[Keita, Corey, Kiwamu]

The alignment of the POP paths in HAM3 are done. We removed the green fiber laser.
The next step is a picomotor test at HAM3 with the Beckoff system.

Forward propagating POP
 The POP path behind PR2 for the forward propagating beam (going from PRM to PR2) were successfully adjusted within a precision of ~ 1 mm or less at the QPDs. Comparing the alignment precision with the QPD diameter of 3 mm, we conclude that our alignment is good enough. This time the issue of the transparent BS [1] was mitigated by fully opening the aperture of the irises to make the HR beam as bright as possible. Luckily the HR beam became visible enough to work with. We tweaked the two steering mirrors to get the beam aligned on the QPD sled.   
 Actually we repeated this alignment twice. After the first round it turned out that the first steering mirror (a 3" mirror) which we established its alignment yesterday [2] gave a beam too low for the other POP path (i.e. backward POP path). The first steering mirror is relatively tall by design because it needs to be at the same height as the PRs and therefore the beam reflected by this mirror has to be angled downward such that the beam height gets lowered to the standard height of 4". Yesterday we adjusted the angle such that only the forward beam hits the center of its second mirror but apparently this wasn't a great idea. It resulted in such a low beam in the backward path due to the fact the backward beam propagates a longer distance than that of the forward beam by 6-ish inches. This too-low-beam was found after we removed the green laser setup at HAM2. Therefore we had to setup the green laser again and repeated the same alignment process. We adjusted the first mirror such that the beam is a bit too high for the forward path by 5-ish mm and a bit too low for the backward path by the same amount.


Backward propagating POP
 The backward propagating POP beam behind PR2 was aligned using the green fiber laser and we confirmed that the beam got through to HAM1. The same fiber laser was used to simulate the backward path. The laser was set up at HAM3 and injected at the front of PR2. We let the reflection go through the two irises. The position of the second steering mirror (2" reflector) was intentionally displaced toward east by ~ 7 mm in order to keep a large clearance for the downstream beam which passes by the west side of the mirror. Hence the beam spot on this mirror is also off by the same amount which shouldn't be a problem because the mirror is large enough and also the incident angle is small (about 10 deg ?). Although the green beam diverges a lot when it arrives at HAM2 we aligned the beam such that it passes by any of the towers in HAM2 with the same amount of the clearance. This resulted in a beam off-centered at the viewport in between HAM2 and HAM1 but this is OK. Some alignment of the optics at HAM1 will be performed later.


BS wedge on POP QPD sled
 This is a minor issue and we write this simply as a record here. We found that the BS on the POP QPD sled had its wedge at a wrong side. Ideally we want the fat side to be at the QPD side to let the ghost beam diverge away, but it seems we placed the BS in the opposite way --- skinny side at the QPD side. We checked the engrave mark on the barrel of the BS and found a "W" mark at the far side from the QPD, indicating that the fat side is at the far side and this is wrong. We are not going to fix this as the ghost beam will not hit the QPD anyway since the QPD is close to the BS.

[1] LHO alog 5774 "50-50 IR beam splitter is a good AR for green"
[2] LHO alog 5911 "Alignment of POP QPD path : ongoing"