[Laser] pulsed laser comms

[F1AVYopto at aol.com]

Hi Glenn and  all
>The difference is that instead of a CW laser with a few milliwatts 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.
It was my first  theory before I begin to experiment in real conditions.
- First  example:
I want to receive a 10 nS 100 W 1 KHz rate pulsed laser.
I need  to conserve the pulses amplitude in the RX chain.
The minimal bandwidth to  keep the peak amplitude without to much integration 
is F=1/T
F=1/ 10.10^-9 =  100MHz
The average power during one second is 1000 x 100 x (10.10^-9) = 10^-3  W or 
1 mW
- Second example:
A CWM 2 mW laser is modulated at 1 KHz  50% duty cycle.
The average power during one second is 2 mW /2 =1 mW
The  minimal bandwidth to keep the peak amplitude without to much integration 
is 1  KHz.
- Comparing:
Suppose the 2 lasers are lens fitted to have the  same beam.
The peak power ratio is 100/ 2.10^-3 = 5.10^4
The bandwidth  ratio is 100.10^6 / 10^3 =10^5 
The 1s average power ratio =1
The 1s  photon flux ratio =1
- Conclusions
If not cooled and if not in  Geiger mode, all the APDs add an excess noise 
comparing to the best PIN or PN  photodiodes.
The bandwidth ratio avoids the pick power profit by a dramatic  noise 
increasing.
The synchronic APD RX process (synchronised Geiger mode to  receive only 
during the expected receive pulse arrival time) is the single way  to approach the 
same quantum limits as for the FFTDSP extraction for low rate  pulses at 50% 
duty cycle.
To have a low dark count in Geiger mode, the  minimal APD temperature must be 
under -40°C and the TX and the RX pulses strobes  must be synchronised by a 
very accurate common clock.
This principle is very powerful for  pulse flying time measurement but not 
for ultra low level and low rate  communication?
 
At  ambient temperature, the best I got with my APDs in Geiger mode was more  
than 3000 dark pulses by second without input light and with a simple  RC 
quenching circuit.73, and thanks for the  comments.
Yves F1AVY