Sheila, Daniel, Vicky
On Tuesday, Sheila and I took some beam profiles of the squeezer pump beam after the pump AOM used for ISS (GAOM1). This is trying to understand and fix the squeezer pump ISS issues that result from GAOM1 becoming misaligned on SQZT0.
Summary and proposed plan: the vertical beam profile we measured looks reasonable, the horizontal profile looks off (potentially related to measuring after the AOM). We could move GAOM1 closer to SHG by 1-2 holes to get closer to the beam waist of ~100um that is about 1.3m after SHG. While GAOM1 is out, we could re-measure the horizontal beam profile to make sure it is OK.
Here are plots of the pitch beam profiles we took before, and the very quick a la mode plot code that adapts Sheila's and Georgia's previous codes. I think the vertical beam profiles we took line up with Georgia's previous measurements and D1201210, which both say there should be a waist like 1.25m - 1.3m after SHG. In real life, we measured from z=0 at the EOM mount edge closest to GAOM1, to the face of the beam profiler (may need to take into account beam profile distance..).
So it seems possible that GAOM1 is like 0.02-0.05 meters (~2 inches?) downstream of the waist. From photos and SQZT0 layout D1201210, there is room to try moving GAOM1 closer to L15 (closer to SHG) by 1-2 holes.
- At LLO (example photo), GAOM1 misalignments are not a pump ISS problem - and their GAOM1 is also several holes closer to L15 than LHO's (which is what these profiles suggest to try).
- Kinda ignoring the horizontal beam profiles here since we measured after GAOM1, and the beam's yaw profile was not fitting well while measuring. The pump beam after GAOM1 looked kinda double-lobed with some smaller outer fringes even with the AOM drive at 0V, where it should be "passing everything" and not diffracting, but maybe there was some RF leakage.. The profiler's fitting function was fitting both lobes at once and being thrown off by the fringers, so likely over-estimating the waist in yaw (blue). Worth checking the beam profile without the AOM in place.
Useful DCC references:
- SQZT0 layout D1201210 and mode-matching document here: see pg.4, should be a waist of w0~100um at z~1.3m from SHG.
- SQZT0 Green AOMs for PUMP + FCGS paths: E2000561. GAOM1 for pump ISS is M1250-P200L-0.75, a 200 MHz AOM with aperture of 0.75 mm. Updated +200MHz label on SQZT0 medm screen. GAOM1 has a visibly smaller hole than the other AOMs.
On Friday, Sheila and I measured some beam profiles of the SHG output path without the pump AOM and EOM, then moved GAOM1 ~1" closer to SHG than before (as we wanted to try: photo), and finally re-aligned through GAOM1 and the pump fiber. Overall, the squeezer pump ISS is working and ready.
With GAOM1 an inch closer to SHG, maybe GAOM1 alignment got smoother than before. We have an alignment that can optimize both 0-th order throughput and diffraction efficiency, which seemed problematic in recent realignments (eg 80266, 79993, 78519). With 43mW into GAOM1, we transmitted 40mW at 0V (so 40/43 = 93% throughput), while being able to diffract ~15mW at 5V (28.5 dBm), so ~35% diffraction efficiency. Seems ok-good compared to recent re-alignments.
On Friday, with 20 mW launched into the fiber, we had about 2 (BS50:50) * (2.2mW (opo_refl unlocked) + 1mW (opo_refl_rejected)) = 6.4mW through the fiber, so 6.4/20 ~ 32% throughput even with ~0.1mW mis-polarized light.
Given the questionable yaw beam quality when no parts are installed on the SHG output path, and the dependence of yaw beam profile on SHG temperature, it might require work on SHG to totally solve the beam quality issues. Unclear that further mode-matching adjustments will fix it.
- First attchment here plots all the beam profile measurements from this round (friday and tuesday). Same as Tuesday. Pitch looks good. Yaw does not look consistent with pitch, and not quite gaussian either.
- See photos of the beam profiles in the second attachment here: the yaw profile degrades as we go further from SHG, while the pitch profiles look consistently good. Yaw profile looked bad even with no EOM or AOM installed, so it is a bigger issue. Could be related to SHG problems, since the yaw profile depends so much on SHG temperature. After adjusting the EOM alignment, the yaw beam looked more single-peaked after the EOM, but was still not a great gaussian (see photos), and it got worse as the beam propagated farther from SHG.
Update from Monday: Camilla and I went to SQZT0 and touched up pump fiber coupling. Also touched the alignment on pump GAOM1 to check.
- GAOM1: with 47mW into the AOM, measured 40mW after the AOM (0V, ~85% throughput), with 5V diffracting 14.4mW into the 1st order mode (~35% diffraction). Slightly worse than Friday, but touching alignment didn't improve it (original alignment today was already the best).
- Pump fiber: improved OPO_REFL from 1.7mW to 1.95mW.
