[Laser] pulse modulation

[ERIC GUINN]

James,


There is an INCREDIBLE advantage to using a Q-switched laser or ANY system
which sends out a brief pulse of power.  Think for a moment what would
happen to signal-to-noise if you could take your 20-meter CW transmitter at
100 watts, take all the average power over one second (100 joules) and put
it into one millisecond-long pulse. What would the PEAK power be?  100
Joules / 0.001Sec = 100,000 Joules in that pulse.  That's a PEAK power of
100,000 watts!!!  I sure wish I could do that with my Ten-Tec Paragon!
Q-switching a laser does the same thing-- it compresses the average power
into a very short pulse.  A receiver could easily detect quite a high data
rate of these pulses, but a slow-cw receiver requires integrating a
continuous signal and offers a verrrrry low data rate.  As you can see, it
is far easier to detect a 100,000 watt signal than a 100 watt signal.

Q-switching gas, dye and ruby/YAG lasers is pretty difficult and requires
expensive equipment, detectors and tuning, but it can be done.   Nd:YAG is
probably the easiest flashlamp-pumped laser to construct, but again,
Q-switching requires spinning mirrors or high-voltage RF generators and
Pockels cells and may be financially out of our reach.  It has been done,
though.  (but not by me!)  See:
http://www.repairfaq.org/sam/lasercps.htm#cpstoc
One caveat: if you are using laser diodes, the only way -- the only
inexpensive way, that is-- that you can take advantage of high-current
pulses is to build a LED laser pulse driver which puts out more peak power
than the usual diode current regulators allow.  The problem is that the LED
laser maximum output isn't determined by a thermal limitation, but by
instantaneous mirror facet damage induced by the intensity of the laser
light.  So don't try stuffing 10 times the current into a laser LED for 1/10
the time.  The Magic Smoke will get out for sure!  On the other hand, you
CAN pulse infrared (non-laser) LED's to many times their Maximum Average
Current rating, (for very short pulses) but since that isn't really Laser
Communications, that would get me onto my ARRL "coherent light with one
stage of electronic detection" soapbox, and that's another topic.

Have fun!



Eric Guinn, AC4LS
Sevierville, TN