[Laser] AM versus PAM

[F1AVYopto at aol.com]

With LED or lasers the pulse modulations give the same problems.
One  can easily pulse a LED at low duty cycle and 1 to 10 watts peak power 
can be  output.
But ....if the pulses have T duration, the bandwidth must to be 1/2T  to get 
the full pulse amplitude in a linear mode RX.
If we increase the  bandwidth, we increase the noise.
A threshold detector can detect the pulse  and a boxcar averager can increase 
the S/N from a pulse constant rate but the AM  modulation from pulses is not 
extract with these devices.
To use the real  energy from a short pulse we have to use a none linear 
quantum detection with  APD in Geiger mode operation.
The system works like photons detector and can  be triggered at the photons 
peaks.
The pulse rate could be modulated with a  FM sub carrier or with a numerical 
coding frame.
The PWM (pulsed width  modulation) with a LED is a good system to get a good 
modulation linearity. It  needs a low pass filter to reject the commutation 
frequency.
For the noise  rejection in your experiment, it seems to me the best will be 
to use a 10 KHz FM  sub carrier to modulate your LED diode and to use an IC 
demodulator in the  RX.
All the parasitic AM noise received will be rejected.
Yves  F1AVY
http://pageperso.aol.fr/yvesf1avy/