We are looking at the RIN measured by the reflection port of the interferometer and comparing it to the RIN of the input beam as measured by the ISS second loop detectors. For both 75W and 60W input we are measuring this transfer function using high frequency ISS noise injections.

One thing to notice is that the filter modules for LSC-REFL_A_RIN and LSC-REFL_B_RIN contain a calibration factor of 0.3055, which we removed from the measurements presented here.

We also used the calibration factor from alog 70979 to correct the ISS outer loop detector.

Finally, we derive the power reflection coefficient of the interferometer by dividing the observed power in reflection by the expected prompt reflection value. This is using scaling factors from the above alog.

For the 75W measurement the interferometer was not fully thermalized. We expect more like ~8.5% of the power to be reflected after a couple of hours locked.