Coupling into the fast axis of the fiber:
The PM couplers I bought require that the light is coupled to the fast axis. The Thorlabs part that Andrew ordered, and which Anjali was using for the MZ experiment, was the opposite configuration, and so the input coupler K6XS mount was rotated to accommodate this polarization. The HWP was also rotated to cut the power into the fiber. I undid these changes. Mode-matching is ~65% (2.42mW/3.70mW) which isn't stellar, but good enough. The PER is ~15dB (ratio of power in fast axis to slow axis is ~40), which I verified using another collimator at the output, and a PBS + two photodiodes. Again isn't stellar but good enough.
EX laser temperature adjustment:
Rana adjusted the temperature of the main laser to 30.61 C. According to the calibration, the EX laser temperature needed to be ~32.8 C. It was ~31.2 C. I made the change by rotating the dial on the front panel of the EX laser controller. Fine adjustment was done using the temperature slider on the ALS screen. With an offset of ~+610 counts, I found a beat at ~80 MHz.
First look at PM beamsplitters:
From my initial test, the beat amplitude was stable to my moving of the fibers . The NF1611 DC monitor reports 2.6 V DC with only the EX light, and 3.15 V DC with only the PSL light. So I should probably cut the PSL power a little to improve the contrast. Assuming the 10 kohm DC transimpedance spec can be believed, this means the expected signal level is 4*sqrt(260uA * 315uA)*700V/A ~0.8 Vpp, and I see ~0.9 Vpp, so roughly things add up (this is actually more consistent with an RF transimpedance of 800V/A, which is maybe not unreasonable). The RF amps for routing this signal to the delay line has been borrowed for the 2um frequency noise experiemnt - I will reacquire it today and check the ALS noise performance.
So overall, I am happy with the performance of the current iteration of the BeatMouth. |