Before doing the full T vs. angle test on the 6-m mirrors we have been using (so that we could send them back to CVI), I figured I would just test them at 45°, outside of the cavity, using the same simple configuration I used to measure the new 9-m mirror. What I found was that---for both 6-m mirrors---the transmission was in the range of 50 - 100 ppm, roughly independent of angular tuning away from 45° on the order of +/- 5°. This is pretty much consistent with the part specifications from CVI for Y1S, especially considering that they have been on our table for a couple months.
So.. why did we observe a disproportionate amount of light escaping these mirrors with the cavity locked? Good question. We suspect that it might have something to do with polarization issues, as a macroscopic polarization shift in the cavity would certainly account for the spurious transmission.
I don't know much about what could cause this, but I am reading. The good news is that our mirrors are all what they are supposed to be (yippidy doo da).