🚀 go-pugleaf

In article <820@tekred.UUCP> billr@tekred.UUCP (Bill Randle) writes:
>Actually, you can do some modulation without using a variable opaqueness
>device in front of the beam.  There was an article in Popular
>Electronics about 16 years ago on how to build a modulator-demodulator

I built the laser and several modulators from these articles; I believe
the article on laser communication was in June 1970 PE.  The modulator
used a pentode (I used a 6L6) in series with the tube as a variable
current source.  Most laser tubes can be modulated at least slightly
this way, but the percentage of modulation will depend on the laser tube;
Metrologic's standard tubes would modulate ~50% before turning off
(the arc inside the tube goes out), but Metrologic built special
"communications" lasers that would go all the way to zero output before
the arc dropped.  I doubt that a transistor modulator would work well;
the laser supply is ~1500 volts, and the starting spike is ~5 KV.  
Perhaps if you used a high voltage transistor (750 V) with an 
overvoltage bypass (e.g. a string of neon tubes or high voltage Zeners)...

Probably the most common "external" modulators are Kerr, Pockels, and
Faraday cells, using an optically active material (mononitrobenzene, 
KDP) in, repectively, a transverse electric, longitudinal electric, and
longitudinal magnetic field.  For details, see physics/quantum optics
texts, particularly 60's vintage.  None are trivial to make.

For quick and dirty modulation (with poor linearity) consider a
shutter blade attached to a loudspeaker voice coil...

For long range communications, optics are necessary at both ends.  
At the transmitter, a small telescope of good optical quality will reduce
the beam divergence from ~1 mrad (several meters at 4 mi. range) to
a tenth or less of that, PROVIDED you have a good way to focus and align it
(an observer at the far end with a CB??  but then why do you need the
laser to communicate? :-))  Receiving optics can be lower quality, but
should be fairly large (foot-square fresnel lenses are about $10).
For daylight operation, a narrow band interference filter (centered at
633 nanometers) will block
everything but the laser light; Edmund used to sell these fairly cheaply.
(Note that such filters will not work with converging light; you need
additional optics to make a parallel beam again).

Ordinary photodiodes (even "solar cells") will work for audio communications,
but are not especially sensitive. Use them as current sources driving
a current-to-voltage converting op-amp.  PIN diodes and photomultipliers
are better, but fairly expensive.

Back when I did this stuff, I used to get on the ham bands and tell
people that I'd been working "real VHF -- the 633 nanometer band" :-)

		Jordin Kare   jtk@mordor.UUCP