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From: uge@athena.mit.edu (Eugene A Beidl)
Newsgroups: sci.electronics,sci.optics
Subject: Re: Laser Listener - How does it work?
Keywords: Laser, vibration
Message-ID: <1991Mar12.195308.14030@athena.mit.edu>
Date: 12 Mar 91 19:53:08 GMT
References: <1991Mar11.152746.19730@hobbit.gandalf.ca> <1991Mar11.163650.9794@fs7.ece.cmu.edu>
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In article <1991Mar11.163650.9794@fs7.ece.cmu.edu>, coast@faraday.ECE.CMU.EDU (Douglas Coast) writes:
|> 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



I believe the reflected signal is actually phase modulated and added to the
reference beam to produce an amplitude modulated output signal.  I have not
built this device, but it is akin to a laser radar.

--Eugene