shivaraj.kandhasamy@LIGO.ORG - posted 23:34, Friday 25 July 2025 - last comment - 03:06, Monday 28 July 2025(85984)
Some of the non-stationary features in DARM
As part of the DQ shift, I was looking at the DARM spectrorams to identify non-stationary (of the order of hours) features. Below is a list of features that were relatively obvious from the spectrograms. I used GDS-CALIB_STRAIN_NOLINES channel data for July, 21, 2025. In order to see the frequency variations a bit clear, i used 100-second spectrograms.
- 17.8 Hz, wandering line with variation around 0.2 Hz
- 20.26 Hz with linewidth/variation of ~ 0.04 Hz
- This was also reported in the a-log 85856 recently. In addition to the narrow line, sometimes we also see ~1 Hz broad noise around the narrow line. It is not clear whether thest two features (narrow and broad) are related.
- 25.6 Hz with ~0.2 Hz wander
- 27.1 Hz with ~0.1 Hz wander
- 33.0 Hz with ~0.2 Hz wander
- 40.1 Hz with linewidth/variation of ~ 0.02 Hz
- Is this a harmonic of 20.26? Their temporal behaviour look similar.
- The bruco results show that most of the low frequency features listed above including 20.26 Hz come from the input part of the IFO such as HAM2-3, IMC, which was also reported in the a-log 85959. Further studies required to identify the exact coupling location and the mechanism.
- 126.24 Hz with linewidth/variation of ~ 0.02 Hz
- 136.36 Hz with linewidth/variation of ~ 0.02 Hz
- 151.48 Hz with linewidth/variation of ~ 0.02 Hz
- All the three lines (126.24 Hz, 136.36 Hz, and 151.48 Hz) look similar in their variations over time and looking at the time scales the variations may be related to temperature; not sure if the source is the same for all three frequencies.
- 177.1 Hz with variation of 0.2 Hz
- 179.1 Hz with variation of 0.1 Hz
- The temporal variations of 177.1 Hz and 179.1 Hz look similar.
- 272.65 Hz with 0.2 Hz variation
- Its variation looks similar to 126.24 Hz, 136.36 Hz, and 151.48 Hz (the frequency is very close to the next harmonic of 136.36 Hz)
- 540 Hz with 0.5 Hz width; excess noise at the beginning of the lock (input jitter?)
- 660 Hz with 1 Hz width; excess noise at the beginning of the lock (input jitter?)
- We see something similar in LLO (alog 77265)also.
- 3073 Hz with ~2 Hz frequency ring down
- 3131 Hz with ~2 Hz frequency ring down
- The 3073 Hz and 3131 Hz features look similar (the frequency of the feature decreases over time after the lock). Maybe down conversion of HOMs?
- 6556.8 Hz with 0.02 Hz variation
- This one looks a bit different from 3131 Hz and 3073 Hz. It has more stable slope and small frequency variation. Maybe this is also realted to HOM, but one with high Q?
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Comments related to this report
To identiy possible sources for the features listed in the above alog, I have produced spectrograms of some of the important auxiallary and environmental channels. I used the same fft parameters as the GDS plots and also restricted to same frequency regions as that of GDS plots above. All these plots available at link.