With the elevated intensity noise currently being injected into the interferometer, we can passively make an estimate of the CARM pole frequency.
The attachment shows the transfer functions that take interferometer input power to transmitted arm power (as measured by the four end-station QPD sums) with the interferometer locked at 22 W. There isn't good coherence around the CARM pole frequency itself. However, if we normalize each signal to RIN, this fixes the dc value of the transfer function at 1. Hence, the magnitude of the slope is sufficient to extract the CARM pole.
At 10 Hz, the transfer function is 0.0631, which implies a CARM pole of 0.63 Hz. There is about 1.5 % uncertainty from the spread in the values from the four transfer functions, and about 2 % uncertainty from the drifts in dc intensity for the four QPD sums during the measurement period. Together that makes 2.5 % uncertainty. There may also be additional uncertainty from the whitening and antiwhitening of the QPD signals.
If we assume an ETM transmissivity of 4 ppm, an ITM transmissivity of 1.45 %, a PRM transmissivity of 3.0 %, and 50 ppm of loss on each test mass, we expect a CARM pole of 0.64 Hz (see T1500325).
Obviously a superior method is to take a driven transfer function that has good coherence at and around the CARM pole frequency, and to use the phase information to make a true fit.
Kiwamu, Evan
Similarly, the transfer function from input intensity to POP dc can give the recycling gain of the 9 MHz sidebands. This transfer function is attached, in RIN/RIN. The ac magnitude of this TF is about 0.04.
Assuming the 45 MHz sideband power is negligible in the PRC, assuming the 9 MHz modulation depth is 0.22 rad, and assuming the carrier power recycling gain is 35 W/W, I believe this implies a 9 MHz recycling gain of 60 W/W or so.