Naoki, Sheila, Nutsinee, Daniel

Today we found that the CLF_LAUNCH_DC_POWERMON was 270uW and the CLF_REFL_LF_OUTPUT was 30uW, which means the CLF fiber coupling is only 10%. First we thought the fiber coupling got worse so we went to SQZT0 and tried to align the CLF fiber. However, we could not improve the alignment and found that the CLF power before the fiber is 75uW, which should be 270uW according to the CLF_LAUNCH_DC_POWERMON. So the fiber coupling is 40%, which is OK, but the calibration of CLF launch PD has changed. We calibrated the CLF_LAUNCH_DC_POWERMON to 75uW as shown in the first attachment.

The second attachment shows that the CLF launch power was 650uW and the CLF REFL power was 245uW 8 days ago. We reduced the CLF power 8 days ago in 76405 and after that, the CLF launch power was 270uW and the CLF REFL power was 10uW. 

Daniel pointed out it is due to the polarization contamination. The CLF launch light might have some p-pol from the imperfect PBS for CLF rejected and it is a fixed proportion of the light incident to the PBS. Since the reflectivity of BS before the CLF launch PD is different for p/s-pol, when we reduced the CLF power, the p/s-pol ratio increased and effectively changed the calibration of the CLF launch PD.