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From: adp@hp-sdd.UUCP
Newsgroups: sci.electronics
Subject: Re: laser communications
Message-ID: <557@hp-sdd.HP.COM>
Date: Tue, 28-Oct-86 13:16:27 EST
Article-I.D.: hp-sdd.557
Posted: Tue Oct 28 13:16:27 1986
Date-Received: Wed, 29-Oct-86 06:13:29 EST
References: <2200@orca.TEK.COM>
Reply-To: adp@hp-sdd.UUCP (Tony Parkhurst)
Distribution: na
Organization: Hewlett Packard, San Diego
Lines: 43
Keywords: laser

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.  That is, the power supply starts it with about 10kv then it runs
at about 2 or 3 kv at about 6 milliamps.  Trying to modulate the beam by
means of the power supply would likely keep turning off the beam.
The power supply is usually highly regulated so it would be difficult
anyway.  As a matter of fact, the beams intensity is highest at the point
closest to turning off the beam.  That is, if you increase the running
voltage, the beam will dim.  Not a proper way to modulate it.

>	In this case tube is 
>	powered by a 12 volt DC/DC converter. If not then what other 
>	"simple" ways of modulating can be used? 

	"simple" this isn't.  The idea your looking for is something to
put in front of the beam whose opaqueness is controllable.  These devices
are available, but I dont know the cost involved.

>	What about detectors? I would like to be able to detect the beam
>	from a distance of several miles. What kind of detectors work well
>	in this application? Is it possble with a 4 mWatt laser? Can a lens
>	help if the intensity is too weak?

	This can be a toughy.  Any general purpose sensor would have
to be filtered, which will dim the signal.  It helps to have some
directional input like a black tube.  I think you will find a 4 mWatt
HeNe to be too weak for such a purpose, you really want a 10 mWatt to
do any real work with:-).  You might check into using a laser diode
instead of a HeNe.  The diodes can be modulated, the beam is IR insead
of Red, and they should be pretty inexpensive.  I don't think you 
can use one for a several mile transmission tho (unless you use fiber optics:-)

-- 
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*      Tony Parkhurst -- {hplabs|sdcsvax|ncr-sd|hpfcla|ihnp4}!hp-sdd!adp      *
*                        OR   hp-sdd!adp@nosc.arpa                            *
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