Aidan asked me to look at scanning the arm cavities immediately after a lockloss, so that we can look at the higher-order-mode spacing as a function of time as the interferometer cools off. We hope to be able to infer from this the absorption in each of the individual arms, to help separate if there is anomalously high absorption in one of the arms and if so, which one.
The procedure was basically identical to that of Kiwamu's back in 2014 at LLO (LLO alog 13768). The general idea is to lock the arms on green and hold them steady with ALS, then put an offset in the ALS COMM to adjust the PSL frequency relative to the arm cavities.
Since we do have the frequency difference discriminator (FDD) in the ALS COMM system, we can't scan more than 1.5 or 2 FSR in our nominal configuration. During the commissioning period today, Sheila showed me how to bypass the FDD. After she bypassed it, I was able to scan almost 10 FSR per arm. The cold-cavity scan I took today was much too slow, so the data isn't particularly clean. Also, I neglected to add a small misalignment to the input IR beam, which will help the higher order modes to resonate. At the end of the commissioning window, we put the FDD back into the signal chain.
So, our plan for the next time we have a commissioning window, or perhaps just at the beginning of maintenece on Tuesday, is the following:
- While the IFO is locked, gently go into the LVEA ISC racks and bypass the ALS COMM FDD (this way we don't have to acquire lock without it, but can run the scan without it).
- Unlock the IFO, after it's been locked for several hours (so it's nice and thermalized).
- Relock ALS as quickly as possible (I was able to get there within about 2 min this afternoon).
- Run scan script (attached) to step the ALS COMM VCO offset - should run for 20 - 30 min.
- Put FDD back in the ALS comm system.
- Done with test. Relock IFO or begin maintenance or other activities.
