As reported in klog33834, 372 Hz peak in CARM out loop sensor (K1:LSC-REFL_PDA3_RF45_I_ERR) seems to be originated from PMC power fluctuation. I tried to distinguish the cause of PMC power fluctuation, for example the input jitter to PMC by improving the alignment to PMC with PICO motors. Fig. 1 shows the time trend of PMC TRANS/REFL power during the alignment. Fig. 2 shows the image on PMC REFL camera before (left panel) and after (right panel) alignment. I succeeded in about 8 % improvement in terms of PMC TRANS power.

Then, I measured the spectra of the PMC TRANS/REFL power (K1:LAS-POW_PMC_OUT, K1:PSL-PMC-REFL_DC_OUT) and the CARM out loop sensor and took the coherence between them after improving the alignment. Fig. 3 shows the results, the upper left panel in Fig.3 shows the spectra of above sensors around 370 Hz, and the lower left panel in Fig. 3 shows the coherences between the PMC power sensors and the CARM out loop sensors. Also, the spectra labeled REF are the ones before the alignment, the no-labeld spectra are the ones after the alignment.

The 372 Hz peak disappeared from the both power spectra of PMC TRANS and REFL after the alignment work. However, the 372 Hz peak seems to be still in the CARM out-loop sensor. Of course, the coherence between them at 372 Hz got lower.

On the other hand, the 372 Hz peak still remains in IMC TRANS/REFL power (K1:IMC-CAV_{TRANS,REFL}_OUT) in the upper right panel in Fig.3 even though the peak disappeared from the PMC power sensors' spectra. I suspected the 372 Hz peak changed to the 2nd harmonics, that is, the 744 Hz peak in PMC powers' spectra thanks to improving the alignment, but there is no peak around 740 Hz in the spectra of the lower right panel in Fig.3.

From the fact that the peak disappears from the PMC trans power after improving the alignment, the 372 Hz peak may be originated from the PMC input jitter. However, I'm not sure of the reason why the 372 Hz peak still remains in IMC TRANS, CARM out-loop sensor... (IMC alignment is not good due to the offset in WFS as used in IMC ASC?)