[Tanaka, Komori]

Abstract:

We successfully decoupled the pitch {D, C}SOFT modes from the HARD mode by adjusting the sensing matrix of the arm transmission QPDs.

Details:

After tuning the output matrix of the arm WFS signals based on the calibrated TM oplev signals, the next step was to decouple the {D, C}SOFT and CHARD modes from DHARD.
To measure the couplings, we injected a 50-mHz sine wave into the DHARD output while keeping the arm WFS control engaged with the conventional filter.
The excitation frequency was chosen to be 50 mHz in order to avoid feedback to the TMs, which is enabled in the conventional filter, and to isolate the response of the MNs.

The initial result is shown in Fig. 1.
From the beginning, we did not observe a clear 50-mHz peak in the CHARD signal.
This suggests that the TM oplev calibration is still accurate and that DHARD and CHARD are already decoupled at the level of an order of magnitude.
In contrast, clear peaks were observed in the DSOFT and CSOFT signals.
We therefore decoupled these modes from the DHARD excitation by tuning the sensing matrix of the arm transmission QPDs, which serve as the sensors for the SOFT modes.

Figures 2-4 show a comparison of the four arm signals during excitations of DHARD, CSOFT, and DSOFT, using the previous (labeled as “REF” in the figures) and the updated sensing matrix.
After the adjustment, the peak amplitudes in the DSOFT and CSOFT signals during DHARD excitation were significantly reduced, while sufficient responses remained in each degree of freedom when the corresponding excitation was added.
The updated sensing matrix is shown in Fig. 5.