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Path: utzoo!utgpu!water!watnot!watcgl!onfcanim!dave
From: dave@onfcanim.UUCP
Newsgroups: sci.electronics,sci.physics
Subject: Re: Laser eavesdropping
Message-ID: <15260@onfcanim.UUCP>
Date: Sat, 28-Mar-87 22:03:39 EST
Article-I.D.: onfcanim.15260
Posted: Sat Mar 28 22:03:39 1987
Date-Received: Sun, 29-Mar-87 03:38:35 EST
References: <499@sw1e.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 24
Keywords: modulation
Xref: utgpu sci.electronics:429 sci.physics:980

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.  How *could* such a signal
>be encoded?

One suggestion: The coherence length of a stabilized laser can be several
hundred meters.  Thus, you can send a beam 100 meters to a reflector,
have it return, mix it with a sample of the beam coming from the
laser, and get interference fringes.  The movement of these fringes
measures the change in distance between the interferometer assembly and
the reflector to a resolution of less than a wavelength of light.
Of course, the interferometer has to be on a very vibration-free
mount for this to work.

A much simpler possibility: if the mirror is mounted near the
edge of the pane of glass, it will rotate as well as translate as
the window flexes.  The rotation will change the angle of the
reflected beam, and this angular change may be measurable.
However, the window will flex far more due to wind, air pressure changes
when doors are opened, etc, probably making the desired signal
hard to extract.

I haven't tried any of these.