On Sat, 17 Oct 1998, Mark A Moss wrote:

> I think it has something to do with the temperature coefficient of the
> laser output.  For many laser diodes, the power varies so greatly with
> temperature that a drive signal that produces typical output at 25C will
> have no output at 0C and would self destruct at 50C.  There are probably
> other answers as well.

You are almost right. The next time you have a visible laser diode
assembly with PSU in your hands, use electronics freeze spray on it while
keyed on (from the side). The temperature drop from +30 deg. C (my case
here) to -50 deg. C (according to the freezer leaflet I have here) causes
at least a two-fold increase in output before the window fogs over.

The trick also works with common LEDs. SMT LEDs soldered on ceramic work
best ;) The ultra bright orange kind becomes so bright it hurts to look
into it even without a lens, at If ~= 8 mA !.

Be sure not to freeze your fingers off if you do this, and know that some
laser pointer plastics won't take -55 C. Use this at your own risk.

Anyway, the original experiment involves liquid nitrogen, which gives even
MORE output. The method also works with IR LEDs and lasers of course. The
first semiconductor lasers worked in liquid nitrogen at 77 K according to
my books.

Peter

PS: Lasers operated in closed circuit do not have such a strong response
on temperature, but power requirements drop with dropping temp. and the
laser radiation is cleaner (coherence, bandwidth).