[Laser] pulsed laser 101

[F1AVYopto at aol.com]

Hello  Art 
You start a  very interesting subject. 
I ask me the  same questions as the yours a year ago and I exchange many mail 
with DL4YHF  about. 
In fact when  you pass from a 50% duty cycle modulated beam to short pulses, 
the  needed bandwidth increases with the noise in a such factor that the  
FFTDSP "frequency domain" losses all its capability! 
You must to  work in FFTDSP "time domain" with ultra high speed samplers (not 
with sounds  cards!). 
You need to  have a consequent samples number during each pulse! 
With very  accurate synchronised clocks you could average the temporal 
amplitude  spectrum to reject noise and enhance the pulses  display. 
It is not  other thing that the radar techniques. 
With the  light pulse the PIN photodiodes are too noisy and too slow to give 
a good  sensitivity at high speed and you mandatory need APD  devices. 
To reach the  quantum sensitivity with APDs they must bee strongly cooled and 
over voltage  biased in "Geiger mode". 
I built  several APD receivers and AsGa pulsed lasers ('20 W 80  nS). 
In absolute  sensitivity I never succeed to have a better detectivity limit 
than the one get  with a classical modulated CW laser diode and a PGP RX. 
I always  built my pulsed lasers with the classical old schematics: capacitor 
discharging  by avalanche transistors or high speed thyristors. 
They are easy  to built and simple to adjust with a good oscilloscope. 
73 Yves  F1AVY