[Ushiba, Komori]

We have conducted an estimation of the frequency noise contribution to the DARM sensitivity.

Firstly, we measured the coherence between the error signal of the CARM (K1:LSC-REFL_PDA1_RF45_I_ERR_DQ) and the OMC trans DC sum, as shown in the first figure.
As suggested in klog:23945, the coherence is high above kHz, indicating that residual frequency noise dominates in this frequency range.
The CARM contribution was estimated based solely on this coherence (shown in the second figure).

Next, the residual frequency noise was roughly estimated using the CARM cavity pole (assuming 1 Hz) and the laser power on the REFL PD, as shown in the third figure.
The roughly estimated CARM shot noise appears to be mysteriously similar to the measured residual noise.
However, since the in-loop error signal should not be limited by shot noise, further investigation is required.

Finally, we made a very rough estimation of the transfer function between frequency and DARM displacement, as shown in the fourth figure.
This estimation can be modeled by considering the cavity pole difference between the two arms and the phenomenological higher-order mode coupling of the laser light in the interferometer.

All the work presented here requires further detailed discussion in the near future.