Jennie, Sheila
Sheila and I looked at the steps Elenna changed the DARM offset through, the last time the contrast defect measurement was taken (see LHO #69361). To get an idea of how much light seen at the anti-symmetric port (calibrated in terms of power into HAM6) is insensitive to DARM motion I plotted the scaling of the light at ASC-AS_C_NSUM_OUTPUT with DARM offset changing (OMC-DCPD_SUM_OUTPUT).
| DARM offset (mA) (OMC-DCPD_SUM_OUTPUT) |
Power out of SRC (ASC-AS_C_NSUM_OUTPUT) (+/- 0.00945792) |
Time offset changed |
| 19.91 | 0.8655 | - |
| 6.760 | 0.8467 | 1367176162 |
| 10.62 | 0.8519 | 1367176288 |
| 15.15 | 0.8589 | 1367176413 |
| 20.82 | 0.8666 | 1367176538 |
| 27.17 | 0.8755 | 1367176663 |
| 34.15 | 0.8857 | 1367176788 |
Attached is the plot (pdf) of total power into HAM 6 versus the power that gets through the OMC.
The uncertainty in the AS measurement is was estimated using the y cursors on ndscope (shown in png).
0.837W of power is predicted for no DARM offset, and 82% of the light coming into HAM 6 is sensed by the OMC DCPDs.
We also tried to use the total power on OMC REFL to do a similar calculation of the light rejected from the OMC that is insensitive to DARM motion but due the noise on this signal we made need bigger DARM offset steps to see a clear trend.
Since this measurement, whitening has been implemented on the OMC REFL PD by Daniel.
Code is Plot_OMC_REFL_2023-05-12.ipynb in /ligo/home/jennifer.wright/git/OMC_mode_matching/
Figure is in /ligo/home/jennifer.wright/git/OMC_mode_matching/figures
In the SQZ loss budget, HAM6 losses are OM1 (99.3%) OM3 (98.5%), OMC transmission (95/7% measured before installation), and DCPD QE (98% in budget, should be 99%). This gives an expected HAM6 throughput of 92%, which means we are missing 11% loss, including OMC mode mismatch and any degredation of the OMC transmission or PD QE.
See 60885 and the comment above it for a similar measurement from LLO.
Just to clarify, that's 82% of carrier light that gets through the OMC from the input of HAM 6.
Jennie W, Sheila D
We reviewed some alogs and dcc documents about OMC losses, OMC cavity scans suggest that our OMC transmission has degraded from 96% to 92%.
Testing before installation results start on page 140: T1500060 The input output coupler transmission (T) is 7690ppm, 50ppm loss per mirror.
Finesse = pi/(1-r1*r2*rloss) (approximation) with r1=r2 = sqrt(1-T) and rloss is an amplitude reflectivity that represents all the cavity losses.
round trip loss = 1-[(1-pi/F)/(1-T)]^2
cavity transmission = T^2 * (Finesse/pi)^2
- October 2022: 65422 Finesse is 392, implies OMC transmission of 92%, and round trip loss of 653ppm
- April 2022: 64582 Finesse 395, implies transmission of 93.5%, and round trip loss of 530 ppm
- Testing document page 140: Finesse 399.7 implies cavity transmission of 95.7% and roundtrip loss of 342 ppm and
- Testing documents page 143: reports 4690ppm transmission of the input and output couplers, and summing the losses 284ppm, implies finesse of 401 and cavity transmission of 96.4%
I've updated the SQZ Loss wiki, if we assume the OMC transmission is 92%, our known IFO output losses become 13%, and our known squeezing losses become 19%.
Redoing the comparison above of Jennie's HAM6 throughput estimate (82%) to the 12% known HAM6 losses:
- OM1 99.93%
- OM3 98.5%
- OMC QPD 99.26%
- OMC transmission 92%
- PD QE 98%
Implies that we have 7% unknown HAM6 losses, which could be OMC mode matching.
Testing documents page 143: reports 4690ppm transmission of the input and output couplers, and summing the losses 284ppm, implies finesse of 401 and cavity transmission of 96.4%
Minor comment: This line should be identical to P.140. The transmission of the input/output couplers should be 7690ppm. Did I give you a link to an old document...?