Yuhang, Marc, Michael

The lock of both the p-pol and CC phase locked loops was recovered. 

Details

Yuhang adjusted a bit the fiber coupling and noticed a bit of an issue with touching something somewhere and the fiber signal immediately disappearing.

Yuhang adjusted the ML-ppol fiber coupler position while Marc and I looked a bit at the green mode cleaner. The mode matching is 95.4% but the error signal is also a bit low, about 120 mVpk (in the past it was 10x larger). This situation is similar to the IRMC and SHG. We seem to be losing a bit of power from the transmission of the BS90/10 to the input of the MZ. The beam seems quite large and passes close to a mirror post on the way to the MZ input.

Eventually we certified a satisfactory level of fiber coupling for all 4 fibers. Taking the signal from the fiber PD shows a 45 MHz peak at -20 dBm and we could see some optimisation on the spectrum analyzer using the half wave plate.

We were a bit confused about which DDS RF signals required an attenuator before injection to the PLL servo, and by chance found that the DDS3 DAC3 is controlling the p-pol local oscillator - it should be controlling homodyne LO, with p-pol LO coming from DAC0 - with our recent PLL issues we didn't have the correct PLL LO frequency (was 7 MHz, should be ~ 20 MHz) and didn't notice this cause until now. But we decided to leave it for now and just try locking the PLL with the DAC3 port.

We found that some ad-hoc DC blocks (grey boxes on the bottom of the rack labelled #4, #5, #6 etc) used for monitoring the PLL signals were improerly connected. So we double checked all the connections coming from the fiber PDs and confirmed the proper placement of signals going to the "BEAT" port on the servo, and the "MON" wires near the spectrum analyzer. Then we confirmed the sign of the signal on the PLL software and eventually could lock p-pol PLL with fast and slow loops.

For the CC p-pol, we discoverd some silly mistakes like mixing up the CC/p-pol lasers and a disconnected signal wire. Then we saw that the CC LO PLL signal is very large (14 dBm) so we put the -12 dB attenuator back in. There was some issue again with the PLL software but it is now fixed and CC p-pol lock was recovered.