Michael and Yuhang
We have reported that BAB was misaligned for filter cavity.
To increase BAB power, we are amplifying BAB with OPA. The pump phase is scanned with frequency around 100Hz and amplitude of 1V signal sent to high voltage driver.
At the beginning, we were trying to find where the IR beam was located by using camera. Then we found we can see from camera but the position where IR beam hits is not clear and depends on where we put camera. Also we tried to use IR viewer to find IR inside PR chamber, it didn't work either. So we can use camera to check, but this is not really helpful.
On the other hand, we should rely on the reference points outside the PR chamber window. We found we could easily recover the IR beam for these reference points. However, we found reflection was extremly weak (less than 1%).
To maximize IR reflection, we followed the method we used three years ago. We put a sensor card in reflection and set up a camera to monitor the beam on sensor card (attached photo 1 is setup, attached photo 2 is image from camera). By doing alignment with two mirrors, we found reflection power cannot be increased a lot. In addition, after this alignment, we made IR go away from reference points. We also made IR hits almost the edge of last bench mirror. This means we are not going to the good direction.
So the starting point, where IR beams hit on both reference points, should be quite close to good injection. If we can rely on this, the only problem we are having should be inside vacuum chamber. The IR mirror equipped with picomotor before GR/IR overlap is the dichroic mirror. Actually this is the place where IR and GR overlaps, but the tilting of this mirror doesn't affect GR. So we think we should do like this.
1. Align the last two steering mirrors on bench to hit on PR chamber IR references as good as possible.
2. Move yaw of dichroic mirror while checking the first target inside arm and reflection together.
3. In the end, we should see: (1)IR/GR overlap on the arm first target (2)reflection power increase while reflection reaching nominal position
