>>You won't, of course, be able to switch the LASER on and off that
fast. I assume you mean you are looking for a liquid-crystal shutter
that can be used to block/ transmit the beam?

>>What speed do you wish to communicate at?  What (approx.) will be the beam
intensity to me modulated by the LCD shutter?  Just curious, what are you
building?

>>-todd.


______________________________ Reply Separator _________________________________


I doubt a laser module, such as those built into a pointer, would be able to
modulate very fast because they are not designed to do that. However, the laser
alone can modulate incredibly fast!!

Forget LCD shutters. They are miserably slow - tens of milliseconds; you'd do
better with a 1200 baud modem!

Most laser diodes are designed to emit light out of both ends of a cavity (not
necessarily the same power). One end is the beam transmitted out the package and
the other usually points to some sort of photodiode. This photodiode is used to
measure the laser's power. Solid state lasers are very sensitive (I think it's
primarily to temperature, but manufacturing process variations may also play a
role) and without feedback, you'd have to cut back its power a lot to make sure
that you wouldn't over-drive it.

You'll probably need a collimating lens to cancel out the divergence of the
laser diode and keep the spot small over a long distance.

Some other things to think about:

Alignment- how are you going to do this? it's very difficult to find a laser
point if its too far away if you don't know where it is. (I know this because
when I try to shine my pointer at license plates from the 3rd story it's really
tough to do). A good reflective sheet (not shiny like mylar, but reflective like
on car brake lights) will help a lot in initial alignment. Then keeping
alignment (I would think) would also be a problem.

Receiver- You have to be careful to get a receiver that can work at high speeds.
Most are optimized for sensitivity, so make sure you check it's rise/fall times.

good luck!
John Maushammer
jmaushammer@SMTPLINK.CTA.COM

p.s. I haven't built a link; I've just played around a little with the raw laser
diode.