Tomi Holger Engdahl wrote...
Check out the this explanation of an experiment I did at 1996:
http://www.epanorama.net/circuits/laserlink.html
Dave's idea was to simply drive the laser with a switched voltage
through a series resistor. This will work, but typical lasers
have a rather small current region between the lasing threshold
and the maximum allowed light output, so you're likely to either
get less than ideal operation, or damage the laser.
Tomi's idea (ref above) was to switch the laser module, but he
found this greatly limited the maximum switching speed. Still,
if speed is not important to you, Tomi's idea will work fine.
My idea has been to create a custom circuit to switch the laser,
but this has led to a small pile of rather complicated circuits.
That's because even cheap lasers often use light-level feedback
with a sensor mounted inside the laser. This sensor puts out a
current of 30 to 100uA at the maximum operating condition of the
laser, and the laser-driver includes a servo using this current.
Because this servo is usually not very fast (certainly not as fast
as the laser is capable of being modulated), switching it on can
result in a slow laser turnon. One solution I have used is to have
the servo switch from ON to HOLD during the off times, so it waits
at the operating point while the laser is off, and quickly re-
establishes the proper current level when the laser is turned on.
As an element of extra complexity, the high-power laser current
source which the servo drives can also be kept on, but its current
diverted away from the laser during shutoff. This trick further
enhances the switching speed, even as complexity grows.
An alternate idea is to make a very fast servo, and feed an extra
current into the sensor node, tricking it into thinking the laser
is putting out lots of light even as it shuts the laser off. This
simple approach is OK for medium-speed applications.
Finally, for very high-speed laser modulation, maintain a constant
dc current, and simply add an ac-coupled high-frequency current in
parallel. I used this approach to go to 1200MHz with a surplus red
Hitachi DVD laser. Some will remember the discussion here on s.e.d.
where I described the setup with a 47-ohm SMD series resistor right
at the laser, terminating the modulation-signal transmission line,
and the microwave bias-T to inject the current. There was evidence
my optical receiver caused the 1200MHz limit, rather than the laser.
