With the interferometer locked in CITF, we scanned the arm cavities around the second order modes of the 56 and 6 MHz sidebands. The scan in the west arm looks very weird, with very low power and weird shapes (Fig.1). From the north arm scan (Fig. 2) the peak heights for each SB HOM2 are:

LSB56 HOM2: 0.12

USB56 HOM2: 0.09

LSB6 HOM2: 0.255

USB6 HOM2: 0.273

Aftwerwards the CHRoCC power was decreased by 5% and after waiting for ~1.5hrs, re-doing the arm scans show that the power in the west arm scan increased significantly (Fig. 3) and the shape improved a bit, although the shape is still dubious. From the north arm (Fig. 4) the peak heights for each SB HOM2 are:

LSB56 HOM2: 0.44

USB56 HOM2: 0.21

LSB6 HOM2: 0.37

USB6 HOM2: 0.53

So, the mismatch increased for all sideband modes (that is expected from simulations, for CHRoCC changes all SB order 2 modes should follow the same trend). LSB56 mode 2 increased by 3.7x, USB56 mode 2 increased by 2.3x,  LSB6 mode 2 increased by 1.4x and USB6 mode 2 increased by 1.93x .

We can see that the mode imbalance for the 56 MHz increased, with the LSB56 having a higher mode 2 content - according to simulation this is expected for a positive increase in PRM RoC around the nominal case, and indeed by decreasing the CHRoCC we increased the RoC of the PRM, so that is consistent with simulations. Simulations do not predict a mode imbalance for LSB6, so it is interesting to try to reproduce this effect in simulation.

A full analysis with all data points taken and using the B4 phase camera to normalise the sideband power during the scans will be made soon.