Abstract:

A peak at 116 Hz and several peaks between 300–400 Hz in the DARM sensitivity may be explained by the CARM sensing noise.
These peaks likely originate from the IMC LSC.

Detail:

I projected the CARM sensing noise shown in the top left panel of Fig. 2 in the klog:31601 onto the DARM sensitivity spectrum.
The projection is represented by the blue line in the attached figure.
Please disregard the noise floor of the blue line, as the CARM shot noise with the IFO is lower.

The calibration factor from the CARM error signal to DARM should be flat at lower frequencies, due to the cavity pole difference between the two arms, and scale proportionally with frequency at higher frequencies, likely due to mirror birefringence.
For this projection, I used a calibration factor of 3e-16*sqrt(1 + (f/100)^2), which is roughly consisten with previous measurements in the klog:31291, taking into account the difference in power at the CARM PD.

While the calibration remains somewhat arbitrary, the peak at 116 Hz and the multiple peaks between 300–400 Hz in the DARM sensitivity may be explained by the CARM sensing noise.
This sensing noise does not exhibit coherence with the IMMT1 QPD transmission signal, as shown in the klog, suggesting it is unlikely due to the input jitter around IFI.
Instead, it may be imposed on the RF sideband through the IMC transmission.

Identifying the exact noise source is critical.
Possible origins include displacement within the IMC or jitter originating in the PSL room.
In principle, the sensing noise could be mitigated by tuning the IMC offset or adjusting the RF sideband frequency, but previous attempts in klog:31601 works have not yielded further reduction.
It may be worthwhile to attempt these adjustments again, or to work on reducing the RMS of the IMC LSC to prevent nonlinear coupling.

Additionally, as noted in the klog:31636, the jitter coupling on the CARM PD with the PRM aligned differs from that with the IFO, which suggests that even peaks at 126 Hz and 220 Hz could potentially be explained by CARM sensing noise.
To address this, an alignment tuning system for the CARM PD would be beneficial.

Furthermore, tuning the beam spot on the test masses will be another solution to reduce the coupling from the CARM noise to DARM.
Finally, it should be noted that such kind of the CARM sensing noise cannot be measured at the out-of-loop CARM PD because that noise is added before junction of the in-loop and out-op-loop CARM PDs.