[Jenne, Daniel, Stefan]
There seems to be an offset somewhere in the ISS second loop. When the 2nd loop comes on, even though it is supposed to be AC coupled, the diffracted power decreases significantly. This is very repeatable with on/off/on/off tests. One bad thing about this (other than having electronics with unknown behavior) is that the diffracted power is very low, and can hit the bottom rail, causing lockloss - this happened just after we started trending the diffracted power to see why it was so low.
Daniel made it so the second loop doesn't change the DC level of diffracted power by changing the input offset for the AC coupling servo (H1:PSL-ISS_SECONDLOOP_AC_COUPLING_SERVO_OFFSET from 0.0 to -0.5), the output bias of the AC coupling servo (H1:PSL-ISS_SECONDLOOP_AC_COUPLING_INT_BIAS from 210 to 200), and the input offset of the 2nd loop (H1:PSL-ISS_THIRDLOOP_OUTPUT_OFFSET from 24.0 to 23.5 - this is just summed in to the error point of the 2nd loop servo). What we haven't checked yet is if we can increase the laser power with these settings.
Why is there some offset in the ISS 2nd loop that changes the diffracted power?? When did this start happening?
We were able to increase power to 25W okay, but turning off the AC coupling made things go crazy and we lost lock. The diffracted power went up, and we lost lock around the time it hit 10%.
The 2nd loop output offset observed by the 1st loop was about 30mV (attached, CH8). With the 2nd ISS gain slider set at 13dB and a fixed gain stage of 30, this correspond to 0.2mV offset in the AC coupling point. This offset is relatively small.
One thing that has happened in the past two weeks or so is that the power the 1st loop sensor (PDA) receives was cut by about half (second attachment). This was caused by the move from the old position to the new position of the PD.
Since the sensing gain of the 1st loop was reduced by a factor of two, seen from the 2nd loop the 1st loop is twice as efficient an actuator. Apparently the second loop gain slider was not changed (the slider is still at 13dB), so even if the same offset was there before, the effect was a factor of two smaller before.
Another thing which is kind of far-fetched is that I switched off the DBB crate completely and we know that opening/closing the DBB and frontend/200W shutters caused some offset change in the 2nd loop board.