very interesting. ACAV was a brand new cavity when we installed it, coming from the same batch as all other LIGO refcavs. When we unpacked the cavity from the original REO tube you could smell the softeners from the plastic. It was sitting more than 10 years in that tube. But as this is/was the only spare cavity we had no choice and installed it. Later i forgot to check the real finesse as the scans showed a finesse close to what it should be and we never spent time on that again.
I analyzed the pole for RCAV and ACAV from 2010_12_06 entry. ACAV has a pole at 54 kHz, RCAV pole is at 38kHz.
Cavity pole = FWHM/2. Knowing cavity pole and FSR, we can calculate a cavity's finesse (= FSR/ pole). These values
will be used when we simulate the TF of the system
We amplitude modulated the laser intensity via EAOM, and measured the TF between RCAV_trans_PD and PMC for RCAV pole.
Since PMC's pole is ~2 MHz and ACAV/RCAV poles should be around 35 MHz, PMC won't effect much on our measurement.
Cavity is 0.2032 m long -> FSR = c/2L = 738 MHz, then
ACAV's Finesse = FSR/FWHM = 6835
RCAV's Finesse = 9710