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From: dje@datacube.UUCP
Newsgroups: sci.electronics
Subject: Re: Laser communications
Message-ID: <2900006@datacube>
Date: Mon, 10-Nov-86 10:40:00 EST
Article-I.D.: datacube.2900006
Posted: Mon Nov 10 10:40:00 1986
Date-Received: Tue, 11-Nov-86 20:10:04 EST
References: <2201@orca.TEK.COM>
Lines: 45
Nf-ID: #R:orca.TEK.COM:-220100:datacube:2900006:000:2160
Nf-From: datacube.UUCP!dje    Nov 10 10:40:00 1986


I did most of this  in 1972  for the  Mass state  High school science
fair.   Beyond audio  modulation, I  used two  carriers and amplitude
modulation to send two audio signals across a He-Ne laser.  

For  a  modulator,  use  current modulation.   The  current should be
maintained between the working  current of  the laser  (say 10mA) and
0.1 Times that.  If your current drops to 0,  the laser extinguishes.
Since most convenient input  signals are  voltages, and  a current is
needed, a  transconductance amp  (Voltage to  current) is  best.  The
voltage is around 600V, and the power is around 10mA * 600V or 6W.  I
used 600V worth of NE-2 neon lamps to prevent the  plate voltage from
exceeding  600V  (especially  during  laser  firing).    At the time,
Hi-Voltage transistors were hard to get, so  I used  a pentode vacuum
tube.  Other  than the  need for  a 200V  screen grid  supply, it was
perfect.  Grid bias was provided with a resistor in the cathode.  The
circuit was from a late sixties or early seventies Popular
Electronics.

                                            600V of Neon lamps
+1600VDC Supply <-----|     [_______________()-()-()-()------> Gnd
                    He-Ne Laser      |
                                   -----  Plate
                     0.1uF          ----------------> +200V Screen grid
  Modulation In <----| |---------------   Control Grid
                          |         ___   Cathode
                          \         | |
                    1MOhm /         /
                          \         \  5K Ohm Pot
                          |         /
                          --------------> GND

Nowadays, 600V 10W power transistors are widely available, but
they're much less forgiving to overvoltage, etc than tubes.

For a reciever, I used  a photo-transistor.   Biasing  to correct for
ambient light is important. You only want the AC component.

I Didn't go for distance, so I didn't use any optics. Just a dark
shield around the detector.
				Dave Erickson
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Datacube Inc. 4 Dearborn Rd. Peabody, Ma 01960 	617-535-6644
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