To measure effectively the internal optical losses of OPO, we need to inject laser from the in-coupling mirror side (elog2784).

Therefore, I flipped OPO and moved OPO closer to 75mm lens as described in elog2790. However, I found that moving 54mm is far too much, whose reason is not understood yet. After moving OPO farther from 75mm lens, the mode matching situation is better. However, I found that the beam is going into the in-coupling mirror but close to the edge of its hole. So I loosed in-coupling mirror and moved it vertically down. After this, I found it diffcult to recover alignment at the first glance.

However, I realized that it is actually very easy to re-align OPO using transmission camera and injection/reflection beam overlaping (without using periscope/translation stage). The method is bascially the same with the procedure provided in elog2783 apart from different beam parameters and almost not seperating the alignment of crystal and in-coupling mirror but aligning them as almost a whole. And I found it is possible to choose a personal preferred position of in-coupling mirror and just fix it with the crystal side of OPO. Then align them as a whole.

After this easier alignment procudure, checking by eye, I found the laser is far enough from the edge of OPO two sides holes. Then I aligned OPO until mode matching level is around 95%. Now OPO is ready for internal optical losses measurement.