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From: levy@ttrdc.UUCP
Newsgroups: sci.electronics,sci.physics
Subject: Re: Laser eavesdropping
Message-ID: <1639@ttrdc.UUCP>
Date: Sun, 29-Mar-87 04:58:31 EST
Article-I.D.: ttrdc.1639
Posted: Sun Mar 29 04:58:31 1987
Date-Received: Tue, 31-Mar-87 06:04:29 EST
References: <499@sw1e.UUCP> <1505@husc6.UUCP>
Organization: AT&T, Skokie, IL
Lines: 41
Keywords: modulation
Xref: utgpu sci.electronics:435 sci.physics:996

In article <1505@husc6.UUCP>, greg@endor.harvard.edu (Greg) writes:
<In article <499@sw1e.UUCP> uusgta@sw1e.UUCP (uusgta) writes:
<<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.
<My understanding is that a window is reflective enough by itself that you don't
<need to put anything on it in order for this device to work.  This way you can
<bug a room without ever being within 100 feet of it.
<<How *could* such a signal
<<be encoded?
<Probably the reflected laser light is combined with the laser source, and the
<interference pattern is measured.  The coherence length of the laser must be
<much greater than the distance from the laser to the window.

If a reflective dot were not used, the laser would almost certainly need to
be one with an invisible wavelength (infrared or ultraviolet).  Otherwise,
there'd be a telltale spot of light on any solid object in the beam's path
on the other side of the window.  (Or the laser beam could be reflected off
at a sufficiently small angle to the plane of the glass so as not to penetrate,
but that would require the sensor and the laser to be at different sites.)

Come to think of it, sunlight, moonlight, or already-present artificial
sources of direct light might even be used with an appropriately placed sensor.
In this case interference patterns couldn't be used because the light isn't
laser light.  But maybe instead some kind of aperture could be used in front
of the detector, such that as the edge of the reflected light pattern
moved back and forth (presuming some angular displacement of the window),
light would shine through less or more of the aperture and thus produce
a greater or lesser instantaneous output from the detector.  (This would
be like "slope detection" of FM on an AM receiver, with physical displacement
corresponding to frequency).

OBLIGATORY STEENKING DISCLAIMER:
Don't try this at home on your big sister's window, kiddies.  She'll smack
you sillier than you already are, even if it IS for your junior high school
science fair project....
-- 
|------------dan levy------------|  Path: ..!{akgua,homxb,ihnp4,ltuxa,mvuxa,
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