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From: res@sdiris1.UUCP
Newsgroups: sci.electronics,sci.physics
Subject: Re: Laser eavesdropping
Message-ID: <560@sdiris1.UUCP>
Date: Tue, 31-Mar-87 02:45:04 EST
Article-I.D.: sdiris1.560
Posted: Tue Mar 31 02:45:04 1987
Date-Received: Sat, 28-Mar-87 11:23:25 EST
References: <499@sw1e.UUCP> <1505@husc6.UUCP>
Organization: Control Data Corp.(CIM), San Diego
Lines: 27
Keywords: modulation
Xref: utgpu sci.electronics:425 sci.physics:972
Summary: laser modulation

> >I was thinking about the commonly mentioned electronic snooping technique
> >of pasting a reflective dot to a window and listening to conversations in the
> >room by bouncing a laser off this vibrating mirror.
> >How *could* such a signal
> >be encoded?
I have a constuction project for this somewhere in my pile of spy gear
manuals, but until I locate it, my recall is that yes, to get a usable
bug at a real-world distance, you must put a reflective dot on the window.
(At short range, you can do without)

The modulation is simple, good old Amplitude modulation.. that is, a
continuous laser, which will be amplitude modulated by the vibration of
the window. (you can do this with a mirror connected to a voice box and
a reflected sunbeam, with a photodetector for recovering the signal)

The real units use telescopes for beam direction (in both directions)
and usually use photo-multiplier tubes for sensitivity.  Range on a real
world (expensive) unit is about a mile, maximum (with the dot) or a hundred
yards maybe without the dot.

Note that ambient traffic noise will obscure the signal, so there is really
not much use of these gadgets in normal city areas.


-- 
Skip Sanders :  sdcsvax!jack!man!sdiris1!res
Phone : 619-273-8725 (evenings)