aLIGO LHO Logbook

H1 SQZ

camilla.compton@LIGO.ORG - posted 11:11, Thursday 06 May 2021 - last comment - 09:44, Friday 07 May 2021(58875)

VIP alignment - B-path update, light through SFI2

Yesterday, Keita and I continued aligning SFI2, Sheila describes start in alog 58707. Summery of VIP status is alog#58724. Drawing linked here.

Aims: to align through SFI2 backwards (injecting light from off table to B:M4) to get light through both SFI2 iris (DONE), align the wedge polarizer B:P1 (next-step).

We started by making sure the beam was parallel to the table by adding in two dirty adjustable iris on clean sacrificial bases. One before B:M4 and one after B:M3. Nominal beam height is 2.5" from VIP platform. Some adjustment of input optics (fiber collimator and mirror that is off the table) and B:M4 to get correct height.
We has previously been using p-pol light, using a beam splitter off the table. As the optics are for s-pol it created 4 beams at SFI2 from back reflections off B:M4 and B:M3. We maximized s-pol and removed the BS to have a stronger beam that's easier to work with. Will later check that we have the correct beam.
The iris on SFI2 is hard to see so Keita set up a camera to help us, this worked great!! We temporarily removed a beam dump screw to get a better view using the camera.
Aligned beam onto iris using input fiber and B:M4. No light transmitted through SFI2.
We scanned using B:M3 horizontally and vertically while monitoring output. Nothing.
We checked position of beam on B:M3 to have incoming angle to SFI1 of 16deg using trig. We were 8-10mm horizontally too far away. Walked beam using fiber input to correct position. Note beam is now very close to edge of optic as we didn't change B:M3 position. Now had light though SFI2 but as before it was coming OVER iris.
Adjusted B:M3 and fiber pointing to get beam down through iris.
Moved B:M2 to shoot beam off table onto photo-detector and increased power in beam by adjusting B:M3 and fiber input.
Noticed black glass beam dump on B:M4 isn't dumping all the light so we moved a dirty iris to use as a temporary beam dump. Will change orientation of B:M4 black glass beam dump to be closer to optic once aligned. May have to do this with others too.

Now we have the max light going through SFI2, next steps: align wedge B:P1 and check we have correct polarization of beam, add in B:L2. Then once new fiber arrives, align the nominal way from A-path.

Comments related to this report

camilla.compton@LIGO.ORG - 09:44, Friday 07 May 2021 (58893)

Link

Image showing beam path we are using attached.

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camilla.compton@LIGO.ORG - 09:20, Friday 07 May 2021 (58892)

Link

Yesterdays work on aligning polarizing wedge B:P1 following procedure from p22:

As we adjusted the pointing of fiber significantly yesterday to get beam through SFI2, started by checking beam height after B:BS1 using iris and camera and adjusting input steering. Repeated for beam height after SFI2 then maximized power coming through SFI2 with B:M3.
Added B:L2 and repeated above alignment. Note, we haven't got final mode matching data for position of B:L2 yet.
Checked B:P1 roll alignment by measuring how parallel beam is to table with and without B:P1. If roll is well aligned, beam height should not change. See table below, beam height changed 1/16th of inch over 1.5 meters when B:P1 removed. We think this is aligned enough.

	Beam height just off VIP	Beam height at edge of optical table (~1.5m away)	Difference
B:P1 removed	3 15/16 "	3 27/32 "	- 3/32 "
with B:P1	3 31/32 "	3 15/16"	- 1/32 "

Next step is to check B:P1 pitch and yaw alignment using AR reflected beam. Problem: we can't see an AR beam.
- We increased the power before the fiber the wave plate to give us 30mW of power coming out of the fiber and onto B:M4 (note we are loosing up to half power by the fiber coupling).
- Still no AR beam. Specs estimate AR reflectivity of wedge to be 100-250ppm.
- Aimed beam off table without wedge and measured power of 240μW after SFI2. So best case we are looking for 60nW from AR of wedge. This is why we aren't seeing AR beam!
Question: Where is all our power going? Dropping from 30mW to 0.24mW through 2 HR mirrors, one 99% BS and the SFI2.
- Our beam is much too big entering SFI2?
- We are seeing a surface reflection from inside SFI2 and have aligned to that rather than the real beam?