By looking at the spectrum of filter cavity lock correction signal, at the same time, driving pitch/yaw of INPUT/END mirror, the coupling to length could be seen as a peak. By adjusting driving matrix, this coupling could be minimized. This was done for INPUT y/p and END y/p. For each DOF of cavity, the excitation was examined with different driving matrix.

The excitation and spectrum of one test is shown in attached figure 1 and 2. By testing many different driving matrix, I did following modification to reduce coupling to length.

INPUT mirror H4: change from -1 to -0.5

END mirror H1: change from 1 to 0.7

This modification may help to solve the problem of worse locking accuracy caused by AA loop, as reported in elog2227.