Sam's Laser FAQ, Copyright © 1994-2004, Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the Sci.Electronics.Repair FAQ Email Links Page.

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    Laser Experiments and Projects

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    Laser Experiments and Projects Introduction

    Scope of This Chapter

    This chapter provides a variety of suggestions for experiments and projects using lasers ranging from trivial to quite advanced. Some utilize the optical properties of the laser beam like its ability to be well collimated or highly focused while other depend on the unique coherence and monochromicity of the laser light itself. And still others take advantage of the ability to modify or control the lasing process via intra-cavity optical components, magnetic fields, or other mechanisms.

    Currently, they are just suggestions. (If you can't wait, there are also some links to Web sites with educational laser projects below.) Eventually, additional details of the setup and required supplies will be added. Where the particular topic is already discussed elsewhere in Sam's Laser FAQ, a link to that section will be provided. However, in most cases, at least some of the details will be left as an exercise for the student. What fun or challenge would it be if we told you everything? After all, besides its educational value, hands-on experience should indeed be both fun and challenging! However, where more information is available in this document, links are provided.

    A 1 to 5 mW internal mirror helium-neon laser will be suitable for most of the basic experiments (though a somewhat higher power one would be better for those like holography). It should be possible to procure such a laser for under $50, possibly under $25 depending on your resourcefulness and scrounging abilities.

    Some experiments may require a polarized laser but for most, any type will do, even a better quality (one with an adjustable focusing lens) laser pointer - and those are practically given away in cereal boxes these days. :) Where access to the laser cavity is required, an external mirror HeNe or Ar/Kr ion laser will be needed. A one-Brewster HeNe laser setup can be put together quite inexpensively (probably under $100) using a surplus one-Brewster HeNe tube and power supply, the OC mirror from a deceased HeNe laser, and some scrap materials available in any well equipped junk box. Of course, if you have access to a nice lab laser, that would be fine as well but probably not nearly as much fun or as rewarding compared building one (at least partially) yourself. :)

    Alternatives like bare laser diodes and appropriate drive circuitry may be more desirable for projects like laser communications where modulation is required. And, other color lasers (than the boring red HeNe or laser pointer) will be desirable for laser display.

    See the chapter: Laser and Parts Sources. I also have a variety of suitable lasers and components available on Sam's Classified Page.

    Laser Experiments and Projects Acknowledgements

    The material in this chapter has been derived from various sources including:

    Laser Experiments Safety

    All of these experiments can be performed with a fully enclosed 1 mW HeNe laser (or laser head and power supply) which poses minimal risk to vision and no shock hazard from even gross carelessless (though not totally, perhaps, from deliberate abuse). However, some, like those dealing with holography, could benefit from a 5 mW or larger laser. Higher power lasers, especially those above 5 mW, need to be treated with great respect as even momentary eye exposure can cause permanent damage to vision.

    In addition, those experiments requiring access to the interior of the resonator of an external mirror laser may expose the user to potentially lethal voltages in the vicinity. If possible, any exposed high voltage terminals should be well insulated or blocked from accidental access. And, where all you have is an exposed HeNe laser tube and separate power supply, building all this into a safe enclosure is highly recommended.

    Read the chapter: Laser Safety in its entirety and follow its guidelines - particularly in regards to the safety of others who may not be as aware as you in dealing with your equipment.

    Suggested Lasers and Optics Science Museum Interactive Exhibits

    Here is a subset of the experiments listed later in this chapter that might be appropriate for a hands-on exhibit in a science museum that still does science (as opposed to multimedia marketing!):

    Basic principles:

    Applications:

    Other Laser Experiments and Projects Resources

    Here are some optics sites (not laser specific) oriented towards kids: Also see the chapter: Laser Information Resources.



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    Basic Experiments with Lasers

    Beam Characteristics

    Geometric Optics

    The Series Mirror Paradox

    When neutral density filters are placed one after the other, their ND numbers (-log attenuation) add. So two ND1 filters (T = .1) in series results in a equivalent ND2 filter (T = .01).

    Now, what happens if multiple dielectric mirrors are placed in series? Under certain condition, more light will get through than might be expected. For example, using the same example as above, if T = .1 for both mirrors, the resulting output may actually be as high as for an equivalent mirror with T = .05 (rather than T = .01). Why? Under what conditions will this happen? How does the T factor of each mirror affect this behavior? What other factors are important?



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    Diffraction and Interference

    Single Edge Diffraction

    Double Slit Diffraction

    This just requires a pair of narrow closely spaced slits, a laser pointer or HeNe laser, and a screen or white card onto which to project the resulting diffraction pattern. The main problem is in getting the slits to be narrow and close enough together to obtain a nice wide pattern.

    (From: Skywise.)

    I once made a two slit experiment using a piece of glass, two razor blades, some tape, and some water based acrylic paint.

    First I painted the glass with the paint. I chose a dark green color which absorbed my HeNe light pretty well. To make a smooth single layer of paint I put two parallel strips of Scotch tape on the glass about 1/2 inch apart. Then I placed a drop of paint towards one end of the channel formed by the two pieces of tape. With a razor blade resting on the tape, I dragged the blade along the channel thus spreading a nice thin even coating of paint along the glass. With practice on how much pressure to apply I was able to get a very good strip of paint that wasn't too thick or thin.

    While the paint dried I taped two razor blades stacked together. To get the razor edges closer together than the thickness of the blades I used a small piece of folded paper at the back edge of the blades so as to fulcrum the sharp edges of the blades closer together.

    Once the paint was dried I quickly dragged the two points on the corner of the joined blades across the paint strip thus creating two parallel slits.

    With practice on applying the paint, adjusting the gap of the blades, amount of pressure when scoring the paint, etc... I was able to successfully make two slits close enough that a raw beam from my 5 mW HeNe pointed at the two closely spaced slits caused diffraction in the far field.

    It was quite fun and was very useful for demonstrating the wave nature of light.

    More information with photos can be found at: Skywise's Laser Picture Gallery - Two Slit Experiment.

    Interference Involving Multiple Sources

    Foucult Experiment

    Schlieren Optics

    Complex Diffraction and Interference Patterns

    Determining Laser Wavelength Using a Ruler

    Spectral Lines in HeNe Laser Tube Discharge Versus Output Wavelength

    The objective here would be relate the visible wavelengths in the discharge inside the bore to the actual output wavelengths. For example, while red at 632.8 nm is the strongest of the visible HeNe wavelengths, it is a relatively weak line int he discharge. A long HeNe laser with external interchangeable or multi-line optics, or a tunable (e.g., PMS/REO) HeNe laser would be best for this experiment.

    See the sections starting with: Viewing Spectral Lines in Discharge, Other Colors in Output.



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    Experiments Inside the Laser Cavity

    The External Mirror Laser

    These require access to a laser with at least one external mirror. The usual choice would be a modest size lab type HeNe laser. Unfortunately, these aren't the sort of thing one typically has at home. And, even if you find one, it may not be convenient (or permitted if it isn't yours!) to gain access to the cavity. However, it is possible to put together something that is every bit as good for minimal cost using a HeNe laser head with an internal HR mirror and Brewster window at the other end. A HeNe laser power supply and easy to construct mirror mount completes the assembly which provides full access to the inside of the cavity between the Brewster window and external mirror. One-Brewster HeNe tubes and laser heads are available on the surplus market but you may have to ask. As an example of such a laser head, see the section: A One-Brewster HeNe Laser Tube. A complete laser using this laser head is described in the section: Sam's Instant External Mirror Laser Using a One Brewster HeNe Tube.

    Mirrors

    Brewster Windows

    Transverse Modes

    Etalons

    Perpendicular Window Trick

    See: Perpendicular Uncoated Windows in a Low Gain Laser.

    Resonator with Intermediate Mirror

    Scattering

    Particle Counting

    Optical Tweezers

    Single Pass Gain

    Experiments With the Mirror/Optics Test Jig Using One Brewster HeNe Laser Tube

    The setup described in the section: Mirror/Optics Test Jig Using One-Brewster HeNe Laser Tube may be used to perform a variety of experiments requiring access to the inside of an adjustable length resonator using various mirrors or other optics. With the commonly available one-Brewster HeNe laser tubes like the Melles Griot 05-LHB-570, either multimode or single mode operation is possible depending on the external configuration.

    Here are some suggested experiments and questions to ponder using this rig:



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    Fourier Optics

    Fourier Transform

    Spatial Harmonics

    Spatial Filtering



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    Advanced Laser Experiments

    Michelson Interferometer

    High Resolution Interferometer

    Scanning Fabry-Perot Interferometer

    A Scanning Fabry-Perot Interferometer (SFPI) consists of a pair of partially reflective mirrors. The laser under test (LUT) is input to one end and a photosensor is mounted beyond the other end. The coarse spacing and alignment of the mirrors can be adjusted by micrometers. The axial position of one of the mirrors can be varied very slightly by a linear PieZo Transducer (PZT). By driving the PZT with a triangle waveform and watching the response of the photosensor on an oscilloscope, the longitudinal modes of the LUT can be displayed in real time. In essence, the comb response of the SFPI is used as a tunable filter to analyze the fine detail of the optical spectrum of the LUT. As long as the FSR (c/2*L) of the SFPI is larger than the FSR of the LUT (i.e., the SFPI cavity is shorter than the LUT cavity), the mode display will be unambiguous.

    Assuming a function generator and oscilloscope are available, it is possible to build an SFPI that demonstrates basic principles for next to nothing, or one that rivals the performance of commercial instruments costing many thousands of dollars for less than $100. See the sections starting with Sam's $1.00 Scanning Fabry-Perot Interferometer. Even this very simple SFPI using salvaged mirrors will easily resolve the longitudinal modes of a HeNe laser.

    Waste Beam Interferometer

    Monitor the leakage (waste beam) from the HR of a common HeNe laser tube to detect sub-wavelength changes in the distance from the OC to an external mirror.

    The Laser Oscillator

    Holography

    Other Lasing Wavelengths from HeNe Laser Tubes

    By adding an external mirror or grating to a conventional internal mirror red or other color HeNe laser tube, it is usually possible to get anywhere from 1 to over a dozen other lasing wavelengths to appear. With am aluminized or dielectric mirror next to the OC, even a 1 mW red (632.8 nm) tube will probably give 1 or 2 additional red lines. With a 3 to 5 mW tube, 4 or 5, or even more may be produced. Some of these are not normal HeNe laser lines and their existence is not widely known. In fact, being able to do this overall experiment isn't something that's widely known. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.

    Demonstration of Zeeman Splitting

    It is possible to show the Zeeman effect with a relatively simple setup. All that is needed is a short HeNe laser tube (5 to 6 inches is probably optimal) and power supply, a ring magnet into which the HeNe laser tube can be placed producing an axial magnetic field (or electromagnetic solenoid), a photodetector with a response to a couple of MHz, and an oscilloscope. See the section: Two Frequency HeNe Lasers Based on Zeeman Splitting for more information.

    Here is a summary of the basic procedure. The following components are required:

    Set up the components in a reasonably stable manner. The polarizer just needs to be in the beam - its orientation doesn't matter since it's simply converting circular to linear polarization.

    Using a scope (preferably) or frequency counter, look for a beat frequency from the detector. The HP 5501 tube is very stable - its frequency will only vary by a few percent over several minutes. The frequency of the HP 5517 tube varies quite widely in a periodic manner as mode cycling takes place due to heating. The beat may disappear totally during part of the cycle. This behavior is very similar to that of the home-built version.

    Adding some means of cavity length stabilization would be the next step.

    Experiments can also be performed without the tube inside the ring magnets by trying various positions and orientations of external magnets. It may even be interesting to put the output through an audio amp and speaker as the beat frequency with a smaller magnetic field will cover the audio range.

    Beat Frequencies Due to Longitudinal Modes

    Using a 5 mW or larger TEM00 polarized HeNe laser and high speed silicon photodetector, it is possible to monitor the difference frequencies resulting from longitudinal mode beating as well as the differences of the difference frequencies, which are non-zero due to mode pulling. See the sections starting with: Longitudinal Modes of Operation.

    For looking at the longitudinal mode beating, a photodetector and oscilloscope with a response beyond c/2L for the HeNe laser will be required. This would be 500 MHz for a 12 inch long tube (mirror to mirror). So a longer tube would be desirable both due to its lower beat frequencies and more modes. For the second order difference frequencies, the photodetector still has to be fast but the scope only needs to respond to 100 kHz or so.

    Beat Frequencies Due to Transverse Modes

    The setup is similar to that for longitudinal modes beating, above, except that a multimode (non-TEM00) HeNe laser is required. The response of both the photodetector and oscilloscope only needs to be a few MHz since transverse modes are quite close together in frequency. A lens may be needed to force overlap of the mode spots since they won't mix if falling on separate areas of the photodetector.



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    Laser Measurements

    Laser Rangefinder

    Laser Beacon

    Laser Level

    Laser Seismometer



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    Laser Surveillance

    Laser Burglar Alarm

    The simplest type of system would use the beam from low power laser (e.g., red laser pointer, HeNe laser, or IR diode laser module) sent around the perimeter of a building or in criss-cross fashion within an area to be protected. It is best to use front surface mirrors for this but for a reasonable number of bouces (say, less than a couple dozen), ordinary rear surface mirrors will work just fine. However, I can usually scavange front surface mirror bits while taking walks along highways where fender-benders occur frequently. Automotive side-view mirrors are actually quite decent. :)

    Simple Beam Break Detector

    The circuit below will activate a relay (K1) when dark. It will easily detect a laser pointer after many bounces from mediocre mirrors, a flashlight, 100 W bulb at several feet without a lens, etc. All components are probably available from Radio Shack, certainly from DigiKey or Mouser. The only not totally common parts are PD1 and K1. I used a Photonic Devices, Inc. part number PDB-V107 (about $2 from Digikey) for PD1. This has a nice large active area of 17 mm but almost any silicon photodiode will work including those salvaged from computer mice and barcode scanners. K1 is a low current relay from Radio Shack but I don't know if it is still listed in their current catalog. There is nothing critical in this circuit.

                                                          o--- NC (Light)
                                                COM ---o/
                                                          o--- NO (No light)
           +6 V o-------+---------+---------------+----+
       (4 AA Cells)     |         |               |    |
                        |         \              _|_    )|| K1
                        /         / R3    1N4148 /_\    )|| 6V coil
                        \ R1      \ 1.5K          |     )|| 500 ohms
                        / 3.3K    /               |    |
                        \         |               +----+
                        |         |                    |
                      __|__       |      5.6K      B |/ C
         LIGHT ---->  _/_\_ PD1   +------/\/\--------|    Q2 2N3904
              Sensor    |         |                  |\ E
            Photodiode  |     B |/ C                   |
             PDB-V107   +-------|    Q1 2N3904         |
                        |       |\ E                   |
                        \         |                    |
                     +->/ R2      |                    |
                     |  \ 100K    |                    |
                     |  / Sens.   |                    |
                     |  |         |                    |
        Return o-----+--+---------+--------------------+
    

    Laser Listener

    The idea is to bounce a laser beam off of a window pane and detect vibrations from conversation or other sounds inside the room due to the minute vibrations of the glass. (This same approach can be used to build a laser microphone and this would be somewhat easier since the distances are much shorter and everything is within your control.)

    Basic experiements can be performed with just a laser pointer, solar cell, and audio amp. However, keep in mind that to really get any decent performance is not a trivial undertaking. Sound is likely to be distorted and noisy with contributions from both inside and outside. And just getting enough optical return off a window unless at precisely normal incidence will be a challenge in itself. Here is one link that appears to have rather detailed information: Laser Listening.



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    Laser Communications

    Modulation

    Detection

    Free Space Link



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    Laser Display

    Simple Deflection

    Laser Oscilloscope

    Laser Digital Clock

    Raster Scan

    Multicolor Merging



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    Laser Games

    Laser Maze

    Build a covered 2-D or 3-D (or higher if your Universe supports it) maze placing a fixed collimated laser (laser pointer, diode laser module, or HeNe laser aimed into the maze and planar mirrors at various locations on swivel mounts. The objective would be to adjust the mirrors so that the beam passed through the maze and exited at some predetermined location without removing the cover. Perhaps, peep holes could be placed at strategic locations to help. The maze need not be Cartesian. :)

    A typical front surface aluminized mirror reflects about 90 to 95 percent of the light so there can be quite a few bounces before the beam loses so much intensity as to be undetectable. However, the quality of the mirror is also important so as not to distort or scatter the beam. Sources for these mirrors include barcode scanners and laserprinters. Back surface mirrors are considerably worse than front surface mirrors. Dielectric mirrors coated for the specific laser wavelength are by far the best, some reflecting 99.999 percent of the light.



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    Sam's Laser FAQ, Copyright © 1994-2004, Samuel M. Goldwasser, All Rights Reserved.
    I may be contacted via the Sci.Electronics.Repair FAQ Email Links Page.