Eric, Naoki, Vicky, Jennie W, Sheila
We did SQZ OMC mode scan with cold OM2. The PSAMS is 175/100 (strain voltage 8.9/-0.69). The attached figure shows the result (ref 34). The SQZ OMC mode matching is 0.64/(0.64+2*0.03)~91.4%.
We followed the instruction in 74892. Here are some notes.
- We increased the seed power to 76mW and requested the OPO guardian to LOCKED_SEED_DITHER and FC guardian to MISALIGNED. The OPO_IR_PD_DC_POWERMON reached only 0.3mW while it was 0.89mW in 74892. We noticed that the seed was locked on the higher order mode. Vicky fixed the OPO guardian and the seed was locked on TEM00. The OPO_IR_PD_DC_POWERMON reached 1.26mW.
- We opened SQZ beam diverter and requested SUS_SR2 guardian to MISALIGNED (SUS_SRM guardian is ALIGNED). After we engaged DC3/4 centering loops, ASC-AS_{A/B}_DC_NSUM_OUT16 are ~75 and ASC-AS_C_DC_NSUM_OUT16 is 0.0009.
- We manually locked OMC. We turned on input and output switch, increased the gain to 1, turned on boost and integrator.
- For rough OMC alignment, we turned on OMC ASC by increasing the OMC ASC master gain "one click up" to 0.025. After that, we decreased the OMC ASC gain to 0 and manually aligned the OM3/OMC to maximize the OMC-DCPC_SUM_OUTPUT. After the manual alignment, OMC-DCPC_SUM_OUTPUT is 0.63.
- For OMC scan, we set the OMC-PZT2_OFFSET to -50 and ran the scan template in $(userapps)/sqz/h1/Templates/dtt/OMC_SCANS/April9_2024_PSAMS_OMC_scan_coldOM2.xml
PSAMS 175/100, cold OM2
- LOCKED: 1396722738 (2024-04-09 18:32:00 UTC) + 60 seconds
- UNLOCKED: 1396722978 (2024-04-09 18:36:00 UTC) + 60 seconds
- Mode-scan (ref 34): 1396723292 (2024-04-09 18:41:14 UTC) + 200 seconds
DARK
- DARK: 1396723968 (2024-04-09 18:52:30 UTC) + 60 seconds
I used my code to fit the scan and C02 peak for this scan. As we saw with the PSL beam the new OMC means we cannot resolve these two peaks.
But if I assume one is buried in the noise and do the fit anyway we get (0.027 - 0.0038)/(0.027 + 0.64 - 0.0038) = 3.5% mode-mismatch for the squeezed beam matching to the OMC, with a fitted HWHM of 0.33 MHz.
NB: The factor 0.0038 mA is what I had to add to the scan data to shift the baseline to be above zero otherwise the fitting doesn't work well.
If I just use the measured height of C02 from the full scan (which is like assuming both C02 and C20 are overlapped) the mode-mismatch would be (0.029 - 0.0038)/(0.029 + 0.64 - 0.0038) = 3.8 %.
The OMC (001 is the current) parameters are given in this alog by Matt Todd, this gives a value for the half-width at half-maximum of 0.31MHz which matches our fitted one fairly closely.
The C02 peak graph (second image) has the OMC scan data in blue, fit in red, and initial guesses for the fit in purple.
Looking at this SQZ-OMC mode scan (data, mode scan), and running the same scripts as before (e.g. LHO:70866), this loss estimate based on OMC visibility using the squeezer beam doesn't really make sense. Hopefully PSL scans will make more sense. May be worth re-taking this squeezer measurement sometime.
Mode-matching and visibility make sense: Single-bounce SQZ-OMC mode mismatch ~ 4% based on TEM02/20 is consistent with Jennie's peak fitting, HOM content ~ 9-12%, and OMC locked/unlocked visibility = 1-locked/unlocked ~ 90%.
Loss estimate does not make sense: the inferred OMC transmission is ~72%, and if we ignore mode mismatch (ie assume 100% matching) this corresponds to an OMC transmission of 81%. But ~20% omc loss is ruled out by observed squeezing levels.
5.4 dB SQZ corresponds to a total loss of 20-25%, see params below including 12.3% expected sqz losses. This limits OMC losses < 10-15%, if assuming no mode-mismtch. With mode-mismatch, squeezing suggests OMC losses << 10%. This is not saying much, except this measurement is off.
- 5.4 dB of kHz squeezing, e.g. LHO:77073
- 5 - 25 mrad phase noise (Nutsinee's < 5mrad recent estimate from in-loop noises LHO:77124 & O4a 20-25 mrad phase noise from DARM NLG sweep LHO:74318)
- NLG ~ 17.2 (many measurements @ 80 uW green trans)
- ~12.3% known SQZ losses, excluding OMC losses and mode mismatch. This is known sqz injection losses of 7.3% + known readout losses, excluding OMC losses, of 5.4%.
---> 1 - (1-.073)*(1-.054) = 12.3% sqz loss, excluding OMC losses and all mode-mismatch ( O4 SQZ wiki, gsheet )
So squeezing rules out the ~20% omc losses inferred from this sqz-omc visibility measurement. I'm not sure what is exactly is wrong, but in the mode scan, there was considerable TEM01/10 misalignment despite running OMC ASC + manual alignment. Unsure if alignment is the issue, or maybe something wasn't right with the squeezer beam, such that alignment couldn't improve (worth checking the beam round-ness on sqzt7?).
Script output below (code in git). Running this and Sheila's script in LHO:70866, I get the same output. So, the script seems OK, but something else is wrong with this measurement.
--------------- er16 sqz ------------------ processing measurement for er16 sqz P_Refl_on_res*1e3 = 0.12831 mA P_Refl_off_res*1e3 = 1.00626 mA Trans_A*1e3 = 0.62987 mA (I assume OMC-DCPD_SUM_OUTPUT is calibrated into mA with the new OMC, I did not check this calibration.) removed trans blocked of -0.00332 mA Power on refl diode when cavity is off-resonance: 1.006 mW Power on refl diode when cavity is on-resonance: 0.128 mW Trans power when cavity is on-resonance: 0.734 mW Incident power on OMC breadboard (before QPD pickoff): 1.026 mW Measured efficiency (DCPD current/responsivity if QE=1)/ incident power on OMC breadboard: 71.546 % Using 4.0% mode-mismatch from Jennie peak fitting, visibiilty_00*100 = 90.885% assumed QE: 100 % power in transmission (for this QE) 0.734 mW HOM content infered (like mode matching): 11.964 % Cavity transmission infered: 82.057 % predicted efficiency () (R_inputBS * mode_matching * cavity_transmission * QE): 71.546 % omc efficency for 00 mode (incl R_inBS * cavity_transmission * QE, no mm): 81.270 % round trip loss: 1606 (ppm) Finesse: 372.613