Last night we made some measurements in order to better characterize the recycling cavity.

First we locked the CITF on the carrier at different MICH offset: this was done in order to compare the carrier gain in the CITF configuration (carrier not resonating in the arms) with the SB gain in the PRITF configuration (sidebands do not resonate in the arms). The analysis of the data will follow.

The measurement was repeated many times in order to accumulate some statistics. Indeed, the alignment was fluctuating a lot, worsening the cavity buildup. Moreover, there was a very clean ~2Hz oscillation, mostly in the TX of PR (figure 1), for which we couldn't identify the origin.

Then, the same kind of measurement was repeated, this time locking on the sidebands. In this configuration we could see really clear the signal on the B4 SFP (the two sideband peaks), since the carrier is anti-resonant (and is very small in the middle of the two peaks) (figure 2, carrier zoom in figure 3).
Finally, a couple of lock until MICH offset = 0.2 were performed in order to accumulate some data for the B4 SFP which, however, doesn't give a very clear signal for the 6 MHz sidebands, which are hidden in the noise and in the Carrier shoulder (see entry 36661).

To have a qualitative idea of the beam shape as it propagates in the recycling cavity after multiple FSR (see entry 35959) we slightly misaligned the PR in order to see on the B4 camera more than one beam reflection. Of course, due to the alignment fluctuations, the image is not super-stable. However, it can be a reference and can be compared to the RH off configuration, even if any possible difference might not be appreciable. Figure 4

Analysis will follow.