🚀 go-pugleaf

++
In article <2200@orca.TEK.COM> philb@orca.TEK.COM (Phil Biehl) writes:
> >	I have a small Helium-Neon laser that I would like to attempt to
> >	use for voice communication and desire a little help from those
> >	of you who have done this. Is it possible to modulate the laser
> >	simply by modulating the input power supply? 
> 
> 	No.  A HeNe laser tube behaves in much the same way as a flourescent
> light....

The trouble with this experiment is that optical communications technology
is geared towared infrared diode lasers and fiber optics.  It's less
practical to use red light through air, so there's not much you can buy to
do it.

However, since you only want a few kHz bandwidth, you can probably
do it with even crude aparatus.  You need an external modulator.
There are electronic devices called Kerr cells that change their
opacity with an input voltage but, these are expensive and probably
don't work at the right wavelength. Here's an idea that might be fun
to try:

Modulate the beam mechanically using the vibrations of the
sound.  First you need something that will change the amount of light
let through according to a slight movement, and it must be linear
(or at least it helps).  Focus the output of the laser through
a lens, and beyond the focal point put another lens which collimates
the light into a wide beam.  Adjust the lenses to give you a slightly
narrowing beam so that it focuses on the remote detector (more or less).
Now, in the wide beam put your modulator.  One way to make this (I have
no idea if it will work!) might be to take two pieces of window screening,
paint them black to avoid reflections (always be careful about hitting
shinny things with laser light).  Fix one piece of screen in the beam,
and have the other one attached to a loudspeaker, and also in the beam.

               lens 2  screens
      lens 1     |______#_#__________________________
         |    _/ |      # #                          \_______________
------ __|  _/   |      # #
laser  __|><_    |      # #                     beam converges on detector
------   |   \_  |      # #                            ______________
         |     \ |______#_#___________________________/
                 |      # #
               .........J #
               :     _____#_____
               :    (_____X_____)
               :     \ speaker /
              ---     ---------
If the vibration of the speaker is equal to the spacing between the
metal filaments in the screen, and the space between the filaments
is about equal to the width of the filament, you will have linear
modulation of the light with an on/off ratio of infinity - that
is, when the second screen is fully translated in one direction,
the holes in the second screen line up with the filaments of the
first and vice-versa, and when it is fully translated in the other
direction, the spaces line up with the spaces.  If the spaces are
too big, paint it again.  Probably several coats of very thin paint
can give you the right effect.

You could possibly construct an electromagnet to replace the
speaker if the vibration is too small for the amount of noise
you make (you want your receiver to get the information through
the light, not because he can hear you blasting your voice all
over the neighborhood).

One nice thing about voice is that you can understand it even
if you only get a piece of the frequency spectrum.  So even if
your freq response is lousy you can still communicate.

As far as a detector goes, you can filter the light through
a red camera filter (to get rid of interfering light) and
make a long tube to get directionality.  You'll have to experiment
to see how far away you can get with whatever detector you find.
I don't know what's good for red light.

Have fun,
-Mark
mwg@bellcore.arpa
allegra!bellcore!mwg