Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rutgers!sri-spam!mordor!jtk From: jtk@mordor.ARPA (Jordan Kare) Newsgroups: sci.electronics Subject: Re: laser communications Message-ID: <16409@mordor.ARPA> Date: Thu, 30-Oct-86 16:07:57 EST Article-I.D.: mordor.16409 Posted: Thu Oct 30 16:07:57 1986 Date-Received: Fri, 31-Oct-86 02:07:47 EST References: <2200@orca.TEK.COM> <557@hp-sdd.HP.COM> <820@tekred.UUCP> Reply-To: jtk@mordor.UUCP (Jordan Kare) Distribution: na Organization: S-1 Project, LLNL Lines: 49 Keywords: laser, modulators Summary: Not hard 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