Sam's Laser FAQ, Copyright © 1994-2004,
Samuel M. Goldwasser, All Rights Reserved.
I may be contacted via the
Sci.Electronics.Repair FAQ
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Laser Experiments and Projects
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Laser Experiments and Projects Introduction
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.
The material in this chapter has been derived from various sources including:
- A tutorial from Electro Optics Associates, whoever they were. My
apologies if they evolved into one of the major laser manufacturers! This
booklet was apparently provided along with an early external mirror HeNe
laser (LAS101) for educational purposes. This had a wide bore tube running
on only 300 VDC with a resonator length of about 14 inches. The mirrors
could be interchanged to produce a multimode or TEM00 beam. While this is
from a booklet dated 1965 (!!), all the experiments are still quite valid.
- The CORD Lasers and
Electro-Optics Courses. See the section:
On-Line Introduction to
Lasers for the current status and on-line links to these courses.
- Various miscellaneous places including USENET newsgroup postings and
my hazy recollection of fun things that I have done in the past.
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.
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:
- Geometric optics (lenses, mirrors).
- Diffraction and dispersion (diffractions gratings, prisms).
- Comparison of normal and coherent light (speckle, interference,
monochromicity).
- Common lasers: diode, HeNe, pulsed Nd:YAG, DPSS doubled YAG (green laser
pointer).
- Open cavity helium-neon laser (access to internal photon flux, mirror
alignment, modes, variable angle window).
Applications:
- Interferometer (position sensing).
- Holography (readout and visualization of displacement/vibration).
- Laser display and light shows (audio and PC generated graphics).
- Free space and fiber optic laser communications (audio and video).
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
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
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 in a link from:
Skywise's
Laser Picture Gallery.
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
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. One-Brewster tubes, heads, a complete kit with
power supply, as well as a mirror assortment, are available on
Sam's Classified Page.
See: Perpendicular Uncoated Windows in a Low
Gain Laser.
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:
- How does the mode structure vary with L?
- How does the mode structure vary with OC curvature?
- How does the mode structure vary with OC alignment?
- How is the sharpness of the mode structure affected by OC quality (e.g.,
comparing a laser mirror to a barcode scanner mirror)?
- How does the divergence vary with L?
- Can a positive lens always focus the multimode beam to a small spot?
- How does the profile of the intra-cavity beam (mode volume) change with
L?
- What effect does a variable stop (iris), knife edge, or arbitrary pattern
placed inside the cavity have on the output beam? How is this affected by
position? What about at the Brewster window or OC surface?
- How does output power vary with L (assuming the OC alignment is tweaked
for maximum power at each location)?
- Why does that absolutely fascinating thing happen?
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Fourier Optics
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Advanced Laser Experiments
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.
I offer sets of mirrors suitable for a confocal HeNe laser mode display
SFPI. See Sam's Classified
Page under "HeNe Laser Kits".
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.
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.
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:
- HeNe laser tube: Suitable choices are tubes from Hewlett Packard
model 5501 or 5517 laser heads (with or without output optics but including
the original ring magnets), or home-built versions of these using common
non-polarized 5 or 6 inch barcode scanner HeNe laser tubes with powerful
ring magnets polarized N-S along their axis.
- HeNe laser power supply: This can be the original one that went
with the laser but it is not critical as long as the tube current is
reasonably close to its specification. For these small tubes, 3.5 mA
is usually adequate.
- Polarizer: Any material or optic that will act as a polarizer
at the red 632.8 nm HeNe laser wavelength. The usual choice would be a
piece of a linear polarizer sheet.
- Photodetector: If the laser tube came from a commercial laser
head, the best choice is the photodiode and preamp board that was also
present there as it will have an AGC circuit and comparator to cleanup
the detected signal. Otherwise, any silicon photodiode can be used with
reverse bias to improve the frequency response. If there is no AGC,
some means of adjusting the beam intensity incident on the photodiode
will be required. This can be as simple as moving the beam position so
only a portion of it hits the detector.
- Oscilloscope and/or frequency counter: Anything with a bandwidth
of at least 5 MHz will suffice.
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.
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.
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
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Laser Surveillance
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. :)
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-----+--+---------+--------------------+
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
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Laser Display
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Laser Games
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.