Yuhang and Michael

It was found that after flipping OPO, the alignment of OPO got worse and was hard to recover.

Today we checked again the alignment of OPO. We conclude that it is certainly that the flipping of OPO makes alignment worse. This is because that the mode matching condition is very much different depending on which side the laser enters OPO.

It was always questioned how we can make sure a good internal alignment. In fact, the alignment procedure of OPO is not so much different from our filter cavity. The only difference is that we cannot control the angle of OPO reflection surfaces as easily as the filter cavity mirrors.

For OPO input surface, we adjust injection beam to make injection and reflection overlap. But at the same time, we need to make sure the crystal transmission has a good shape.

For OPO end surface, we adjust input-coupler surface position to find interference.

At this step, the alignment is not optimized but we need to fix the input-coupler. Then we need to optimized mode matching. When optimizing mm, we need to pay attention to which lens is more sensitive. Pay attention also that after moving a lens, we basically just need to move the tilt of one steering mirror to recover alignment and compare the new mode matching HOM to understand if the mode matching is getting better or worse. 

After optimizing mode matching, if there is still misalignment. (It was found that misalignment level is different when optimizing mode matching) We need to adjust input-coupling mirror.

The attached two figures show two situations: 1. after optimizing mode matching, before final alignment of input coupling mirror 2. after final alignment of input coupling mirror

There is still space to further optimize OPO alignment. Note that the OPO transmission power is increased in Fig.2.