Abstract:

The CARM sensing noise is probably not CARM shot noise.

Detail:

To investigate the mystery reported in klog:33372, I compared the in-loop (PDA1) and out-of-loop (PDA3) CARM spectra at both measurement times, as shown in the lower panel of Fig.1.
Please note that this comparison was performed before resolving the PDA3 saturation issue reported in klog:33373.

It is reasonable that the DARM sensitivity at high frequencies remained unchanged, as the out-of-loop CARM spectrum at high frequencies also showed no variation (blue and black).
The coherence was approximately 0.1 and the same before and after increasing the power at PDA1, as shown in the upper panel of Fig.1.

These results suggest that the CARM sensing noise may be subdominant at high frequencies; however, it is likely to be limited not by CARM shot noise but other CARM sensing noise.
Further investigation is needed to identify the origin of this CARM sensing noise.
For example, only PDA3 might be limited by other sensing noises.