Hang, Sheila, Kiwamu,

Last night, we have locked PRMI with the sidebands resonant in order to check the behavior of the AS90 signals in a simpler configuration. The measurement suggested that our previous understanding on AS90's behavior (25999) was likely incorrect.

PRMI was locked with 20 W PSL light in order to obtain sufficient signal-to-noise ratio at the AS WFSs. The arm cavities were not included because the gate valves were closed during the measurement. In this configuration, the same behavior as reported in 25999 was observed -- steering BS produces AS90 signal that has the opposite sign against the DC centering signals. Previously we thought this could be explained by SRC introducing imbalance in the sideband amplitudes. However in this PRMI configuration, the imbalance in their amplitudes should be a second order effect and therefore we should not see the same behavior if the theory was right. But we saw the same behavior. This means our understanding was likely inaccurate.

In addition, we confirmed that the AS90 signal responds to the BS angle even if the DC centering loops are closed. This confirms that the AS90 can detect a wavefront tilt. So they are not just RF optical lever, but also wavefront sensors.