Jenne, Naoki, Louis, Camilla, Sheila
Here is comparison of the DARM CLEAN spectrum with OM2 hot vs cold. The second screenshot shows a time series of OM2 cooling off. The optical gain increased by 2%, as was seen in the past (for example 71087). Thermistor 1 shows that the thermal transient takes much longer (12 + hours) than what thermistor 2 says (2 hours).
Louis posted a comparison of the calibration between the two states, there are small differences in calibration ~1% (74913). While the DARM spectrum is worse below 25Hz, it is similar at 70 Hz where we in the past thought that the sensitivity was worse with OM2 cold. From 100-200 Hz the sensitivity seems slightly better with OM2 cold, some of the peaks are removed by Jenne's jitter subtraction (74879) but there also seems to be a lower level of noise between the peaks (which could be small enough to be a calibration issue). At high frequency the cold OM2 noise seems worse, this could be because of the squeezing. We plan to take data with some different squeezing angles tomorow and will check the squeezing angle as part of that.
So, it seems that this test gives us a different conculsion than the one we did in the spring/summer, and that now it seems that we should be able to run with OM2 cold to have better mode matching from the interferometer to the OMC. We may have not had our feedforwards well tuned in the previous test, or perhaps some other changes in the noise mean that the result is different now.
Is this additonal nosie at low frequency due to the same non-stationarity we oberved before and we believe is related to the ESD upconversion? Probably not, here's why.
First plot compares the strain spectrum from two times with cold and hot OM2. This confirms Sheila's observation.
The second and third plots are spectrograms of GDS-CALIB_STRAIN during the two periods. Both show non-stationry noise at low frequency. The third plot shows the strain spectrogram normalized to the median of the hot OM2 data: beside the non-stationariity, it looks like the background noise is higher below 30 Hz.
This is confirmed by looking at the BLRMS in the 16-60 Hz region for the two times, as shown in the fourth plot: its higher with cold OM2
Finally, the last plot shows the correlation between the ESD RMS and the strain BLRMS, normalized to the hot OM2 state. There is still a correlation, but it appear again that the cold OM2 state has an additional background noise: when the ESD RMS is att the lower end, the strain BLRMS setlles to higher values
Here is the same comparison, without squeezing. Using times from 74935 and 74834
This suggests that where cold OM2 seems better than hot OM2 above that is due to the squeezing (and the jitter subtraction Jenne added, which is also on in this plot for cold OM2 but not for hot OM2). And the additional noise with cold OM2 reaches up to about 45Hz.
After we optimized ADF demod phase in 74972, the BNS range seems better and consistently 160-165Mpc. The attached plot shows the comparison of OM2 cold/hot with/without SQZ. The OM2 cold with SQZ is measured after optimization of ADF demod phase and other measurements are same as Sheila's previous plots.
This plot supports what Sheila says in the previous alogs.
- The OM2 cold is worse below 40 Hz for both SQZ/no SQZ.
- Without SQZ, OM2 cold and hot are almost the same above 40 Hz.
- With SQZ, OM2 cold is better between 100-600 Hz, but worse above 1 kHz. This difference could be due to SQZ and we could try to optimize SQZ around 100 Hz with OM2 hot
Echo-ing the above, and summarizing a look at OM2 with sqz in both Sept 2023 and Dec 2023 (running gps times dictionary is attached here).
If we compare the effect of squeezing -- there is higher kHz squeezing efficiency with hot OM2. We can look at either just the darm residuals dB[sqz/unsqz] (top), or do subtraction of non-quantum noise (bottom) which shows that hot OM2 improved the kHz squeezing level by ~0.5 dB at 1.7 kHz (the blue sqz blrms 5). This is consistent with summary pages: SQZ has not reached 4.5 dB since cooling OM2 74861. Possibly suggests better SQZ-OMC mode-matching with hot OM2.
Without squeezing, cold om2 has more optical gain and more low-freq non-quantum noise. Better IFO-OMC mode-matching with cold OM2.
In total, it's almost a wash for kHz sensitivity: heating OM2 loses a few % optical gain, but recovers 0.2-0.5 dB of shot noise squeezing.
It's worth noting the consistent range increases with SQZ tuning + improvements: even in FDS, there is a non-zero contribution of quantum noise down to almost 50 Hz. For example Naoki's adjustment of sqz angle setpoint on 12/21 74972 improved range, same for Camilla's Jan sqz tuning 75151. Looking at DARM (bottom green/purple traces), these sqz angle tunings reproducibly improved quantum noise between about 60-450 Hz.
Here are some more plots of the times that Vicky plotted above.
The first attachment is just a DARM comparison with all 4 no sqz times, OM2 cold vs hot in December vs September.
Comparing OM2 hot September vs December shows that our sensitivity at from 20-40 Hz has gotten worse since September, the MICH coherence seems lower while the jitter and SRCL coherence seem similar. The same comparison for OM2 cold shows that with OM2 cold our sensitivity has also gotten worse from 15-30 Hz.
Comparing cold vs hot, in September the MICH coherence did get worse from 60-80 Hz for cold OM2, which might explain the worse sensitivity in that region. The MICH coherence got better from 20-30 Hz where the sensitivty was better for cold OM2. The December test had better tuned MICH FF for both hot and cold OM2, so this is the better test of the impact of the curvature change.
As Gabriele pointed out with his BRUCO, 74886 there is extra coherence with DHARD Y for cold OM2 at the right frequencies to help explain the extra noise. There isn't much change in the HARD pitch coherence between these December times, but the last attachment here shows a comparison of the HARD Y coherences for hot and cold OM2 in December.
Peter asked if the difference in coherence with the HARD Yaw ASC was due to a change in the coupling or the control signal.
Here is a comparison of the control signals with OM2 hot and cold, they look very similar at the frequencies of the coherence.
