Since Oct 23rd we observe some strange oscillations at 12 kHz on the power stabilization error signal (plots 1 and 2). Looking at the fast channels it seems triggered by a saturation of the corrections occurring at 3.5 V.

This started after having lowered the impinging power on the photodiodes and having increased the gain and it can be explained by some saturation occuring in a component of the locking electronics.

As a temporary solution we increased again the power and lowered the gain, as it used to be before Oct 22nd. The loop seems more robust, even if the rms on the error signal has increased (plot 3).

In the meanwhile J.P. Coulon will do some modifications on the spare locking electronics to overcome this issue.