[sci.electronics] Laser Listener - How does it work?

coast@faraday.ECE.CMU.EDU (Douglas Coast) (03/12/91)

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

uge@athena.mit.edu (Eugene A Beidl) (03/13/91)

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

baskins@csd460a.erim.org (Bob Baskins) (03/15/91)

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 constructio
>   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 phototransisto
>   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?

 	A question I've wondered about is do you have to be at (or near)
the specular angle of the laser probe to detect the reflections?  i.e. can
you be at ground level and evesdrop on a second story window?

--
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Robert Baskins - Environmental Research Institute of Michigan  (ERIM)
P.O Box 8618				NET: baskins@csd460a.erim.org
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