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From: jones@pyrite.cs.uiowa.edu (Douglas W. Jones,201H MLH,3193350740,3193382879)
Newsgroups: sci.electronics,sci.physics
Subject: Re: Lasers for communication
Message-ID: <2312@ns-mx.uiowa.edu>
Date: 12 Sep 90 20:46:30 GMT
References: <153DAVE@ORION>
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From article <153DAVE@ORION>, by DAVE@ORION.BITNET:
> I saw my high school physis teacher put an LED into the earphone jack
> of a simp > le radio shack radio, and about 4 feet away, put a
> phototransistor (?) into the microphone jack of a small amplifier, the
> sound was carried by the pulsing LED

Many commercially available lasers have a modulation input.  All you do is
hook the audio (or RF, for that matter) signal you want to transmit to the
modulation input, then turn on the laser and aim it at an appropriate
photodiode on the opposite rooftop, and you've got a roof-to-root
communications system.

Problem 1) Impedance matching at the laser modulation input.  If it's a BNC
connector, it probably wants a 50 ohm impedance line, expecting a fairly
low voltage signal.  Most audio lines, such as the ones from your tape
deck to your amplifier, are about 600 ohm impedance, with correspondingly
higher voltages.  At audio frequencies, a direct connection between the
two may be OK, if you turn the volume way down.  At RF frequencies, you may
need some kind of matching transformer to get things to work right.

Problem 2) Beam spread.  Over a long distance, it can pay off to get a
telescope for the transmitter.  Shine the laser in the eyepiece, and adjust
the focus so the beam focuses to a reasonably small spot at the receiver.
(You'll have to do this at night).  Among other things, this makes the signal
far more resistant to raindrops, since one drop will only block a fraction of
the beam when you spread it to the size of a telescope aperature.

Problem 3) Noise.  Ideally, the only light falling on the receiving
photodiode should be from the transmitting laser.  For short haul signals,
a simple chunk of pipe may provide enough of a shield, but for long
haul transmission, use another telescope.  With the laser off, focus the
receiving telescope on the laser's transmitting telescope, then put the
photodiode where your eye was and turn on the laser.

If your telescopes are reflecting telescopes, you should not that the
result is the optical analog of the parabolic reflectors traditionally
used in microwave transmitters and receivers.

					Doug Jones
					jones@herky.cs.uiowa.edu