Komori-kun,

Did you change the whitening filter setting of PRD1 during this measurement?
Whitening filter of PDA1 was turned off after locking PRFPMI to avoid saturation during the lock acquisition.
So, if you did not change the whitening filter setting of PDA1, it might be limited by ADC noise.

Also, CARM IN1 gain during the OBSERVATION state is small, so sensing noise might be limited by not PDA1 noise itself but input noise of CMS.
So, it is better to also check the spectrum of K1:LSC-CARM_SERVO_MIXER_DAQ_OUT_DQ to confirm CMS input noise doesn't contaminate the signals.

Thank you for your comments.

During measurement shown in Fig.1, I did not change the whitening filter setting, while I turned on all of the three stages during Fig.2 measurement.
Therefore, I'm also suspecting the possibility that the Fig.1 measurement was limited by the ADC noise.
Thank you for pointing out that the sensing noise might originate from the CMS noise.
 

> Then, we came up with increasing the power on REFL PDs to improve the measurement of the CARM sensing noise, and we did that by adjusting the HWP before PDA 3/4.
> The current power levels on PDA 1, 3, and 4 with a 1.3 W input under the PRM-aligned, others-misaligned state are 33 mW (unchanged), 24 mW, and 44 mW, respectively.

> Please do not increase the IMC output power at this moment.
> Kenta will reset the HWP angle later this week.

This morning, I restored the HWP angle to its original value. From now on, we can increase the IMC output power.

On the other hand, there seem not to be any jitter-like peaks in today's PD spectra before the HWP restoration (fig.1), even though I did not touch the steering mirror in front of PDA3. I'm not sure of the reason for now. But today, I do not have enough time to investigate the phenomenon, so I did not perform the centering on PDA3. We need the centering for PDA3 later.

I checked the spectra of CMS MIXER DAQ channels and compared with RF45 signals to confirm if CARM CMS noise is limiting the sensing noise or not.
Figure 1-4 show the spectra and coherence of K1:LSC-REFL_PDA1_RF45_I_ERR_DQ and K1:LSC-CARM_SERVO_MIXER_DAQ_OUT_DQ with the gain of CMS input of 20dB, 0dB, -20dB, and -31dB, respectively.
Left top graph shows the CARM CMS input equivalent noise of each signal.
If we use -20dB or -31dB (nominal of current OBSERVATION state), the sensing noise seems to be limited by CMS noise while it can be avoided by using 0dB or more.
So, to avoid CMS noise, it would be better to use more than 0dB following the precaution written in circuit diagram of CMS circuit (fig5).

I also checked K1:LSC-CARM_SERVO_MIXER_DAQ_OUT_DQ when ITMX/ITMY are misaligned and the other suspensions are aligned (state1), ITMX/ETMX/ITMY/ETMY are misaligned and the other suspensions are aligned (state2), ITMX/ETMX/ITMY/ETMY are misaligned and PRM is aligned to maximize REFL PDA1 DC values (state3).
Since PRM alignment to maximize REFL PDA1 DC is far from the recorded values (fig6), it might be a clipping somewhere, so the alignment on REFL PDs should be checked at some point.
Figure 7 shows the result.

REFL0, REF3, and REF6 show the spectra of state1, 2, and 3, respectively
Since REF0 spectrum around 10 Hz is larger than the others (prbably due to the interference from ETM reflection at REFL), we need to misalign ETMs to evaluate sensing noise of CARM by using single bounce of PRM.
Also, since spectra in state3 shows the lower noise than those in state2, current good PRM alignment at OBSERVATION state would be not optimized in terms of CARM sensing noise.