Listening to neighbors: Lasers and windows
rema...@bitline.com.br
After reading some ads about equipmets that use a laser beam to listening
to someone's talking, I have some doubts.
The equipment sends a light beam towards a glass window (your neighbor
window).
Then this light beam reflects in one direction and scatters in all directions.
Almost always you will not be in the direction it reflects, but certainly
you will receive some scattered light.
The equipment analizes this scattered light and extracts some audio
information.
This audio information corresponds to the sounds heard nearby that window
and had modulated the light reflected and scattered.
I was wondering about some topics (does someone have any experience about this
subject?):
1) Can I use a IR emitter (TV remote control) or must be LASER light?
2) The scattered light is focused on a photodiode, then amplified and
filtered using a band pass filter (300Hz..3kHz)? Or does it takes a more
complex optical arrangement?
Thank you for any ideas!
Marcus Ponce
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Marc H. Po
Marcus,
A real laser is required. This is because the information will be recovered
by detecting the phase shifts between the outbound, and the recovered and
modulated (by the motion oft he glass) light beam. A Telescope is used to
collect and deliver the optical beams. The detector is an interferometer,
and stability is critical. BTW this technique absolutely does work.
--
Regards,
Marc H. Po
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rema...@bitline.com.br wrote in article
<6gmpfb$c6p$1...@nnrp1.dejanews.com>...
Ralph and Diane Barone
In article <6gosum$8...@bgtnsc02.worldnet.att.net>,
How about modulating an IR LED with a (say) 1 MHz crystal oscillator, then
taking the reflected signal and FM demodulating it? Would that work?
rema...@bitline.com.br
In article <B1566067...@user85.mdi.ca>,
bar...@mdi.ca (Ralph and Diane Barone) wrote:
>
> In article <6gosum$8...@bgtnsc02.worldnet.att.net>,
> "Marc H. Po" <LVM...@worldnet.att.net> wrote:
>
> >Marcus,
> >
> >A real laser is required. This is because the information will be recovered
> >by detecting the phase shifts between the outbound, and the recovered and
> >modulated (by the motion oft he glass) light beam. A Telescope is used to
> >collect and deliver the optical beams. The detector is an interferometer,
> >and stability is critical. BTW this technique absolutely does work.
> >--
> >Regards,
> >Marc H. Po
>
>
> How about modulating an IR LED with a (say) 1 MHz crystal oscillator, then
> taking the reflected signal and FM demodulating it? Would that work?
>
>
Interferometry seems a good aproach, but we have an interesting problem here:
a)If we use a 632nm laser, seems to me that if the glass window moves more
than 316nm along the light beam direction (yes, 1/2 wave)we will have trouble
recovering the audio. For example: if a 1kHz sound hits the window and the
surface of the glass vibrates with an amplitude of 3.16um (micrometers), we
would get back a 20kHz modulated beam! Seems too much sensible! And if the
window is 50 meters far away from the equipment, the light would travel 100
meters in the open air. Warm/cold air moving in the light path would modify
the total path lenght constantly, generating noise.
b) If we use a IR LED and modulate if with 1MHz XTAL oscilator and we make
electronically the interferometer (multiplying the oscilator signal for the
received signal) the 1/2 wave would be 150 meters. Now seems to me that would
be very hard to detect the window vibrating... Or maybe not so hard?
Going further in this second idea: Suppose we could modulate using a 1GHz
oscilator, then the 1/2 wave would be 150mm. Or maybe a 10GHz oscilator and a
1/2 wave of 15mm. Sounds very interesting, but modulating IR emmiters with
10GHz and receiving 10GHz on an photodiode? How much money is involved?
Is there an easy way of doing this??? Or maybe the plain and simple
optical interferometer (632nm) does a good job? OR the LASER must
have a long wavelenght (...maybe a 10um CO2??)
Thanks!!!
Marc H. Po
Well as it works out you can create an artificial optical carrier beat
frequency by offsetting the interferometer legs. This is equivalent to
adding an electrical subcarrier to make demodulation easier.
BTW the phase modulation can be greater than 1/2 wave it is the rate of
motion that is pickup in the interferometer anyways. Consider that an FM
broadcast station has a 20 kHz base band spread out over a 75 kHz bandwidth
so the modulation index is greater than 1 which is OK and desirable.
The modulation of the air current occurs slowly and effects mostly the
amplitude not the phase, as that would require changes in the dielectric
propagation constant along the path and occurring at an audio rate!
--
Regards,
Marc H. Popek
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rema...@bitline.com.br wrote in article
<6gs1vp$bjk$1...@nnrp1.dejanews.com>...
rema...@bitline.com.br
"Marc H. Po" <LVM...@worldnet.att.net> wrote:
>Well as it works out you can create an artificial optical carrier beat
>frequency by offsetting the interferometer legs. This is equivalent to
>adding an electrical subcarrier to make demodulation easier.
>
>BTW the phase modulation can be greater than 1/2 wave it is the rate of
>motion that is pickup in the interferometer anyways. Consider that an FM
>broadcast station has a 20 kHz base band spread out over a 75 kHz bandwidth
>so the modulation index is greater than 1 which is OK and desirable.
This is a very good idea!
If I understood correctly, we must send a light beam of frequency f1 (a LASER
must be the source) towards the target window. Then we collect the scattered
or reflected light and think about it like frequency modulated. Perfect!
We don't have the 1/2 wave limit this way!
Now we must route to the photodiode this modulated light and (if I understood)
another laser light but of frequency f2. The f2 and f1 should be very close,
like 200kHz apart? This way I would get from the photodiode a 200kHz signal,
a frequency modulated one, and that's easy to demodulate it to get the audio!
Well, I am sorry for my little background in interferometry: How can I
produce f1 and f2 ? I must use two lasers or one laser and some device?
Thanks a lot!