Michael and Yuhang

This Friday (20220114), we changed optical set-up from what described in elog2788. This is because we found one of the steering mirrors in front of OPO cannot effectively align laser beam inside OPO. We checked beam parameters in elog2486, which shows that the non-effective steering mirror is located just around the beam waist after 40mm lens. This explains why this mirror shows anomaly while OPO alignment procedure.

To solve this problem, we modified the optical set-up. We add one steering mirror just before OPO while leaving enough space for a BS between it and OPO (Fig.1). To confirm this set-up is capable of aligning beam and the OPO still has good internal alignment, we aligned OPO while laser beam is injected from the crystal side of OPO. We found the newly add steering mirror helps to optimize alignment.

In the end, we achieved a decent alignment by moving the position of lenses. The TEM02 mode is optimized by moving position of lenses. The position of lens are shown in Fig.2 and Fig.3.

We also characterize the mode matching level by measuring the TEM00, TEM01 and TEM02 modes. Their seperate power is shown in Fig.4, 5, and 6. This indicates a mode-mismatch of (5.6+18.8)/(5.6+18.8+282) = 8%

Using parameters of OPO, we simulate the beam parameters as Fig.7. From this simulation, we see the beam waist should be located in front of crystal when injected from the crystal side. But if we inject from in-coupling mirror side, the beam waist should be located inside OPO. Therefore, to measure optical losses and flip OPO housing, we need to flip OPO and move it OPO by 54mm closer to 75mm lens.