victoriaa.xu@LIGO.ORG - posted 22:58, Wednesday 14 September 2022 (64982)
HAM7 today- retro+QWP installed, B:BS1 splitting ratio depends a bit on polarization (p-pol will cause overestimation of ifo power)
Camilla, Sheila, Vicky
Summary: In HAM7, we aligned the retro-reflection mirror + QWP before ZM4, and varied the return beam's polarization between s/p-pol. This let us check that s/p-pol IFO light dump as expected on the VIP. Rotating the waveplate, we found that OFI_PD_A reads 20x higher for p- than s-pol, which may have been confusing us to overestimate IFO power in HAM7.
This afternoon, we:
- Placed retro mirror just before ZM4 (within 1 foot of ZM4, we estimate ~1.25m from SFI2)
- @ SFI2, we see the retro is within 1-2mm of forward going beam, with misalignment looking mostly in yaw.
- With a card at OFI_PD_A (see picture), looks mostly off in yaw, maybe 1-2mm offset from center of PD? Cannot tell yet if we'll need to move the PD, it might just be our retro-alignment is off. We will check more tomorrow, and bring the IR camera.
- Then we placed the quarter wave plate (QWP) in front of the retro-mirror, and began checking how the return s- and p- pol lights are dumped on the VIP, following SFI2.
- We see beams dumping as expected. Sheila rotated the QWP while Camilla watched spots be minimized on an IR card, at the 2 +X-side apetures from SFI2.
- We found rotating the QWP changes power on OFI_PD_A by a factor of ~20x. We're taking this as the pickoff B:BS1's splitting ratio is actually a bit polarization-dependent, and seems to transmit ~20x more p-pol than s-pol.
- This means if IFO/OFI sends us p-pol light, the OFI_PD_A pickoff ratio is underestimated ~1%, and causes us to overestimate IFO power in HAM7. IFO is supposed to send us s-pol from the OFI TFP, but imperfections in this TFP can send us p-pol too. Makes sense that we would receive p-pol, as the AS port sends ~600mW of p-pol light into HAM6.
Because it's confusing, here's tabulated how s- and p- polarizations are dumped after SFI2 on the VIP. See also Camilla's nice drawing.
| from IFO | Dumped where after SFI2 | @ OFI_PD_A | B:BS1 (E2000450) |
| s-pol | dumps @ apeture #2, further = B:L1 | min @ 0.005 | Spec'd R=99% for 45-deg AOI, s-pol |
| p-pol | dumps @ apeture #1, closer = unlableled empty lens mount | max @ 0.1 | reflects less, transmits more |
- IFO s-pol dumps at the 2nd apeture (B:L1) and min's OFI_PD_A @ 0.005.
- IFO p-pol dumps at the 1st apeture (unlableled, empty lens mount) and max's OFI_PD_A @ 0.1.
To-do's tomorrow include 1) look at VIP and OFI_PD_A with IR camera, and 2) determine if we need to move OFI_PD_A.
Images attached to this report
