Yokozawa, Akutsu
Summary
Following 28776. Checked the IR beam way to PRM and back to REFL; no big issues found so far but (1) off-centered beam spot at PRM AR, and (2) REFL beam might be mis-aligned. Now laser is S-pol, and about 1 W to IMC, so do not try to let the IMC resonate.
Preparation with S-pol high power
After putting a screen in the IMC optical axis to prevent occasional IMC resonance, I removed the tentative HWP installed on 28402 in the PSL room to get back the input polarization from P to S. At the same time, the MCE flanges were covered again by Yokozawa-san. As reported in 28402, the IR power is 70-80 mW. Despite with this power, we could see beam spot up to PRM, but it was so faint at PRM. So we determined to increase the input power to aout 1 W but rotating a HWP (of course this is different from the HWP removed above) in several degrees.B
Beam at IMMT2
The nominal beam height in IFI chamber is 210 mm from the top surface of the optical table (Fig. 1). The beam reflected from IMMT1 seemed on this reference (Fig. 2). It was hard to set a vertical line on IMMT2 as it's HR edge was covered by the beam shield, so I feelingly checked the yaw offset on IMMT2 and may be ok so far. Note that IMMT2 reflects the beam to PRM in pitch up about 1/300 (Fig. 3). Then we set the dedicated beam target (as was before O4a) to PRM AR.
Beam at IMMT1
Looked back IMMT1 then. The height of the beam in the IMM chamber was checked with a green laser level. Note that the optical table in the IMM chamber is about 3 mm heigher than the others for some reason (Fig. 4; also see 16946). Anyway, the beam from IFI to IMMT1 seemed on this reference line (Fig. 5). In the same reason as above, we omit to set a vertical reference to IMMT1. Maybe we can use IMMT1T QPD, which did not lost the beam even after the Noto earthquake (see 28170).
PRM
Referring to the PRM AR target, the beam spot seemed off-centered in yaw about a few mm in the minus Y direction (Fig. 6). In pitch, it seemed matched to the reference line on the target plate. Note: to confirm the optical axis fully, we need to check it also at PR2, but today it was not possible. IMMT2 centering or IMMT2 yaw might be the cause of this mis-centering. We called ALIGNED to IMMT1 and 2 but the situation did not change so much.
Anyway, for our sake today, it would be sufficient with this accuracy of the "centering". So we did not tweak any optics, and removed the target. Then we called ALIGNED state to PRM. Then we confirmed that the reflection beam from PRM coming back to POM1 through IFI; good news; thank to the robust oplev.
POM1 in MCF
We confirmed that this reflected beam hit around the very center of POM1 in the MCF chamber. Very good.
POM1 to POM2 and REFL
We found the beam from POM1 was a little bit off centered at POM2 (Fig. 7); it seemed shifted plus X direction, though still within the POM2 effective aperture. Then we found the beam reflected at POM2 over the REFL table. At the first periscope and the following iris diagphram, the beam was consistently a little bit off-centered in the plus X direction (Fig. 8). Maybe due to this mis-alignment, neither REFL camera nor REFL PD show responded.
If simply adjusting POM1 and/or POM2 to recovering the beam on the REFL table, a beam dump would fail to catch the ghost beam due to the REFL high-quality viewport window, so we need to consider the way carefully to resoleve this issue.
Note and todo
- Take care of the beam mis-centering in yaw at PRM-AR. Need to pass the beam to PR2.
- Take care of the beam mis-alignment at REFL table as described above. To check the REFL table, maybe we need to remove the tube connecting the IFI chamber and REFL table.
- By the way, Miyakawa-san's camera coud see the PRM AR target (through the mid baffle) together with IR beam spot, but unfortunately it was difficult for me to take a nice photo due to camera shakes (tebure); Fig. 9... please help me some photograph experts!
