Here's a 1-minute video of a PRMI sideband lock, from the AS port camera. Kiwamu, Rana, and Sheila will write more about this later in the evening.
Rana, Kiwamu,
In response to Sebastien's study of the BS motion, tonight we took a close look at the BS motion when the PRMI was locked and unlocked.
The conclusion is that:
it is the LSC feedback that has been shaking the angle of the BS below 100 mHz through some kind of L-to-A couplings.
(What to do)
We need to re-design better L-to-A decoupling filters to reduce the coupling. According to a transfer function measurement, the coupling coefficient was about 0.1 [rad at the bottom stage / meters at the M2 stage]. Because of this coupling, both pitch and yaw motion in the BS increased their RMS by a factor of three, mostly from components below 100 mHz. In order not to excite the angular motion more than they normally are, we roughly need another factor of three improvements in the existing L-to-A decoupling filters.
(Some plots)
The 1st plot show various spectra when the PRMI was locked on the sidebands. The upper plot of the 2nd attachment shows a comparison of the oplev signal with and without the PRMI locked. It is evident that the LSC feedback makes the spectra worse below 100 mHz. The rms increased up to 0.1 urad in rms. Also, the bottom plot of the same attachment shows the transfer function of the couplings from the M2 stage witness sensors to the bottom stage oplevs.
