Summary: BRUCO gave us a few interesting clues about our current noise, including a broadband contribution to the DARM noise that seems to couple through the 9 MHz sideband.
We had 20 minutes of glitch free data from 1231532246 to 1231533411. This was before Georgia and Danny fixed the ISS second loop clipping 46412, which improved the noise from 27-15 Hz and around 350-400 Hz. Gabriele and Joe Areda managed to run BRUCO (and update the code to deal with some other updates), a report is here: https://ldas-jobs.ligo.caltech.edu/~gabriele.vajente/bruco_lho_1231532246/
Some interesting features (other than the ISS coherence)
- The peak at 90 Hz which has appeared in our spectrum since December has coherence with accelerometers and ISI sensors around HAM5. This is where PEM injections on the HAM5/6 chamber walls showed some noise that was well below DARM in the past, so we can try exciting those ISIs, or the OFI to see if that has gotten worse.
- Some templates used in the past for shaking HAM5 are in /ligo/home/sheila.dwyer/noise_budget/simple-noise-budget/restructured_nb/input/ISI
- The broad coherence of the OMC QPD sums with DARM (OMC QPD A sum). Some of this is just the intensity noise that is also witnessed by the ISS second loop out of loop sensors (PD7) The attached screenshot shows a comparison of this coherence before and after the ISS fix, as you would expect the ISS fix reduces the coherence at the frequencies where the ISS noise was reduced, but the broad coherence is unchanged. The legend in this screenshot is wrong, the gold DARM spectrum is not a 92 Mpc reference. We aren't sure of our calibration at the moment.
- We increased the modulation depth for 45 MHz by 3dB from 5:29 UTC to 6:28 UTC on Jan 16th. There were many loud glitches in this period, but there is a quiet time that starts at 5:47, the coherence of the OMC QPD with DARM was unchanged at this time. So this noise isn't coupling through the 45 MHz sidebands.
- We increased the 9MHz modulation depth by 3 dB from 6:40 UTC to 7 UTC, and the coherence increased, roughly scaling with the sideband power. (Our range also dropped by about 10 Mpc) Koji reminds us that we've seen a 9 MHz higher order mode in the OMC camera before: 29395 We can't see much in the camera tonight, if we increase the exposure we saturate the camera to the edge of the screen, so if we want to recreate Jenne's images we probably need to go to the new camera housing and move the camera or add a lens.
- We could add more whitening to these QPDs, and will plan to do that next time we are unlocked.
- We hadn't redone the SRCL FF since the increase in power and changes to TCS, so this showed up in BRUCO. Georgia, Craig and Danny have improved the feedforward above 60 Hz (separate alog).
Sheila just engaged a second stage of whitening for the OMC QPDs, so hopefully that will address the coherence there. We can't just add DC gain, at least until the 45 MHz modulation depth is reduced, otherwise we'll saturate the diodes.
Our nominal powers on the EOM driver right now are 20dBm for 9 MHz, and 23.6 dBm for the 45 MHz. (Those were the driver powers before we started changing things last night).
By looking at the DC power on the OMC QPDs while we were changing the modulation depths last night we can find the relative powers in (carrier+118), 9 MHz and 45 MHz.
| carrier | 45 MHz power | 9 MHz power | |
| OMC QPD A | 23% | 67% | 9.7% |
| OMC QPD B | 19% | 74% | 7.4% |
The attached spectrum comparison shows that the noise on the OMC QPD didn't increase in when the 9 MHz modulation depth was increased. This means that the 9MHz amplitude noise is not the dominant noise in the OMC QPD, so we can't make that assumption to make a projection of the 9MHz amplitude noise into DARM.
