I'm aligning double pass AOM. After maximizing the power of the 1st order of the transmitted beam, I place the R=0.3m mirror to reflect the beam  back to the AOM.

The mirror is mounted on a translational stage for a fine adjustment.

At the right distance L away from the AOM(L = ROC), the size of the reflected beam at the AOM should be the same as the incoming beam.

Thus, there are 3 things to adjust.

First is the angle of the quarter wave plate that rotates the polarization of the beam after 2 passes by 90 degrees.

Second, the angle of the mirror, and

third, the distance of the mirror. At right position the power of the 1st order beam should be maximized.

I might have to change the position of the PBS that reflected the AOM double passed beam. Currently, the PBS is placed before 2 mirrors that move the

beam to the side of the table to avoid the insulation box. The problem is the double passed beam might clip on the mirror. So now I put the PBS after the steering mirrors, just in front of the AOM, but this limits the space for mode matching. I'll have to check which one will be better. From the attached picture,  two PBS's are placed on two possible locations. On the bottom right the, and down at the middle next to the AOM.