Sam Goldwasser wrote in
:

A dead laser would produce the same symptoms, right?

Yeah, that's always a possibility. I'd use a scope to check the laser
drive, but I've no idea what it would look like as this is my first
experience with this type of stuff. The laser does get comfortably
warm to the touch after a few minutes and seems to vary thermally
depending on the output power I dial in, so I'm ASSuming and hoping
that it's still functioning. Given those conditions, would the
laser appear to be working?

Likely working but no guarantee.


At risk of going beyond my ground.....

That laser seems to be accepting a drive correctly, but the output might do
one of two things, either it will have died by retroreflection from a large
return of light from the damaged point in the fibre, or it will be active,
because energy can get out (as heat) without returning to kill the diode.

If this laser is powerful enough, you might be able to detect heat at the
damaged point in the fibre. Try taping it to a small Peltier (that has lots
of couples in it) and seeing if you see a few millivolts on the Peltier
wires when you switch on the laser. Even a 5 mW laser ought to read
something consistent if you control for ambient heat and watch a few
switching transitions.

The one thing the Peltier test can't tell you is if the laser is low power,
and the fibre is reflecting energy back to the diode, it can't tell you if
the diode is dead or not, but for power greater than around 50 mW it should
be a reliable test.