[Yokozawa, Komori]

Abstract:

We implemented a new PRCL control filter, resulting in improved PRCL and DARM sensitivity at low frequencies.

Detail:

This is also a report of the work conducted yesterday.

As discussed in klog:33437, increasing the UGF of the PRCL control may be beneficial.
In O4a, the UGF was set to approximately 20 Hz to avoid injecting sensing noise through feedback.
However, due to the high-power operation, the actual PRCL signal is now clearly observable below 50 Hz, and the contribution of sensing noise feedback is much smaller than the current DARM sensitivity.

We re-tested the same filters used in klog:33437 in each lock acquisition state and confirmed that the interferometer can reach DC readout with these filters.
The DARM sensitivity and coherence between DARM and PRCL are shown in Fig. 1.
Implementing the new PRCL filter successfully reduced the coherence and improved the DARM sensitivity at several frequencies in the 10-40 Hz range.

Additionally, Fig. 2 compares the calibrated PRCL and PRCL error signals before and after applying the new filter.
The RMS of the PRCL error signal was significantly reduced by a factor of approximately six.
Furthermore, the calibrated PRCL spectrum also shows improvement.

This improvement in the calibrated signal is unexpected, as the current calibration method should not depend on the feedback filters.
This result suggests that a nonlinear effect was contributing to the calibrated PRCL, and that the new, more robust feedback design helped mitigate it.