[Keita, Corey, Kiwamu]
We worked at HAM2 and HAM3 to get the POP QPD path aligned and this is ongoing.
We have set up a fiber-coupled green laser with a telescope at the stage by HAM2. This laser mimics the actual forward infrared beam (going toward PR2 from PRM). Since the laser launching setup is on the stage which is not quite stiff we found that the injection angle of the laser can be statically misaligned when a person stands on the same stage (typically a few mm shift of the beam spot at the iris in HAM3).
After tweaking the green laser and a temporary in-vac steering mirror to get it through two existing irises (one at HAM2 and the other at HAM3), we moved on to alignment of the optics behind PR2. The position of the first steering mirror (3 inch mirror) was corrected such that the green beam hits a point which is 9 mm off from the center of the mirror in yaw. This is because that this mirror is supposed to catch both forward and backward POP beams who are separated by 18 mm from each other in yaw. Then we started tweaking two steering mirrors to let the beam go in to the QPD sled. An issue at the moment is that the green light is too dim on one of the QPDs and this is a known issue : the HR surface of the pick-off-BS is extremely transparent for 532 nm [1]. We tried a half-wave-plate to make the HR beam brighter like we did at EY [2], but this didn't help. We will try aligning them using the AR beam since the AR beam is bright enough to work with (although we have to do a math to predict the beam position of the HR beam from that of AR).
[1] LHO alog 5774 "50-50 IR beam splitter is a good AR for green"