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From: uusgta@sw1e.UUCP
Newsgroups: sci.electronics,sci.physics
Subject: Re: Laser eavesdropping
Message-ID: <503@sw1e.UUCP>
Date: Mon, 30-Mar-87 16:48:22 EST
Article-I.D.: sw1e.503
Posted: Mon Mar 30 16:48:22 1987
Date-Received: Wed, 1-Apr-87 06:51:30 EST
References: <499@sw1e.UUCP> <1505@husc6.UUCP> <560@sdiris1.UUCP>
Organization: Southwestern Bell Telco GHQ St. Louis, MO
Lines: 46
Keywords: modulation
Xref: utgpu sci.electronics:438 sci.physics:1000
Summary: AM? seems difficult.

In article <560@sdiris1.UUCP>, res@sdiris1.UUCP (Robert Sanders) writes:
> The modulation is simple, good old Amplitude modulation.. that is, a
> continuous laser, which will be amplitude modulated by the vibration of
> the window. (you can do this with a mirror connected to a voice box and
> a reflected sunbeam, with a photodetector for recovering the signal)
> Skip Sanders :  sdcsvax!jack!man!sdiris1!res

If/when you locate the construction project I would appreciate a reference.
AM modulation doesn't seem terribly possible unless I misunderstand. Correct
me if I'm wrong but I would think that the only amplitude variability would
be attenuation due to different return path length (i.e. as the window vibrates
inward the beam travels further to get back to the xmitter.)  For any real
distance I would expect this signal to get swamped quickly.  Interferometry
seem more reasonable.  Asking a physics type person around here cleared up
some things for me.  Where I had thought that this sort of translation was
many quick measures of distance he points out that the number of constructive/
destructive interferences per unit time is an indication of velocity.
I hadn't heard of interferometers being used for anything other than precise
measurements of distance, on the order of a part of a wavelength.
He also points out that there are two components here. The gross travel
of the window, as a function of sound level, and the "wiggle" of the window,
as a function of frequency.  He also points out that a reflective dot helps
in determining which surface of the pane is reflecting.

I have a few questions.

	1) Can someone think of a method for determining the sign of the
	velocity of the window at a given time.  It seems that waiting 
	for acceleration to pass zero is the only indication of the window 
	passing back through an earlier position.

	2) Does anyone have a "feel" for the travel distances of the window?

	3) Any thoughts on using a second imposed frequency on the laser? How
	 could this help? Also what of lower frequency beams? Limits on
	 frequency?

	4) Are fourier transforms the appropiate tool to seperate volume and
	frequency information.

	5) Could someone decribe the inerferometer setup.  How would you
	prevent vibration at the measuring site from ruining the measure?
-- 
#			---Tom Adams---
# {bellcore,ihnp4}!sw1e!uusgta	St. Louis MO	314-235-4237
# Opinions expressed here are mine, not those of Southwestern Bell Telephone