[Ushiba, Akutsu, Komori]

We measured the MICH coupling on the DARM sensitivity, confirmed the appropriate feed forward gain again, and evaluated the impact.

At first, the transfer functions from the MICH out to DARM and from the ITMX out to DARM were measured by the sinusoidal injection like in klog24239 without the feed forward.
Both of them had the familiar f^(-2) dependence.
From these measurements, the appropriate constant gain for the feed forward was calculated by taking the ratio of them to be 0.0266, which is slightly larger than that measured in our previous work (klog24001).
In the first figure, the transfer functions from MICH to DARM without and with the feedforward are shown as the blue and red lines, respectvely.
We succeeded in reducing the MICH coupling by roughly 20dB at 50-200 Hz, and actually this feed forward largely contributed to the improvement of the DARM sensitivity.
The diaggui file is located at /users/Commissioning/data/PRCL/2023/0309/TF_MICH2DARM_20230309_MICHFFon.xml.

Next, we measured the transfer function from MICH to PRCL and PRM to PRCL.
From this measurement, we similarly determined the feed forward gain from the MICH out to PRM to be 0.0324 to reduce the coupling from MICH to PRCL.
This feed forward is also important because we have to avoid the coupling path of MICH -> PRCL -> DARM.
The second figure shows the improvement of roughly 20dB around the same frequencies (from the blue lines to the red lines).

We also measured the PRCL to DARM coupling, but just measured it without the feed forward.
It still seems much smaller than the MICH coupling.
Due to the frequent lock loss of the interferometer today, we stopped the work and will resume next week.