As a next noise-hunting activity, we will hunt noises imposed by the PR2 optical lever control loops.

Inspired by Yutaro's log on the periodically appearing noise (4891) and also as part of updating the latest noise budget (4834), we had a close look at the high band width optical lever control loops on the PR2.

The PR2 control loops show suspiciously high coherence with the interferometer readout in the frequency band from 10 Hz to about 30 Hz as shown in the attached screenshot. In particular, the pitch loop shows the most prominent coherence among the BS and PR2 angular control signals. To see how similar the spectral shape is to the Michelson displacement spectrum, we made a 1/f² noise-coupling model (due to the suspension response) with a fudge factor. It seems that the PR2 angular control noise can explain several peaks in the Michelson spectrum. Note that, despite our recent effort of reducing the BS angular control noise (4868 and 4879), the BS optical lever controls still contribute to the Michelson signal above 10 Hz.

If I remember correctly, the PR2 loops don't have a roll-off low pass filter and we should add it to reduce the contribution to the Michelson signal.