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From: coast@faraday.ECE.CMU.EDU (Douglas Coast)
Newsgroups: sci.electronics,sci.optics
Subject: Laser Listener - How does it work?
Keywords: Laser, vibration
Message-ID: <1991Mar11.163650.9794@fs7.ece.cmu.edu>
Date: 11 Mar 91 16:36:50 GMT
References: <1991Mar11.152746.19730@hobbit.gandalf.ca>
Sender: news@fs7.ece.cmu.edu (USENET News System)
Organization: Electrical and Computer Engineering, Carnegie Mellon
Lines: 18

In the Oct 89 issue of Radio Electronics, an article by Richard L. Pearson
entitled "Laser Listener" appeared.  This article describes the construction
of a laser "bugging" device which works by bouncing a low-power laser
beam off of a reflective surface (like a window) and using a phototransistor
to measure intensity of the reflected beam.  According to the article,
the reflected beam is amplitude modulated by vibrations of the reflecting
surface.

My question is: How does a vibrating reflector result in an amplitude 
                modulated beam?  Is the effect simply due to the dispersion
                of the beam (causing intensity to decrease) or is some other
                phenomenon responsible?

I am interesting in building a remote vibration sensor based on this concept.
Has anyone actually built this device?  How does this approach compare
(in terms of accuracy) to an approach based on laser interferometry?

                                  Thanks, Doug Coast  coast@bioeng.asri.edu