded like too much work! Perhaps, if I had been more patient, those renegade gas molecules would have been reabsorbed but I didn't want to wait. So, I decided to try reactivating the getter of tube #1 by putting it in a microwave oven. Hey, what the heck - with a half dozen otherwise useless HeNe tubes, I could experiment! :) Unfortunately, I tried pressing my luck too far by leaving the tube to cook for just a bit beyond well done - and the glass cracked (what can you do with a capillary attached to a mirror?). If I had gone a little easier on it, the outcome would likely have been positive. It took a couple of hours to build up the courage to try the others (with shorter bake times of a few seconds and checking for hot spots after each one). However, the results were mixed and I'm now somewhat confused. The patient status list follows:

           Patient                 ----------- Power Output (1) -----------
            Number     Original     After Treatment    2 Days    1 Month
          ------------------------------------------------------------------
                 1      0.0 mW                  NA - Cracked (2)
                 2      0.0 mW          1.5 mW         1.7 mW    1.4 mW
                 3      0.1 mW          0.6 mW         0.3 mW    0.3 mW (4)
                 4      0.5 mW          0.5 mW         0.5 mW    0.5 mW (5)
                 5      1.0 mW          0.8 mW         0.7 mW    0.7 mW
                 6      1.2 mW          1.1 mW         1.0 mW    0.8 mW
                 7        --            1.7 mW (3)       --      1.2 mW
        
        Notes:

        1. The power output of each tube is listed for its initial test, immediately following microwave treatment, and for subsequent followup tests the next day. In all cases, these data are for the maximum power after a 10 to 20 minute warmup or where its value had obviously stabilized.

        2. Patient #1 died from excess abuse in microwave before treatment protocol could be determined. R.I.P. (Rest In Pieces - Organs including mirrors and capillary may be useful for transplants).

        3. Patient #7 was thought to be DOA since it wouldn't start. It was put in the microwave oven just to confirm. Since the light show unexpectedly looked fairly normal, treatment was quickly aborted. However, as a result, no initial output power reading is available. (I expect the starting problem was unrelated to the tube's condition.) The decrease in power after 1 month may simply have been back to its pre-nuked value.

        4. Patient #3 has had additional treatments. See the section: Followup Experiments with a Low Power RF Source on Patient #3.

        5. Patient #4 was retested on a return visit and found to be outputting a TEM10 beam. The cause was possibly a speck of dust on the inside of the OC mirror. Some gentle tapping has at least partially dislodged or moved this flake (it can still be seen and won't go any further) with a resulting TEM00 beam and power output increased to .7 mW. The of the still low power (for a good SP084-1, it should be at least 1.5 mW) may still be due to this cranky bit of dust as it can still be seen in the internal beam.

          While patient #4 was on the treatment table, the RF exciter was turned on with absolutely no effect. Since there is no evidence of gas contamination, this isn't surprising.

        As noted, rated power of these tubes is probably about 2 to 3 mW. From this data, it would appear that the tubes in the worst shape are likely to benefit the most.

        Something that can be seen from the data and appears somewhat peculiar is that cooking certain tubes just long enough so that the microwave induced discharge glow reaches full brightness resulted in a substantial increase in output (when powered in the normal manner). However, after a few minutes (or maybe a day or so), the output power would decay back to its original value (or below as with tubes #5 and 6 - though the original values may be suspect and the decay may have happened regardless of whether the microwave treatment was attempted). Tube #3 peaked at about double its final value but still retained a 3-fold improvement compared to its condition upon arrival. In any case, I now believe that whatever is going on isn't strictly related to the getter - maybe also a combination of the heat resulting from the microwave treatment releasing trapped helium and/or neon from the walls of the tube (low gas pressure originally), helium deficiency due to diffusion through the tube walls/seals, or a phenomenon that is totally independent (more discussion below). Since this behavior can be repeated at will for those tubes that exhibit it - a quick shot in the microwave and you get a nice, but temporary boost in power output, which may have it uses. :) (Patient #3 has agreed to some additional experiments to determine if extended operation or other more advanced treatments can actually clean up contamination.)

        The result with tube #2 was impressive (in a relative sort of way, and more so since it appeared to improve further with a day's rest) but I have no idea why. I imagine that at least with respect to the getter, some of these tubes probably had no available un-activated getter material remaining in the getter electrode, nothing to activate. Tube #2 must have had a wad of the stuff hiding somewhere. :)

        (From: Consulting Laser Physician Daniel Ames (dlames3@msn.com).)

        About Sam's microwave HeNe/getter soup recipe:

        From what you have described above with the 6 patients (data for patient #7 wasn't available at the time of the consultation. --- Sam), I can only surmise about the results showing power peaking and decaying. If the tube was tested within only a few minutes after being removed from the microwave oven, then I would suspect one or more of the following to have ocurred:

        1. He and Ne were released from the aluminum cathode and some from the anode metal, plus maybe some from the inner glass walls.

        2. O2 and other impurity gasses were absorbed by the getter.

        3. Output power could start out higher than before the getter baking treatment, but here is what I suspect might have caused the output power to decrease after a few minutes: The aluminum cathode most likely absorbed more of the microwave energy than any other part of the tube and therefore it also became the hottest. As it began to heat up, it released some He and Ne but the treatment only lasted for what - 2 seconds approximately? Successful outgassing of metals in a vacuum even as high as 2 to 4 Torr requires more than just 2 seconds. I suspect that one of two things caused the power drop off:

          • The cathode remained hot enough for a few minutes and continue outgassing He and or Ne and possibly raised the tube's He. and/or Ne partial pressures to (above) the desired pressures and caused a power drop off.... Maybe.... Before and after discharge voltage and current tests should confirm this, either way, see below. Or:

          • The tube's gas temperature was elevated by the microwave energy which caused the He and Ne partial pressures to elevate too. If the tube originally (before treatment) was He deficient, than the elevated He pressure could actually cause the output power to temporarily increase until the tube cools back down to a normal operating temperature and the partial pressures of both the He & Ne have decreased again. One way to test this could be to wrap the tube with a thermal insulating material (e.g., Fiberglas) to trap the heat generated by normal use in order to raise the gas pressures, then monitor the output power.

          • The Weird Science Theory: If the tube was immediately powered up with its normal power supply after being removed from the microwave oven, could it be possible that the He and Ne gasses are behaving in a similar fashion to food when it is cooked in the microwave oven - I.e., the gas molecules are caused by the microwave energy to vibrate and possibly just like nuked food, its molecules are still vibrating for a few minutes after the tube is removed from the M/W energy and immediately powered up by its normal power supply and behaving differently until they stop vibrating???????

        What about measuring and comparing the operating voltage and current on tubes #5 & 6 above with the reading from tube #2, since #5 & 6 actually dropped in output power below that of their respective (originally) observed power. This could give us a clue as to whether tubes #5 & 6 are actually higher in pressure or lower than tube #2.

        It would probably be easier on the glass tube and it's geometry if it was powered up and thus heated up to normal operating temps (just) before subjecting it to the intense heating of the metal parts of the tube by he microwave. The aluminum cathode will expand in diameter in the microwave, the metal anode too, so by allowing the normal power supply to heat up the glass and metal parts first at a normal rate of expansion, then it should have a better chance of survival in the microwave.

        HeHeHe..... and I thought this would be a 1 paragraph reply..... hehe :) I'll submit my usual bill for services. :)

        (From: Sam.)

        I could believe partial pressures increasing or He being released (I've been more convinced that He depletion may play a part though it isn't obvious from the discharge color). However, the metal parts of the HeNe tube actually remain quite cool. This could mean that any effect on the getter may actually due to the glow discharge and not the actual microwave heating though on tube #1, the getter glowed orange hot after a couple seconds once the tube had cracked - I suspect it doesn't get heated nearly as well with the surrounding gas competing for microwave attention. The glass between the cathode and anode of the tube gets hottest (which is what cracked with patient #1) but the cathode itself doesn't appear to get very warm at all.

        I really doubt any molecular vibration effects apply here - those sorts of phenomena have time constants measured in small fractions of a second. The behavior seen with patient #3 was on the order of 20 minutes.

        My current feeling is that the odd behavior is due to a combination of heating and release of gasses from the tube walls and that the fundamental problem is one of low gas pressure but not a particular lack of He or Ne. I do expect to measure tube operating voltage and current producing maximum output (what of it there is). I may also attempt a helium soak starting with the tubes having the lowest output power (though none of the tube's spectra appeared to be obviously abnormal).

        Followup Experiments with a Low Power RF Source on Patient #3

        • Patient: Weak and Variable Spectra-Physics model 084-1 HeNe tube designated patient #3.
        • Symptoms: This is a followup after initial microwave treatment.
        • Vital signs:
          • Output power: Varies between .1 to .3 mW over the course of a few minutes.
          • Optimal current: 5 mW.
          • Beam quality: TEM00.
        • Initial tests: See the section: Attempting to Revive Some Soft-Seal HeNe Tubes.

        A few weeks after the original microwave revival experiments, patient #3 returned for some more extensive tests.

        Operating a HeNe tube is supposed to result in the scavenging of residual gas molecules due to the cathode acting as a sort of getter. So, I decided to perform a very scientific experiment on the most bedraggled of my assortment of Spectra-Physics 084-1 HeNe tubes - the one with the lowest output power and most off-color discharge - patient #3.

        I started by operating patient #3 for hours on end at 5 mA. Early in these tests, the output power would fluctuate quite substantially - dipping to as low as 0.1 mW at times. After perhaps a total of 24 hours of actual running time over the course of several days, the power has tended to stabilize somewhat, remaining over 0.25 mW at all times and peaking at 0.4 mW with an average of about 0.35 mW.

        However, additional operation hasn't resulted in any substantial improvement beyond this point. A few of observations:

        • The initial output beam power when running from a cold start was typically 25 percent higher than the 0.35 mW average but would decay over the course of a few minutes to the average or below.

        • The initial discharge color was nearly normal. But, along with the drop in power was a successively more off-color discharge. Eventually, while the overall bore color was only slightly pink and perhaps a bit more toward the blue approaching the anode, the color of the fuzzy discharge in the funnel at the anode-end of the tube was nearly neutral white with little orange or pink. Therefore, it would appear that the funnel contains high concentration of the 'bad' gas at this point.

        Thus, I concluded that since gas-metal reactions at the anode electrode are minimal, further improvement wouldn't be likely in any case since none of the rogue gas molecules were bumping around at the cathode where they might be taken out of circulation. Reversing polarity would sweep them to the other end of the tube but (1) running a HeNe tube with reverse polarity will quickly damage the anode mirror from sputtering and (2) the molecules will again congregate at the wrong end of the tube and stay there!

        Based on these observations, some other treatment would be required - something that would facilitate reactions at the getter and/or cathode but which wouldn't damage the mirrors.

        Using the microwave oven approach, that tube could be temporarily boosted to 6 times its original output power with it remaining more or less at a 3X improvement. So, I decided to do some additional experiments similar to these but under more controlled conditions.

        I repeated the glow discharge treatments on patient #3 but using a flyback based high frequency RF exciter instead of the microwave oven. With this approach, both the location of the discharge and the power level could be selected at will. In particular, the power could be set low enough that the discharge could be maintained indefinitely without fear of physical damage to the tube due to overheating.

        For the RF exciter, I adapted the circuit described in the document: Simple High Voltage Generator. The new schematic, with the high voltage rectifier removed is shown in Flyback Based RF Source and the major parts in ASCII, below (shown attached to a modern HeNe tube):

           +Vcc     Q1   +----------------+                               A||
             o           |                 )::                           .-''-.
             |       B |/ C                )::                           |\  /|
             |  +------|    2N3055         )::                          || || |
             |  |      |\ E             5T ):: +------------------------|| || |
             |  |        |                 )::(                         || || |
             |  |       -_-                )::(                          | || |
             |  |                          )::(                          |G|| |
             +--|-------------------------+ ::(                          |_||_| LT1
             |  |   Q2  _-_                )::(                          | || |
             |  |        |                 )::( Secondary (HV) winding   | || |
             |  |    B |/ E             5T )::(                          | || |
             |  |  ----|    2N3055         )::(                          |    |
             |  |  |   |\ C                )::(                          | C  |
             |  |  |     |                 )::(                          |____|
             |  |  |     +----------------+ ::(                          '-..-'
             |  |  |                        :: +--------------------------+||
             |  |  -----------------------+ ::
             |  |                       2T )::
             |  |               +---------+ ::
             |  |               |       2T ):: T1 - Flyback transformer from B/W or
             |  +-------------------------+         color TV or computer monitor.
             |                  |
             |            R1    |    R2
             +----------/\/\/\--+--/\/\/\--+
                          110        27   _|_
                          5W         5W    -
        

        With no high voltage rectifier, the output is radio frequency AC at between 10 and 20 kHz. This was applied between the cathode mirror mount and a 2" strip of aluminum foil wrapped around the tube to provide capacitive coupling for the return path without involving the anode-end mirror mounts (and thus avoiding the possibility of sputtering). There is absolutely no glow inside any part of the bore or near either mirror mount. In addition to allowing capacitive coupling through the glass of the tube, the AC would also assure that the gas molecules wouldn't get stuck in one spot. This circuit produces a nice glow when powered from only about 5 VDC at 1 A or so it runs cool. The visual effect is similar to that of a plasma globe operated at low pressure and as with those gadgets, the glow could be influenced by touching the glass of the tube.

        The physical connections to one of our patients is shown in RF Treatment of SP084-1 HeNe Laser Tube. Note that this way of exciting the gas in the HeNe tube will not cause the tube to lase as there is no high intensity discharge in the bore.

        CAUTION: If you try this, take care not to use too much voltage or the glass may be punctured! Spectra-Physics HeNe tubes have nice thick glass walls so the risk is quite low but don't press your luck - it isn't voltage but power transferred to the plasma that should matter so really high voltage isn't required. In fact, I'll be trying a coupling coil instead of capacitance through the glass next.

        The most effective position for the aluminum foil wrap to have any effect on tube performance was about midway between the end of the cathode-can and the anode mirror mount. This resulted in the glow discharge bathing the getter and end of the cathode. A test with the foil wrapped around the cathode area of the tube resulted in minimal effect despite the close coupling and nice glow.

        As with the microwave oven treatment, the RF also resulted in a dramatic increase in output power for patient #3. In fact, although my records are non-existent, I believe that this resulted in even more of a boost to over 0.8 mW. Of course, I could run the RF discharge for a long time (several minutes in this case so far) compared to a few seconds for the microwave treatment (before there was risk of overheating and killing the tube). But, as before, the output power still decayed back to its original value over the course of a half hour or so.

        Accompanying the power increase was a distinct improvement in discharge color. Recall that originally, the discharge was somewhat pink and charged to a somewhat blue color at the anode with almost a neutral white in the funnel next to the anode. The new color was much more normal though possibly a bit on the orange side indicating an excess of neon or lack of helium (as before with the microwave oven treatment). In fact, the funnel discharge color was distinctly orange - more so than is typical of healthy HeNe tubes. Another change was that the tube's operating voltage declined by up to about 100 V when compared to its value with the off-color discharge. (This is the opposite effect observed with the "Northern Lights" tube - see the section: Repairing the Northern Lights Tube. One thing that has been confirmed is that heating plays little or no role in the power boost - the RF approach results in very little heating of any part of the tube.

        Next, I set up the RF exciter to run at the same time as the normal HeNe power supply so I could monitor the beam power while tickling the gas outside the bore. With this configuration, output power could be maintained at a much higher level, though not at the absolute maximum that could be achieved by 'off-line' RF treatments.

        • By shutting down the HeNe laser power supply for a minute or so for every 5 minutes or so of operating time while allowing the RF exciter to continue to run, power could be consistently peaked at about 1 mW (!!). Of course, when powering up, there would be a minute or two delay before this output was again reached. My assumption is that by allowing the tube to 'rest', the residual gas molecules that had collected in the bore and near the anode would redistribute themselves so they could be scavenged by the RF glow discharge.

        • The combination of the HeNe laser power supply and RF exciter could not maintain maximum output power together. I suppose that while much of the gas in the outer reservoir could be cleaned up by the RF, when any of the contaminant molecules happened to wonder near the end of the bore, it would be sucked in by the stronger DC field gradient. So, the net effect is still a small flow of these molecules toward the anode where they aren't wanted.

          The equilibrium power output with both the HeNe laser power supply and RF exciter running with minimal input (about 3 VDC resulting in perhaps 1 W of RF power) is around .7 to .8 mW. This was confirmed both by starting at a high output power level after an extended RF treatment (about 1.05 mW - the maximum I have seen so far on patient #3) and letting it decay, as well as running it on the normal power supply without RF until the output power had dropped to aroudn .3 mW, turning on the RF, and watching the output power climb back up. The actual amplitude of the RF doesn't seem to affect the equilibrium output power very much. However, some of the RF must couple to the HeNe tube bore because turning the RF up causes the normal discharge to become unstable resulting in a flashing HeNe tube unless the HeNe laser power supply voltage is also increased (resulting in a higher current as well). The sensitivity to this effect depends to some extent on the location of the coupling foil - nearer the anode reduces the amount of RF that can be tolerated while maintaining stability. This would make sense of sorts since more of the bore is between the RF source and the grounded cathode.

        • Performing the same experiment on another SP084-1 that exhibited low power (patient #4) but no evidence of gas contamination (the color of the discharge was normal), resulted in no significant change in output power one way or the other. (The cause of this tube's problem may be dust on the inside of the OC mirror).

        So, it appears as though maintaining a modest glow discharge outside of the bore can be used as a means of life support for these marginal soft-seal HeNe tubes. Too bad about the additional high voltage (the foil) that needs to be well insulated. :) If only HeNe tubes had gas return channels from the anode to the gas reservoir! (A helical capillary longer and narrower than the bore would prevent the normal discharge from taking that shortcut.) Then, there would be a steady flow of gas and even without the RF, there would be no concentration of contaminants in the bore or near the anode. With the RF active, there would be continuous cleaning and instant purifying action!

        More to follow. :)

        Treatement for the Yellow HeNe Laser Tube with a Warped Bore

        • Patient: Hughes style (manufacturer unknown) yellow (594.1 nm) HeNe laser tube about 14 inches long.
        • Symptoms: Starts fine, no output.
        • Vital signs:
          • Output power: 0.0 mW.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial tests: Checked for output by gently rocking mirror mounts. Results inconclusive. There was some indication of flashes but they were not consistent. However, gentle tapping on the tube (not the mirror mounts) would result in flashes.

        This is the upper tube in Three HeNe Tubes of a Different Color Side-by-Side. The OC (and anode connection) is at the left with the cathode terminal and getter visible below it. No attachment is made to the OC mirror mount on the right.

        The fact that gentle tapping affected the behavior suggested that something was loose inside. And, even pointing the tube up in the air at various angles would occasionally result in at least a weak output beam - perhaps the tube would be useful as an inclinometer. :)

        At first, there was no visible indication of loose parts - its general condition is quite good. However, upon close examination, the bore is supported at the OC-end of the tube by a cup affair which had a set of fingers that look sort of like the pedals of a tulip and these were actually loose around the bore. Either the tube had been used to hammer nails, or the mirror mount next to the cathode can had been accidentally used as the cathode connection. Since most modern HeNe tubes use the mirror mounts for both power supply connections, a natural mistake is to attach the negative of the power supply to the cathode-end mirror mount. While this would result in the tube appearing to operate normally, there will be serious overheating of the mount and possible sputtering of the OC mirror. The overheating could cause the petals to relax and loose their grip.

        In fact, pressing laterally on the HR-end mirror mount - not to deflect the mirror but to actually move the entire bore slightly by flexing the glass of the tube - would result in a strong good quality beam. Interestingly, even with careful adjustment of the mirror alignment at both ends, but without this external force, would produce a weak beam but no where near what was possible with the added deflection. The OC mirror mount could be easily rocked without affecting anything else. However, for the HR mirror mount, I had to construct a Melles Griot style three-screw locking collar for this test to be able to make slight adjustments in the alignment without permanently bending the mount. Otherwise, any effect would be a combination of the bore being moved and the mirror alignment with respect to the bore changing.

        It appears as though the HR mirror was correctly aligned as just changing this relationship would only result in lower maximum output though it was possible to reach a compromise where the tube produced a steady beam by also tweaking the OC mirror alignment. However, this was less than 1/2 the possible power available by just the bore movement technique.

        My theory is that the bore is actually slightly warped - though I can't tell by looking at it. If it were just improperly positioned, realignment of both mirrors should have resulted in a strong beam equal or nearly equal to its original performance. Given that this didn't happen, I am forced to the conclusion that the lateral deflection not only moves the bore but also unwarps it to some extent. Another indication of a bore problem is that just adjusting the mirrors tends to result in a TEM10 rather than the expected TEM00 beam. However, as the lateral force is applied, the beam starts out TEM10 and thenthe two sub-beams merge to form what looks like a TEM00 beam though I haven't confirmed that this is actually so. With the cathode can obscuring most of the interior, it is impossible to see if there are other internal problems. It needs to have an X-ray or CT scan. Is there medical insurance for sick lasers? :)

        To deal with the chronic condition - there is after all no practical way to actually go in there and really fix the problem - I intend to construct a mount for the tube that will also have a lateral force adjustment. Some experimentation (actually quite a bit of it) has revealed that the optimal force seems to be low enough that there is minimal risk of breaking the tube, though I'd be happier with some other solution. With this arrangement, it should be capable of producing more than 1 mW at 594.1 nm which isn't bad for a tube of this size.

        Reviving a Spectra-Physics Model 130B Antique Laser

        • Patient: My third Spectra-Physics 130B HeNe laser.
        • Symptoms: Starts fine, discharge is red/blue, no output.
        • Vital signs:
          • Output power: 0.0 mW.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial tests: With the red/blue discharge, no output would be expected but being optimistic, I tried some jiggling of the one mirror mount that appeared to have been twiddled - nothing.

        The SP-130 may have been the most solidly constructed of any small gas laser in history! See: A Typical SP-130 (Note original manual). The case, which is also the support chassis for the tube, external mirror mounts, and power supply is built of precisely milled aluminum panels. Everything fits together like a fine watch (if you remember those!). Versions of this laser were produced as early as 1965 (that is the date on one of the diagrams in my original "Spectra-Physics Model 130 Gas Laser Operation and Maintenance Manual", the one in the photo, above.) More information can be found in the section: Description of the SP-130 Laser.

        This is the third sample of the Spectra-Physics 130B laser that I have acquired. I don't know if there ever was an SP-130A but the SP-130 may have been an earlier version using a tube with a heated filament instead of the more modern cold cathode design.

        SP-130B #1 initially started and had a discharge that was weak though approximately the correct color, but died on the operating table - cause unknown. The discharge winked out, never to return. All indications are that the tube is up to air except that the getter hasn't changed to the "white cloud of death" appearance.

        SP-130B #2 (the actual laser in the photo, above) was DOA with an up-to-air tube and some prior dissection attempts including cut wires. The mirrors were also totally ruined, possibly from poor storage conditions or careless handling or both.

        Which brings us to SP-130B #3. This one started and ran fine but the discharge color was initially red/blue, along the lines of the example labeled "Moderate - no output" in Color of HeNe Laser Tube Discharge and Gas Fill. These are normally hopeless and terminal but I figured it wouldn't hurt to run the laser for awhile just in case a miracle occurred. In fact, over a period of several hours, the color did gradually change eventually approaching something reasonable, at least in the bore. (Normal is defined as "salmon" or white-ish red-orange and more of an orange color in the expanded areas.) The color in the expanded areas was not as orange as would be normal but was fairly close. But there was still no output.

        Next step: Check mirror alignment and clean optics. First, I removed the HR mirror and used a working HeNe laser on an adjustable platform to check OC alignment by passing its beam down the bore and looking at the reflection back to its output aperture. This appeared to be slightly off center, so a bit of tweaking was in order. Then, I replaced the HR and adjusted it to also place the reflection squarely back into the alignment laser's output aperture. Still no output.

        During this time, I also attempted to clean the optics as best I could knowing that the mirrors are soft-coated and thus can't be cleaned with anything stronger than breath-fog. :) The mirrors and Brewster windows were cleaned without incident but the Anti-Reflection (AR) coating on the OC mirror didn't survive so there would be slight ghost beams if the laser was to work at all. The sticky tape method of mirror glass retrieval recommended in the SP-130B manual also removed the coating. :(

        Next, I decided to actually consult the manual with respect to alignment - what a concept! :) Their procedure is even simpler than mine: Using the curved mirror set, just tighten both mirror mounts down so they are flush with the case. The machining is precise enough that this should produce a beam. I only have a curved OC, the HR is planar. So, I tightened down the OC mirror mount and checked it with my HeNe alignment laser - at least as good as doing it my other way.

        Doing the same with the HR mirror mount didn't produce a beam, but when I loosened it slightly, I could jiggle the mirror just enough... And, for the first time in perhaps 20 years, I detected a few coherent photons in a flash from the OC. After somewhat more tinkering and letting the system bake, it was doing between 10 and 40 microwatts depending on the setting of the current adjust pot. Maximum output is at the full clockwise position which suggests that there is still gas contamination or possibly just low helium - it doesn't peak as expected at some intermediate value. After cleaning the Brewster windows (at least they probably won't disintegrate like the AR coating if looked at the wrong way!), output power has exceeded 0.25 mW, not up to spec (0.75 mW) but still a lot better than 0.0 mW and a bit amazing considering the age of this laser.

        So, this patient will be held in intensive care for some time to determine if any more cleanup takes place. I also suspect that a shot of helium would be beneficial. Given that air (probably) has leaked in, helium has likely leaked out. Also, the blue-green portion of the spectra of the discharge appears a bit weak - that is mainly from the helium. What I don't know is the age of the tube (it was probably a replacement) but it is probably at least 10, possibly 20 years old. When I do get around to a helium soak, I'll probably start with 10 days (1 day/year of life) to be on the safe side and see if that helps. It's bad form to overdo it by much - you can't reverse the process except by waiting 1 year for each day of extra helium!

        Two Melles Griot HeNe Laser Heads with Terminal Sputtering Disease

        • Patients: A pair of 05-LHR-991 (10 mW) HeNe laser heads.
        • Symptoms: Hard starting, low output, discharge color too white.
        • Vital signs:
          • Output power: 0.1 to 1.0 mW and erratic.
          • Optimal current: Off scale.
          • Beam quality: Good.
        • Initial tests: Just attempting to run these tubes on my test power supply was a real treat. They just barely started with the Variac cranked up to 140 VAC, and then only if the humidity was low and positive leads insulated except at the tube anode. Operating voltage is probably somewhat high as well. (This supply will easily run longer higher power tubes.)

        These were probably nice high power laser heads at some point in the past but now were clearly in deep trouble. (Thankfully, someone else had already removed the HeNe tubes from the aluminum cylinders so the diagnosis could be made a lot more easily.) At first I thought the gas fill was contaminated somehow (because of the funny white-ish color) and even went so far as to try activating the getter with my Solar furnace with no change at all.

        The key symptom which didn't register at first but is obvious in retrospect were several silvery metallic spots around the tube next to the cathode end-cap/mirror mount. On many Melles Griot tubes, the cathode has a set of 4 holes punched through it equally spaced around its periphery. Normally, it is possible to vide the interior of the cathode and end of the bore through these holes. Not now. What the metallic spots must be are deposits of aluminum on the glass due to very serious sputtering taking place inside the cathode. Inspecting the bore as best I could (until an autopsy can be performed), it would also appear as expected that there are similar deposits on it near the end inside the cathode can. Whether the sputtering was simply from normal end-of-life when the cathode can pickling (oxide) gets used up, from some manufacturing defect, or from abuse, I do not know. The laser heads had closely spaced serial numbers so it's possible they were from a bad batch, or just from a set of lasers shipped to the same customer and used under similar circumstances.

        Unfortunately, prognosis is poor and salvaging the organs for transplant (e.g., the mirrors) may be in their future. :)

        Four Melles Griot HeNe Laser Heads with Broken Bores

        • Patients: Four 05-LHR-201 (5 mW) HeNe laser heads.
        • Symptoms: Bore broken between spider and cathode, low/no output.
        • Vital signs:
          • Output power: 0.0 to 1.0 mW and erratic.
          • Optimal current: 6.5 mA.
          • Beam quality: Good.
        • Initial tests: One of the tubes had been removed from its laser head cylinder before I received it so making the general diagnosis was trivial. Before powering up, I tapped on each head while observing the bore from the cathode-end. On all four tubes, the bores were jammed down into the cathode end-cap - which conveniently helped to stabilize them somewhat. Then, they were each powered up to determine condition. Depending on orientation, jiggling, tapping, etc., power would vary from 0.0 mW to 1.0 mW.

          The usual cause of such trauma is either all four having been dropped onto a concrete floor without adequate padding or passing too deep into the gravity well of a neutron star or black hole. :)

        All four of these laser heads must have suffered some terrible trauma though there was no external evidence of bruises, scratches, scrapes, or dents. Perhaps an entire rack of HeNe heads had dropped to the floor. :( The location of the breaks were also interesting. On these tubes, the bore is supported at three places: the fused glass at the anode-end; the main spider about mid-way, and another spider which is part of the cathode. The breaks were between the two spiders, right at the main spider on the one sample that was naked, probably the same place on the others though I haven't extracted them yet. It's also possible that the double spiders in close proximity resulted in too much stress or a peculiar resonance under the wrong conditions. The bore is made of very thick glass and its extension into the cathode isn't that long. However, these are the frosted variety (inside and out) which I imagine to be weaker than those of similar size which is made of polished glass.

        By tweaking the mirror mounts while the tubes were oriented optimally, I was actually able to get one sample up to 2.75 mW which was stable as long as the tube wasn't moved or rotated. The others peaked at 1.0, 0.6, and 0.25 mW respectively.

        Except for the 2.75 mW tube, the others are destined for my organ, err, mirror bank. I'll probably pull the 2.75 mW tube from its cylinder and keep it as a sort of curiosity and warning to any other HeNe heads that might be tempted toward recklessness. :)

        Spectra-Physics Model 120 HeNe Laser Head with Terminal Gas Leakage Disease

        • Patient: SP-120 with very gassy tube.
        • Symptoms: Red/blue discharge, Uck. :)
        • Vital signs:
          • Output power: 0.0.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial tests: The laser head was run on my HeNe laser test power supply for several hours with no improvement.

        With a diagnosis of terminal gas leakage disease, the only course of action is a tube transplant. Fortunately, I had another good tube (in a resonator) for this purpose. Either the bare tube or the entire resonator could be replaced. I chose to remove the entire resonator and install my spare intact rather than swap tubes since it is slightly lower risk but a replacement tube can be installed in about 5 minutes without requiring anything more than a touch-up of mirror alignment.

        The transplant went smoothly with the patient making a spectacular recovery. :)

        CAUTION: Don't be tempted to touch any of the coarse mirror alignment screws (the ones at 120 degrees around the mirror mount flanges - their setting is very critical and if you lose the beam, alignment from scratch will probably be needed. Use the pan and tilt screws (in the end-plates, on horizontally either side of the mirror mount flange) for all alignment. These shift the center of the bore with respect to the curved mirrors. If you can't get a a beam, the tube is bad, the Brewster windows or mirrors are dirty, or someone else messed with the coarse adjustment screws!

        Spectra-Physics Model 120 HeNe Laser Head with Moderate Gas Leakage Disease

        • Patient: SP-120 with somewhat gassy tube.
        • Symptoms: Overly pink discharge.
        • Vital signs:
          • Output power: 0.0.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial tests: The laser head was run on my HeNe laser test power supply to check for signs of life. Resuscitation was begun immediately.

        This SP-120 has a getter electrode but no obvious getter spot. Since every other SP-120 tube I've ever seen had a very noticeable metallic getter spot if still good, or the "white cloud of death" spot if beyond hope, I can only assume that for some reason or just lack of quality control, the getter in this tube was never fired - that may be an option if needed.

        The laser came in with no signs of lasing at any reasonable current setting but after 10 minutes of a steady 6.5 mA drip, coherent red photons started appearing in small quantities. Patient's chart of accumulated treatment time:

              Arrival  0.2 hour  3 hours   13 hours  24 hours  34 hours
            -------------------------------------------------------------
              0.0 mW    0.1 mW   1.7 mW     4.0 mW    4.4 mW    4.6 mW
        
        The output power of 4.6 mW is less than 65 to 75 percent of what a new SP-120 will produce at a current of 6.5 mA. Presently, the tube will output 5 mW at 7.50 mA and 6 mW at 9 mA. But I don't know the recommended maximum current for the SP-120 and 9 mA was still not the peak, rather the limit of my power supply. In any case, 6.5 mA is always a safe value for this size HeNe laser. Although the other SP-120 tubes I've tested also peaked at a current higher than 6.5 mA (I don't recall what it was), as noted, their output was still much greater at 6.5 mA than the patient. Though 5 mW output at 7.5 mA might actually meet spec, treatment will continue for a few more days. :)

        For more on reviving soft-seal HeNe lasers, see the section: Care of HeNe Laser Tubes.

        Spectra-Physics Model 907 With No Output

        • Patient: SP-907.
        • Symptoms: No output.
        • Vital signs:
          • Output power: 0.0.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial tests: Patient was run on my SP-255 (after initial repairs) and produced no output even with gentle pressing on mirror mounts.

        The SP-907 is the OEM version of the SP-127/107 laser tube and resonator with an overall length of more than 38 inches and a nominal output power of 35 mW. This patient came in with a cut power cable, broken cathode-end ballast resistor tube (only really affects appearance), and no power supply. The SP-207 (both linear and switchmode versions) is the recommended exciter but I don't have one. So, I had to adapt my SP-255 to the task.

        First, I just connected it as best I could with alligator clip leads to see if the laser would do anything. It didn't even flash with the input voltage cranked up to 140 VAC on a Variac. (The SP-255 is a linear power supply so boosting the input would boost the starting and running voltages as well.) I wasn't particularly surprised as the SP-907 tube is about 50 percent longer than the SP-124 for which the SP-255 is designed.

        On a hunch, I grounded the frame as it was not grounded originally. Then, exactly once, it started and continued to run until I backed the Variac down below 110 VAC or so. However, while lit, there was no sign of red output. The discharge color looked reasonable - perfect in fact - so this confirmed that the tube was gas intact and had no serious leakage. (A small getter spot was also present and looked reasonable as well. I don't know if the rest of the getter spot turned clear when used up of if this small spot was all there was.)

        But I couldn't get the tube started this way again no matter how long I held my breath. :)

        As a test I wired the tube backwards since with reverse polarity, the starting voltage is often somewhat lower though the operating voltage is higher. With this arrangement, it would occasionally flash but that's about it.

        Next, I returned the wiring to the correct polarity and applied some RF from my flyback HV widget via a strip of aluminum foil to the bore trying a few different places. When in contact with it relatively near the anode-end of the tube, the laser would flash on momentarily with the Variac to the SP-255 cranked all the way up but would never "catch".

        By accident, I did find out one interesting thing: If left alone for an hour or more, applying the full 140 VAC to the exciter suddenly without slowly turning the knob up on the Variac would result in it starting. But, only if allowed to sit for that hour (or longer). Hmmmm... Maybe it likes the output to climb quickly from near 0 V to its starting voltage, this somehow coupling via the tube capacitance and initiating the discharge. To verify this, I took a 400K ohm resistor and carefully discharged both the power supply and laser tube capacitance. And, presto! The tube started even without the hour's wait. In fact, it would now start at 125 VAC or sometimes even 115 VAC after only the time it took to apply the resistor.

        Great! So, I added a 200M ohm bleeder resistor across the power supply output and attached a nice Alden cable to the laser head. This enabled it to start and run reliably but it might require 125 VAC for starting after which it could be backed off to 115 VAC while running to reduce stress on the SP-255 pass-bank. Later, I added an external pod with a stage of boost circuitry to increase the SP-255 starting voltage. (See the section: Enhancements to SP-255.) This eliminated all starting problems and the need for the Variac. I set the operating current at 10 mA which should be enough (11.5 mA is nominal but I'd rather run it a bit low until later).

        I did dust off the Brewster windows - at least they are accessible after pulling back a rubber boot (unlike the SP-120 where it's impossible to clean them in place). No change.

        At this point, it is almost certain that the major problem is mirror alignment. I emailed the person I got it from and asked: "Before I attempt to align this beast, do you know if the mirrors have been touched?". Reply: "Well, maybe someone attempted to peak the power and totally lost alignment." "Duh, thanks for telling me." :)

        My first approach was to use the "bore sight" method of mirror alignment because I felt there was no way to get a HeNe alignment beam cleanly down the bore. The "bore sight" method allows all alignment to be done by reflecting from the mirrors externally, using a pair of cards with small holes positioned at the tube's axis to align the alignment laser to the tube. (See the section: Major Problems with Mirror Alignment, earlier in this chapter.)

        I used my trusty little 05-LHR-911 HeNe laser head on an adjustable platform to align its beam through the cards, which had previously each had a hole drilled precisely at the location of the center of the SP-907's mirrors. This worked reasonably well for the OC-end and confirmed that the OC mirror was way out of alignment - by 1 or 2 whole turns of the 1/4-28 adjustment nuts! (There would be no lasing on this long a resonator if the nut was off by even 1/10th of a turn!) So, someone really messed things up. :(

        However, I didn't count on what I found next: The outer surface of the HR mirror is coarse-ground (frosted), not polished, so there is no way to reflect a beam from it which this method of alignment requires. Why did SP do that? :(

        So plan A didn't work.

        Plan B is to do everything from the OC-end starting with removing both the HR and OC mirror mounts (just 3 nuts each so at least that's easy) and start by getting as much of my HeNe alignment laser beam through the bore, then installing the HR and aligning for a reflection back from there, and put the OC in and do the same.

        I fabricated some precise micrometer (80 tpi, the mirror adjusters from a large ion laser) adjustment plates and attached these (2 screws at one end, 1 screw at the other) to the laser head. This will provide the degree of control I need to align the tube's bore with the alignment beam. Providing fine pitch screws centered at each mirror of the laser being aligned rather than on the alignment laser results in a much more intuitive setup since there is almost no interaction between adjustments.

        Finally we have lasing!

        What a pain. In addition to the mirrors being all out of alignment, there are adjustments on bore straightness which were also messed up and it was was impossible to get any resemblance of a clean beam down the bore from my HeNe alignment laser. But, with a bit of careful tweaking, a spot was detected on the wall acting as a screen that was clearly from the alignment beam. Then, I replaced the OC mirror mount and aligned its back-reflection to coincide with the HeNe alignment laser's aperture, with dancing interference patterns. Finally, replacing the HR mirror mount and after a few minutes of gentle rocking, flashes where detected. :) Once a stable position was found for the HR (just sitting on the rods), the OC mirror was carefully adjusted to maximize power - still probably less than 1 mW. Then, the HR mirror mount nuts and washers were installed and carefully adjusted to tighten up the mount, never losing sight of the beam! Finally, I walked the mirrors to peak power. I will say one thing, these mirror adjustments are very smooth and repeatable with little backlash even though the entire range of lasing is probably less than 1/10th turn on the nuts.

        Note that I didn't follow the original Plan B procedure exactly taking the short cut of using the OC back-reflection to align it first rather than attempting to get a clean return beam back down the bore from the HR. Fortunately, it was successful.

        This SP-907 currently peaks at 18+ mW but will probably do 25 mW, maybe more, when run at the optimum current (it's still at 10 mA) with a proper cleaning of the Brewster windows - which is still a pain since they attract all sorts of stuff as soon as they are cleaned, and my operating suite isn't exactly a Class-100 clean room. Power typically drops way down just pushing the rubber boots back in place because that dislodges dust and guess where it goes! :) The mirrors could probably also use some cleaning but I'm not inclined to tackle those just yet.

        Melles Griot GreNe with No Output

        • Patient: 05-LGR-170.
        • Symptoms: No output.
        • Vital signs:
          • Output power: 0.0.
          • Optimal current: NA.
          • Beam quality: NA.
        • Initial tests: Patient was run for a short while but produced no output even with gentle pressing on mirror mounts. Discharge color is pink and dim.

        This green HeNe laser tube came from a self-contained rectangular Melles Griot laser, "GreNe" model 05-SGR-871, about 24 inches long with an internal brick power supply (which appears to work fine). The tube is interesting in that it has a frit (hard) seal at the cathode-end but an Epoxy (soft) seal at the anode-end. This was probably done to reduce thermal stress (in the frit oven) on the very delicate OC mirror. In fact, I am in contact with the person who may actually have worked on the design or manufacturing of this laser at Melles Griot. :)

        Normally with a green (or other "other-color") HeNe laser tube having a discharge color/gas fill problem, there is little hope of recovery. The gain is so low that even trace contamination results in no output at all. However, for some reason, I got the feeling that this one was close enough to warrant some effort.

        Regardless of treatment options, the tube had to be removed from the chassis. This required unscrewing two aluminum mounting blocks, unscrewed some nylon set-screws, and pealing away at the black RTV Silicone holding the tube in place. This accomplished, Mr. GreNe was moved to my diagnostic facility (e.g., my adjustable HeNe laser power supply, tapped ballast resistor, and current meter).

        Initially, the tube operating voltage was about 20 percent low and variable - getting even lower as the tube warmed up. The color was obviously wrong but I suspected that there was still some hope. It was very pink but not deep red or blue.

        So, the first treatment procedure was to run the tube for awhile to see if that alone would result in at least some recovery. And, each time the tube was powered-on, the discharge color showed some definite improvement, though after running for a few minutes, it would tend to return to its former condition.

        However, after a total of about 8 hours of a 6.5 mA IV drip over several days, a few green photons started appearing for about 30 seconds shortly after powering up. During that time, I gently pressed on the cathode-end mirror to determine if alignment could be improved. It seemed fairly decent though I would tweak it later. Successive power cycles (with a cool-down period) appeared to result in somewhat more green output and for a longer time.

        I then applied several radiation treatments to the getter from my solar heater. I just set up the tube so a part of the getter ring was at the focus of the solar heater (about 1/4" focal spot from a 7"x10" or so Fresnel lens) and let it bake for a few minutes. Probably a total of 1/2 hour in a half dozen sessions of that around noon on a cloudless day, powering up in between to check condition. :) After a few of those, there is no further improvement. That is the basically the same thing I did to a contaminated red HeNe tube over a year ago (see the section: Repairing the Northern Lights Tube) but that was hard-sealed (it is still doing fine).

        I tweaked the alignment of the HR (cathode-end) mirror using the three-screw adjuster that was already there. That increased the output by about 10 percent. The adjustments were not super critical (as would be the case with a tube having marginal gain) and were repeatable. The beam is TEM00 and nice and circular.

        Following the solar treatments, there was a sustained green output between 0.4 mW (when first powered) dropping to about 0.32 mW steady state with very little power variation due to mode sweeping. The operating voltage has stabilized, probably close to its spec'd value, changing only very slightly during warmup. The discharge color now looks normal for a red HeNe tube, maybe a bit more saturated red than usual but it is stable and hasn't changed with additional getter treatments. (The color may be normal. If I recall correctly, the discharge color of my green 1-B HeNe laser tube looks similar.) The color in the expanded section of the bore near the anode is close to a normal orange. I suppose with the Epoxy seal, helium has likely leaked out in addition to air leaking in. Low helium pressure might explain both the discharge color (if it's really incorrect) and somewhat low output (a modern 05-LGR-170 tube is rated at 0.8 mW but see below). After running for a few more hours, the power has stabilized around 0.4 mW with little change during warmup. This probably means that additional benefits from doing anything with the getter will be negligible.

        However, I'm going to run the tube for a few more days. The power still appears to be climbing - very slowly but steadily. Though at this rate, it may be a few years before the tube achieves rated power. If that doesn't help after a few days, I will perform a helium soak. It should be a simple matter to enclose the anode-end only in a plastic bag filled with helium and even be able to power the tube to check progress. There is little risk of overfilling doing this for a couple weeks (the manufacturing date of the laser is 1988 and this is almost certainly the original tube) - 1 day for every year of age. For now, I have reinstalled the tube in the laser case using the set-screws but no RTV Silicone so it can be removed if needed. According to my contact at Melles Griot, it's possible that this laser had a minimum power spec of only 0.2 mW. Mr. GreNe is already doing twice that. :)

        Followup: After a year or so of occasionally turning the laser on for a few minutes to check that it still worked, I must have missed a couple months with the result that there was no green output at all. However, letting it run for a several hours restored it to nearly the same health, without needing any getter treatments. So, indeed the recommendation to run a soft-seal HeNe laser tube periodically is confirmed!



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