Marc and Yuhang (Eleonora remotely)

As reported in elog2322, we had oscillation in SHG length control error signal. Those peaks appear around about 20kHz and show harmonics.

The attached figure shows SHG transmission spectrum, which shows the same peaks found in error signal spectrum. But those peaks were not present in SHG transmission old measurement (elog1276).

We need to investigate more about this issue. One check could be measuring sound spectrum. Since these peaks also appear in OPO error signal spectrum, another check could be measuring the main laser amplitude noise.