[Laser] pulsed laser comms

[Art]

At 07:47 PM 1/21/2007, you wrote:
>Hi all.
>
>I'm not sure I've read  on this thread anything that describes how 
>the pulsed laser comms works and why we should expect better SNR 
>with a pulsed laser than with a CW laser and FFT processing.
>
>The difference is that instead of a CW laser with a few milliwats of 
>power, the pulsed laser will have peak amplitudes of tens to 
>hundreds of watts - a 40dB to 50dB improvement. The trick is to look 
>at the received signal only when it might be there and ignore the 
>noise the rest of the time. The available bandwidth of the 
>modulation will be something like half the laser PRF. The SNR of the 
>signal will be improved because the pulsed laser is 40dB to 
>50dB  more than a typical CW laser.


Almost Glen..............

You do not look at the signal during a time when you think it is 
present although there are gated receivers.

You do use time synchronization on both the receiver and the 
transmitter. But, the sampling is quite constant. The kicker is that 
you do not throw away the samples that should contain a weak pulse 
buried in noise (as you do with fft). Instead, you average them, and 
over a given number of samples, the desired signal begins to become 
visible. In this manner, the noise averages out so long as you can 
maintain time synchronization.

I don't know if it can work any better than FFT.

Yves says he tried it and it doesn't do nearly as well as FFT.

Regards,

Art