We've done three measurements assess OMC loss, but we've found that there's something weird about DCPD transient response (will attach another alog, but it seems as if something is railing inside the chamber, or maybe the OMC DCPD inductor is responding nonlinearly causing some kind of soft saturation, or maybe it's just a whitening-dewhitening mismatch).
Because of this, Measurement 1 below is suspect, Measurement 2 is definitely OK, Measurement 3 is probably OK too.
Analysis will come later.
Throughout the measurement, RF sidebands (9, 45 and 118) were OFF, H1:OMC-DCPD_A_GAINSET and B were set to low (a factor of 10 smaller than nominal), and IMC-PWR_IN was 10W.
Measurement 1. Scan OMC PZT and measure the MM loss.
Lock OMC, align OMC reasonably well, unlock, scan the PZT slowly. The best scan is between 19:26:25 and 19:34:44 UTC (t=[-31m,-23m] in the 1st attachment).
"Align reasonably well" was a challenge as we were using the OMC QPD for OMC ASC, changing alignment meant changing QPD offset, and doing so frequently railed OMC suspension. While changing the offsets, maximum DCPD_SUM I could reach was 16.9, but I kept bumping OMCS so I gave up (sensor correction was off for the first half of this effort and that didn't help). In the end, usable data was obtained with default offset that gave us ~16.6 when locked, but we know that that was NOT the best alignment.
As you can see, the peaks in DCPD_SUM during the scan are all about ~14, much smaller than 16-something. (At t=-36m, the OMC was held at resonance and DCPD_SUM was ~16.6. After the scan at t=-20m, DCPD_SUM was ~16.45. So the alignment drift wasn't much of a problem.)
It turns out that we had to scan slower than this even though this was REALLY slow (~8 minutes for one cycle) and/or lower the laser power.
Measurement 2. OMC throughput.
Lock the OMC to 00 resonance (19:37:27-19:38:28 UTC, roughly t=[-20m, -19m] on the 1st attachment).
Measure DCPD_SUM, input power (via ASC-OMC_A and ASC-OMC_B SUM) and reflected power (via OMC-REFL_A).
With the same alignment into OMC, find the time where the OMC was off-resonance (DCPD transmission was minimal 19:25:58-19:26:20 UTC), and measure DCPD_SUM, input power and reflected power.
Calculate the throughput.
Measurement 3. OMC Finesse
I roughly kept the OMC at resonance by adjusting pzt voltage. DCPD_SUM was slowly drifting but it was about 16.0.
I started injecting into PSL frequency with an amplitude of ~+-600kHz (1.2MHzpp) via IMC-L_EXC. I slowed down the injection frequency until the peak value gets back to 16.0. (Second attachment.)
Best scan data is obtained 19:56:20-19:57:20 UTC.
OMC DCPD transient response
Let's see the 2nd attachment of the above alog (which is attached again).
At t=[-2m40s, -2m20s], nothing is railing at ADC. You can tell that as ADC saturation means that OMC-DCPD_A_STAT_MAX and/or OMC-DCPD_A_STAT_MIN hit +-512k (MAX only went to 120k and MIN only went to -80k in this case).
However, note that the DC value for MAX and MIN during the time OMC was kept on resonance were about 48k and 47k, respectively. The fact that MIN went to negative 80k and MAX went to positive 120k means that the transient response was huge.
Even though it was huge, if nothing was railing nor saturating, you'd still expect that the peak height in DCPD_SUM is the same as when the OMC was kept on resonance, but clearly that's not the case. At first when the scan was fast the peak value was maybe 60% of what it should have been, and as I slowed down the scan the peak gradually went back to ~99% or so.
In the case of the PZT scan, we're talking about the slow velocity where each 00 peak is ~0.6sec wide (and that was too fast, we should have reduced the laser power, anyway see 2nd attachment).
Where does the mismatch come from?
A simple whitening-dewhitening mismatch? Is something railing inside the chamber, or maybe soft-saturating because of large transient (e.g. big coil?). I think we need a help from Hartmut.
I'm using Keita's times from the OMC visibility measurement above to runthe script described in 73873, and using the dark offset times from that alog as well. This time includes more higher order mode content, and slightly higher overall efficiency, than the time in 73873, which is somewhat confusing.
Results:
Power on refl diode when cavity is off resonance: 22.764 mW
Incident power on OMC breadboard (before QPD pickoff): 23.207 mW
Power on refl diode on resonance: 2.081 mW
Measured effiency (DCPD current/responsivity if QE=1)/ incident power on OMC breadboard: 82.7 %
assumed QE: 100 %
power in transmission (for this QE) 19.187 mW
HOM content infered: 8.979 %
Cavity transmission infered: 91.712 %
predicted efficiency () (R_inputBS * mode_matching * cavity_transmission * QE): 82.676 %
omc efficency for 00 mode (including pick off BS, cavity transmission, and QE): 90.832 %
round trip loss: 685 (ppm)
Finesse: 392.814
assumed QE: 96.0 %
power in transmission (for this QE) 19.986 mW
HOM content infered: 9.099 %
Cavity transmission infered: 95.660 %
predicted efficiency () (R_inputBS * mode_matching * cavity_transmission * QE): 82.676 %
omc efficency for 00 mode (including pick off BS, cavity transmission, and QE): 90.952 %
round trip loss: 348 (ppm)
Finesse: 401.145
Tagging
- CAL (because of the suggestion that there's a mismatch between analog whitening and digital compensation for it [I doubt it]),
- CDS (because Ali James -- who was Hartmut's student that built up the in-vac transimpedance amplifier -- now has been hired as our newest CDS anlaog electronics engineer),
- DetChar (because understanding this transient behavior may be a clue to some other DetChar / GW channel related transient features, since Keita refers to studing *the* DCPDs -- the OMC DCPDs).