Marc, Michael

We attempted to fix the CC PLL locking issue but no luck. It seems that the issue is on the electronics side as the optical level coming out of the fibers is quite good.

Details

I aligned a bit the ML->AUX2 fiber. I made about a 20% increase in the voltage on the oscilloscope coming from the fiber PD monitor. Then, we checked the couplings. The power levels are as follows:

ML in 4.6 mW
ML out 0.75 mW
ML coupling = 0.75*2/4.6 = 33%
ML scope 7.5 mV
CC in 1.0 mW
CC out 0.3 mW
CC coupling 60%
CC scope 2.0 mV

Seems like way too good CC fiber coupling and yet not much signal on the oscilloscope. I tried doing a full rotation of the waveplate in from the the CC fiber but it didn't change noticeably. I also tried switching PDs but the result was the same. Likewise, both PDs showed no appreciable signal on the spectrum analyser.

I tried calculating the expected voltage from the PD. The sensor is not transimpedance (Thorlabs DET01CFC/M) and the output voltage depends on the load resistance V_out = I_out * R_load. The current conversion is ~ 0.7 A/W at 1064 nm. We use 50 Ohm terminators, so the expected voltage would be about 26 mV for ML and 10.5 mV for CC. But the measured value is about a factor 4-5 below this. For comparison, the p-pol PLL has a total voltage of about 45 mV (~ ML 20 mV + ppol 25 mV).