[Laser] pulsed laser 101

[Chuck Hast]

On 1/19/07, Art <KY1K at verizon.net> wrote:
> Greetings all,
>
> I've had a most enlightening week, and I need some suggestions on
> pulsed lasers.
>
> In a nutshell, I got into an email discussion with a highly
> experienced DSP person regarding very deep FFT sound card analysis
> for extremely weak laser signals. In the course of examining the
> ultimate limiting factors in FFT, the person suggested that I might
> be able to receive weaker signals using pulses, with the receiver and
> the transmitter synchronized and by using averaging to add the
> results of each repetition together (rather than tossing out the
> previous samples as Is done for FFT).
>
> Obviously this requires a wideband optical front end and short
> pulses. And, the wideband front end will let in much more noise than
> a narrow band front end like most of us use.
>
> But, by adding the time synchronized results of each cycle together
> (averaging), the desired signal will slowly 'grow' out of the noise.
>
> I was considering FFT with .001 Hz (or .0001 Hz) bins as being the
> ultimate weak signal mode........
>
> But, after this email exchange, I began to wonder if this time
> synchronized (both rx and tx) with averaging method would provide
> better weak signal reception (better than deep FFT)? In terms of the
> receiving system, the pulsed laser receiver is much easier to
> fabricate for sure!
>
> But which method is likely to provide better ability to dig out weaker signals?
>
> Also, does anyone have suggestions on how to produce very short
> pulsed lasers at 850 nM (to coincide with photodiodes best
> sensitivity)? Regarding the pulsed lasers, I can only think of the
> old stud mounted pulsed diodes, but perhaps there is another method
> that is practical.
>
> All comments appreciated.
>

Art,
This is interesting as in my digging into high speed FSO links I have found
that they are using pulsed lasers to carry the data, the pulses are quite
narrow compared to the actual data bit widths. Indeed while digging into
this I ran across several articles discussing optical SETI work being done
and they are basing their search on the same sort of idea, that any group
attempting to send optical data across the distances of space would do so
using narrow pulses from lasers, and they are building their optical receivers
accordingly.

-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."