Michael and Yuhang

Today, we have successfully closed OPO cavity. The OPO no.28 component issue is solved by replacing it with a longer screw.

After cavity closed, we optimized OPO cavity alignment relative to its incident beam. However, we found the scan spectrum is as Fig.1.

1. We can see that there are apparently two higher order modes in the scan spectrum. By scanning OPO cavity with 10mHz ramp and looking at a designated camera , we found that the higher order modes are second-order Hermit Gaussian modes. (The cavity scan video is here https://drive.google.com/file/d/1Od2jI40D_3oZuZksuI474tunwtt7znIs/view?usp=sharing , note that it seems cavity mode is not centered in a circle. But this circle is just a mirror. The cavity mode looks to be well centered when mirror is removed.)

2. We can also see that there are modes overlapping with TEM00 mode. It indicates that we are having a cavity length which offers a non-ideal Gouy phase.

3. When we misalign pitch or yaw of the incident beam, we found the first order mode appears but the existing second order mode decreases. This is wired for me.

4. We have identified a potential beam clipping in the optical path before OPO. After fixing the clipping issue, we didn't get better OPO scanning spectrum.

5. We suspect that we may fix either crystal or in-coupling mirror too tightly, so that mirror has distortion and introduces higher order modes. After loosing a bit the screws, we didn't see improvement.

6. We suspect that there maybe some contamination. Especially, the crystal was in air for several months. But we are not sure if contamination is really true because the cleanroom in ATC is designed to be a class 10 cleanroom.

Personally, I think we need to open OPO again and adjust maybe crystal position.