Dhruva, Naoki, Camilla, Kevin Kuns, Vicky

Summary: We added a HWP in front of the SHG isolator, and now have much more IR pump for SHG, resulting in higher green OPO pump powers available. HWP aligned our input polarization to the SHG isolator, improving its transmission from ~30% to ~86%. We previously had < 30mW available, now we can produce over 60mW SHG green; we are leaving the SHG green output at 50mW.

From the SQZT0 diagram (D1201210), not shown is a large isolator at the SHG's IR pump input, situated between MR22 and DS1. We measured >400mW going into the isolator, and only ~130mW out of the isolator, so its throughput was only about 30%?? Crazy. We first thought there was clipping at the isolator, tried aligning and it didn't help much. Also we saw the isolator was rejecting a lot of power. So, Camilla grabbed a HWP, and we installed it before the isolator, specifically between MR21 and MR22. This HWP aligns the input polarization to the isolator-- but why wasn't the isolator aligned to the input polarization already?? The light on this path is already from the rejected port of a PBS, so polarization here is fixed. We decreased power on this path from 400mW to 100mW for safety, then added and rotated the HWP to improve our output power up to 86mW. With this ~86% isolator transmission, we can get much higher SHG IR pump powers, we have set power on this path to ~240mW pre-isolator. With a little pico-ing on SHG steering mirrors to align IR into SHG cavity, SHG is now producing >60mW green for ~240mW IR input. We have left SHG output powers ~50mW, with ~5-6mW going to FCGS.

Our SHG power has been low and drifting a lot with LVEA temperatures; a plausible explanation would be that the SHG pump isolator is considerably misaligned from the input polarization, so we are on the slope and thus excessively sensitive to any temperature-related polarization drifts. It'll be interesting to see whether SHG output powers are more stable now.

We will update the SQZT0 dcc drawing to include isolator and HWP.