Vicky, Sheila, Camilla, Dorotea, Naoki
We went to SQZT7 this morning with the new homodyne (73241). In the end we wer able to see decently flat shot noise and decent visibility. On the way, we ran into some difficulties that caused some confusion:
- There was a cable from the homodyne very close to the seed beam path.
- The beam on homodyne PDA (LO transmitted through BS).
- There is some seed leakage from the OPO path, even when that path is blocked by the flipper. This means that the shot noise is very noisy unless the flipper is blocked and the seed is also blocked on SQZT0.
- We were able to improve the visibility by moving the alignment of the lenses. This gave us better visibilty than we found just by adjusting the alignment of the steering mirrors.
- The half wave plate on the seed path is still in place, it is set to make the polarization vertical (checked with a PBS) which also results in a close to balanced power ratio on the two paths.
- After we were satisfied with our flat shot noise and visibility aroun 98% on each arm of the homodyne, we went to the control room. Then there was an alignment shift mostly in pitch that badly misaligned the seed beam from the homodyne. The ZM sliders and PSAMs values hadn't changed.
In the end we have flat shot noise, and a visibility of 98.5% measured on PDA (3.07% loss) and visibility of 97.8% (4.44% loss) measured on PDB. The nonlinear gain of 11 measured with seed max/ no pump. A comment to this alog will contain the measured sqz/asqz/mean sqz.
Screenshot summarizing homodyne measurements today. With measured carrier NLG=11 (for generated squeezing ~14.7-14.8 dB), we observe
Comparing sqz/anti-sqz to generated sqz: ~7% unexplained homodyne losses. This is consistent with our last estimate of excess HD losses (8/29/2023, LHO:72802, ~7% mystery loss). Since then, we swapped the HD detector and improved readout losses (visibility). We now measure more homodyne squeezing at 6 dB, consistent with expected loss reductions. That is compared to 8/29 (LHO:72802), we have less total loss, less budgeted hd loss && more squeezing, but the same unexplained hd losses as before.
Comparing mean-squeezing to generated sqz: could be consistent with sqz/asqz losses. I think there is a mis-estimate of the generated squeezing level from non-linear gain. If we ignore our NLG11 measurement, and instead use the generated squeezing level to match observed 13.5 dB of anti-squeezing, then we allow losses to determine the measured 6 dB squeezing level, we would have an NLG=10 (not 11) for a generated squeezing level of 14.5 dB. This would suggest 7% unexplained losses, same as the sqz/asqz measurements.
For ~7% mystery losses, this is compared to total HD losses of 21%, of which we budget 15% losses. From the sqz wiki, the budgeted losses are:
If we include phase+dark noise that degrades squeezing but is not loss, then 21% total loss can explain the 6dB measured squeezing, see e.g. from the gsheet calculator (edited to include ranges for NLG=10 and NLG=11):
DTT homodyne template saved at $userapps/sqz/h1/Templates/dtt/HD_SQZ/HD_SQZ_101023.xml .
Edited to include some history of homodyne measurements:
It could still be interesting to vary NLG to see if we can obseve any more squeezing, or if an additional technical noise floor (aside from dark noise) is needed to explain the NLG sweeps.
We revised the sqz loss wiki table again today, and are including it to explain what we think our current understanding of losses is.
It seems likely that the 7% extra losses we see on homodyne measurements are in HAM7, so we've nominally added that to the loss budget.
In addition to this, there would be an additional 8% loss on the sqz beam if we didn't correct it's linear polarization with a half wave plate. 72604 At the time of the chamber close out, (65110) we measured throughput from HAM7 to HAM5 that would implies that two passes through the OFI were giving us 97.6% transmission, so this is not compatible with the polarization being wrong by this much. We haven't included this as a loss in the loss budget because it seems incompatible with our measurement in chamber.
The wiki currently lists the OMC transmission as 92%, and the PD QE as 98%. The PD QE may be worse than this (see 61568), but measurements of the product of QE and OMC transmission for 00 mode seem to indicate that is in the range 90-92%, so this is close.
With the infered losses of from the measured sqz/anti-sqz in the IFO, the plausible range of losses is 30-35%, we are using 32%. With only known losses (including the values for OMC trans and PD QE), we have 14% unexplained loss. If we include the 7% apparent HAM7 losses, we have 9% unexplained losses in the IFO. This does seem similar to the 8% polarization problem, but it would also include SQZ-OMC mode matching.
Possible future scenarios: We may be able to reduce the 7% HAM7 losses, and we may be able to swap the OMC to reduce those losses from 92% to 97%.
These numbers come from the aoki equations that Vicky added to the google sheet here: gsheet
Don G. and Sheila have very likely resolved the homodyne polarization issue as being due to the SQZT7 periscope. So, the mis-polarization is likely not an issue for squeezing in the interferometer.
The sqz beam leaves HAM7 via reflection off the sqz beam diverter. From the latest CAD layout from Don, the outgoing reflected beam (blue) is ~75.58 degrees from global +X. The periscope re-directs the beam to travel along SQZT7, approximately along +Y. The CAD layout thus suggests that the SQZT7 periscope re-directs the beam in yaw (counter-clockwise) by an estimated 90 - 75.58 = 14.4 degrees.
From recent homodyne measurements LHO:72604, of the sqz light leaving HAM7 and arriving on SQZT7, ~8% of the power was in the wrong polarization, this calculates to a ~16.5 degrees polarization misrotation. Compared to this 16.5 degree misrotatation we were searching for, the 14.4 degrees polarization rotation induced by the periscope image rotation can plausibly explain the misrotation.