- Modulation frequency did not change anything drastically. Even I changed the seed frequency by 300 Hz, the error signals of MICH and SRCL are contaminated by the arm cavity resonance.
- I measured a spectrum with each modulation frequency. They are shown in the attached file. No difference can be seen between the spectra with nominal (+/- 200 Hz) modulation frequencies. I don't know the peak 400 Hz comes from.
- I checked the REFL_RF51I/Q with the following configuration:
* No light on PD -> No signal
* Single bounce from an ITM -> No signal
* pure MICH -> only Q phase has a signal ( Here, I adjusted the demodulation phase)
* SRY -> I/Q has a signal
* SRYarm -> I/Q has a signal and both signals are contaminated by the cavity resonance. Q signal seems to be contaminated more than I.
* MICH with Yarm -> No contamination from the resonance.
- The contamination might come from the carrier and 3rd-order sideband.
So I stopped to investigate it and changed the strategy as follows:
1. Lock the MICH with Yarm by using ALS, and SRM is misaligned. (MICH is locked at the bright fringe for the AS. Not in mid-fringe as like Virgo.)
2. Bring the SRM to aligned position gradually. And at some point, start to feedback to the SRM.
It works more or less. I attached the IR trans and the MICH and SRCL error signals. I tried several times and all the time, the lock seems to faile by a glitch-ish signal in SRCL error signal.
I guess it might be a mode hop, but still, we need an investigation.
