We had a problem with power coupling efficiency to the PMC. Sometime it changes from 80% to 40% or less.
When the temperature of the NPRO is varied by a voltage calibrator @ slow actuator input of the laser controller,
at certain T(V), the coupling efficiency drops from 80% to 40% or less. This might be mode hopping. However, the transmitted beam power still oscillates ,
and one can see the beam spot pulsating on the screen. I thought it might be mode hopping too, but after changing T, the fluctuation of the power has not gone, there is some suppression though. So it might be the servo. I'll check the TF of the servo and compare it with the schematic, D980352-B-1.