[Laser] pulsed laser comms

[Art]

OK all,

It looks like I stirred up some ruckus-I'm glad to see the spirited 
discussion on the list though, as it is better than no discussion as at all!

This whole topic of using pulses and time domain vs FFT detection 
started with my question to a DSP savvy group-although they know 
little about lasers and have experience in radio as a general rule.

I asked about creating and using ultra narrow bins in order to allow 
reception of very very weak mono frequency carriers with a soundcard 
and FFT. I suggested that .001 and .0001 Hz bins would be necessary 
to dig some signals out of the mud and asked what is the ultimate 
limit in the soundcard hardware that would prevent me from using narrower bins.

After I eliminated atmospheric effects, the stability of the 
soundcard and transmitter's frequency and the computers processing 
limitations, I wanted to know just what would limit me if I tried to 
use these ultranarrow bins.

The answer to the question was that FFT at these narrow bins will 
tend to show coherences that are present in the receivers various 
local oscillators, mixers and various stages. Since there are allot 
of stages that don't have perfect linearity, these very weak 
artifacts can become a problem at narrow bin widths and show up as 
lines on the waterfall display that can be confused with the desired signal.

When this was mentioned, it was also suggested that using pulses and 
time domain detection might be a way around these problems and it 
might work just as well (or better) than very narrow bins FFT.

It appears that Yves has tried using pulsed laser and found that FFT 
with a PGP front end worked better. It also becomes clear that 
serious laser power is necessary if pulses are used-not sure amateurs 
want to get into these types of transmitters as they are expensive 
and present much more danger to innocent bystanders. It also appears 
that the gains possible by using time domain detection are lost 
because the shorter the pulses become, the wider the bandwidth of the 
receiver has to be-and a wider bandwidth receiver means more noise is admitted.

In a nutshell, I'm not sure there is anything to be gained by going 
to time domain receivers and transmitters-at best, we can only expect 
the reception of time domain and FFT methods to provide equal weak 
signal performance and I'm not sure either method has an edge.

Regards,

Art


>I absolutely does not agree this point of  view.
>In my last mail I compared between the peak power and the noise versus  the
>short time of the pulses.