Akutsu, Aso, Michael
We made an estimate of the OMC transmissivity by measuring the power just after the OFI (63 mW), then doing a scan of one FSR of the OMC and checking the power of each of the clearly visible modes. We guess that the transmissivity is close to 90%, but until we pump down the chamber we can't really have a good estimate due to the saturation issue of the DCPD from the 800 Hz vibration.
We saw the following modes while scanning the OMC PZT slowly, and checking OMC_TRANS_DC_SUM_OUT_DQ channel on ndscope. The lock isn't particularly stable so the accuracy is +/- 10%, probably. Some of the modes also look quite bad, probably due to birefringence.
TEM00 - 40 mW
Strange mode (TEM22 + something else?) - 1 mW
TEM00 of RF sideband - 0.1 mW
TEM02 - 4 mW
TEM43 (?) - 0.8 mW
Similar mode - 0.6 mW
TEM10 - 10 mW
Strange mode - 1 mW
63 mW (OFI) - 17.5 mW junk light ~ 45.5 mW TEM00 incident at OMC -> 88% transmissivity of TEM00
There were some other modes of order 0.1-0.2 mW that we ignored, and there also may be some loss at the OSTM. So there is a possibility of underestimating the transmissivity. Regardless, the OMC transmissivity is workable but could be a tiny bit better.