Notch
I'd be happy to explain our application. I am a research physicist at
LANL who uses InSb and PMT detectors to measure the radiance of shocked
surfaces to determine their temperature (typically less than 2000K). We
also use 1550 nm lasers to measure the Doppler shift of the moving surface
to determine particle velocity (see attached paper).
We need holographic notch filters at 1550 nm (even 5 or 10 nm wide would
be fine) that will reject the light from the low power lasers at that
wavelength so we can see the much smaller blackbody radiation from the
hotter surface. Typically a 1 inch diameter filter would do nicely, but
smaller would be workable in a pinch.