These days, I feel the PRMI is more sensitive for the alignment fluctuation than before. We can lock PRMI stably when the seismic motion is small, but once the seismic motion got large, the build-up sideband power fluctuate a lot.

I concluded that it is caused by the HOM resonating in PRC. We need to move PR2/PR3.

• First, I doubt our modification of suspension control. but it is not really reasonable. We know that the longitudinal motion is surely reduced by comparing the IFO control signals. So, it is difficult to understand if the PR suspensions would get noisy only for the angular direction.
• Also, the build up sideband power is stabler than before above 1Hz. This indicates the suspension angular motion might be quieter than before at high frequency.
• Then, I found that the PRC error signal is contaminated by the HOM. The attached figure shows the time series of POP90 corresponding to the build-up sideband power (upper), and the error signal of the PRCL (lower). The PRMI is aligned and all mirrors are free swinging. As you can see, the zero-crossing point is not the resonance. The second figure shows several resonances and all resonances has same structure.
• We estimated the round-trip gouy phase (Kenta will report soon), then we found the current round-trip gouy phase is 23.4 deg. The one measured before by Yutaro was 31.4. So, we don't know when, but the gouy phase is smaller than before and the distance between HOM and the 0th-order mode is closer than before.
• I think we should move the PR2/PR3 again. According to Yutaro's calculation, we need to move the PR2/PR3 by ~5 mm.