Re: laser FM detector
- From: miso@xxxxxxxxx
- Date: Sun, 24 Feb 2008 14:02:55 -0800 (PST)
On Feb 23, 9:46 am, John O'Flaherty <quias...@xxxxxxxxx> wrote:
On Sat, 23 Feb 2008 06:18:53 -0800 (PST), j2.macali...@xxxxxxxxx
wrote:
Greetings! I'm a fresh college student currently learning electronic
communications. I have been planning on a project that makes use of
lasers to transmit signals through the laser beam, shifting a beam
using a mirror mounted on a vibrating speaker cone.
For the receiving end, I'll be making a simple slope detector + audio
amp circuit that would hopefully demodulate the signal. But instead of
an antenna (which is particularly useless in my project), I will be
using a simple photodiode that would change the frequency of a signal
(from an AC source, most likely a signal gen) according to the angle
shifting of the beam, even though there's a little difference.
I'm currently stumped in how I would configure the photodiode + AC
source part before the signal would go to the detector. Any help on
this would be appreciated.
(PS. I know, AM would've been better and easier, but still, I chose
FM.)
If you are doing FM by doppler, look what deviation you can get. If
you have a speaker with 1 mm peak-peak excursion, and it is working at
1 kHz, you will have 1 m/sec peak velocity. The relative change in
frequency will be 1 m/sec divided by the speed of light, or about one
part in 300 million times the frequency of the light. Since
photodiodes don't respond at light frequencies, the slope detection
has to take place before the photodiode, which would require an
extremely selective optical filter. The only chance you would have of
doing it would be to detect the signal before the photodiode with the
unmodulated laser beam, as MooseFet suggested.
--
John
Oh now I see what Moosefet meant. That was baffling. I though he
meant the photodiode could reflect the beam and modulate it.
This is what makes thing confusing. I thought the goal was to send a
signal on the laser beam, i.e point to point communications. Now if
the goal is to send information via a laser beam, then we have a
different situation. Bear with me here.
Lets label the areas point A and point B. At point A, there is a
speaker and a mirror. At point B, there is a laser system. Now I see
how to make the laser system. Modulate the laser intensity at a high
frequency (a few MHz), i.e. put a carrier on the laser. Split the
beam. Sense the direct laser energy with one photodiode, and the
reflected laser energy with another photodiode. The wiggling of the
speaker will phase modulate the reflected signal.
.
- References:
- laser FM detector
- From: j2 . macalinao
- Re: laser FM detector
- From: John O'Flaherty
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