[Jon, Gautam, Johannes]
We did the following today:
- Dither align arms such that ITMs were reliable arm references.
- Configure the IFO such that ITMX single bounce was the only visible beam reaching the AS port from the symmetric side - ITMY, both ETMs, PRM and SRM were misaligned.
- Do coarse alignment on the AS table using the usual near field / far field overlap technique, with "near" and "far" dictated by arm reach on the AS table. In this way, the ingoing AUX beam and the PSL single bounce from ITMX were collimated on the AS table.
- Lock the AUX / PSL PLL. We expected a beatnote on AS110 at eithe (80-50)=30 MHz or (80+50)=130 MHz. 80 MHz is the AOM driver frequency, while 50 MHz is the PLL offset. (Marconi was actually set to 60 MHz, prolly Keerthana forgot to reset it after some remote experimentation).
- Beat was found at 30 MHz.
- Input steering of AUX beam into the IFO was tweaked to maximize the beat. Johannes claims he saw -35 dBm on AS110 last week. But Jon reported a best effort of ~-60 dBm today. Not sure how to square that circle.
- Once we were confident that the input of the AUX and PSL beams were well aligned, we decided to do a scan. PRC was chosen as PRMI can be locked but I don't yet know the correct settings for SRMI locking, and DRMI seemed too ambitious for daytime.
- PRMI was locked on carrier.
- Jon can comment more here, but the measurement with AM sidebands does not rely on any beatnote on the AS110 PD, it is just looking for coupling of the AM sideband into the IFO from the AS port at resonant frequencies of the PRC.
- For a coarse sweep, we swept from 1-60 MHz, 801 points, and the IF bandwidth was set at 30 kHz on the AG4395.
- Transfer function being measured was the ratio of AM signal detected at AS110 PD, to RF drive applied to the AOM driver.
- We were expecting to see dips separated by the PRC FSR (~25 MHz, since the PRC RT length is ~12.5m), when the AM sideband becomes resonant in the PRC.
- But we saw nothing. Need to think about if this is an SNR problem, or if we are overlooking something more fundamental in the measurement setup.
This measurement seems like a fine candidate to trial the idea of looking for the FSRs (and in general, cavity resonances) of the PRC in the phase of the measured TFs, rather than the amplitude.