[Laser] MFSK for Light?

[F1AVYopto at aol.com]

Dans un e-mail daté du 19/08/2006 05:58:01 Paris, Madrid (heure d'été),  
TWOSIG at aol.com a écrit :

"I am wondering about your experience with laser  comm and JASON.  Was  the 
light modulated with sine waves, that  is, linearly?  Or were the  frequency 
tones 
square waves, that is,  turning the laser fully on or off?"

James
JASON from I2PHD works with  IFK. (Incremental frequency keying)
Three consecutive audio tones give two  frequency offsets that code one 
character.
It is a very powerful program able  to extract a -40 dB under noise signal in 
a 2500 Hz global audio bandwidth.  (slow mode).
We used the JASON V099 program to communicate by night over 40  km by 
bouncing light off a hillside near Lyon in  France.
(http://us.share.geocities.com/f1avy/NLOS_LASER_EXPERIMENTS2.pdf)
At  normal speed only 2 to 3 characters are sent each minute…
This program is  well adapted for very low level laser echoes on a fix target 
or for a very far  large area beacon (ex. columns of LEDs). 
We use a 75 Hz central frequencies  to escape the strong spurious from public 
lights (in Europe 50 Hz 100Hz 300 Hz  are the strongest lines) and to keep 
the best sensitivity point on the slow PGP  RXs.
Our lasers are basically powered by constant current generators near  their 
maximum output optical power.
The constant current generator supply is  modulated by a chopper.
This chopper is driven by the PC sound card output  (sin wave).
With a 500 mV peak to peak PC output, the current generators give  perfect 
square waves on the laser diode.
With a lower audio level and a  reduced current, our chopper becomes a linear 
modulator that can pass audio  voice or multi-tunes modulations.
In this last case the current generator works as safety current  limitor.
For a square wave, the fundamental term is amplitude equivalent to  the one 
gives by a linear driver in sine wave so JASON works perfectly  well.
For a powerful laser the driver and the chopper schematic can be found  in :
http://pageperso.aol.fr/F1AVYopto/CLOUDBOUNCEUK2.pdf
Yves
F1AVY