keith.riles@LIGO.ORG - posted 13:59, Sunday 02 April 2017 - last comment - 20:40, Monday 03 April 2017(35274)
EX and EY optical lever lasers produce combs in magnetometers, probably DARM too
Following up on this earlier observation of a change point in DARM combs on March 14 and on comments added by Evan and Ansel, I tried looking at possible correlations between optical lever laser power levels and combs seen in EX and EY magnetometers. The bottom line is that there are such correlations, as documented in detail in the attached annotated slides with posted spectra at a variety of power levels (as measured by H1:SUS-ETMX/Y_L3_OPLEV_SUM_OUT_DQ). The figures below show EX and EY before/after spectra. In summary, the spectra show narrow lines before March 14 in combs of ~1-Hz spacing with various offsets (EX worse than EY). During periods when the laser powers were lowered, the spectra were contaminated by broadband noise (motivation for keeping the power high, I believe). When the power was raised to higher than it was prior to March 14, the broadband power dropped again, the lines persisted and were joined by new lines with 1-Hz spacing offset from integer frequencies by ~0.25 Hz, a comb observed to be present in DARM following March 14. The apparent creation of a new comb in DARM from these lasers may hint that other combs seen for a long time in DARM and in magnetometer spectra may also arise from these lasers. Whether the mechanism is though the optical lever damping, as suggested previously for glitches, or through another coupling, such as ESD power supply contamination from the laser supplies, is not clear to me. Fig 1: EX magnetometer spectrum before maintenance work on March 14 (Interval "A" on 1st attached slide file) Fig 2: EY magnetometer spectrum before maintenance work on March 14 (Interval "P" on 1st attached slide file) Fig 3: EX magnetometer spectrum before maintenance work, on March 15 (Interval "Q" on 1st attached slide file) Fig 3: EY magnetometer spectrum before maintenance work, on March 15 (Interval "V" on 1st attached slide file) Attachment 1: Annotated slides showing spectra before / after many change points on March 14/15 of EX laser power Attachment 2: Annotated slides showing spectra before / after a handful of change points on March 14/15 of EY laser power
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Heads up on this: Jason plans to swap the ETMY laser tomorrow (see WP 6555), not necessarily because of this study, but it'll be another "before vs. after" data point. Also -- do we suspect a coupling mechanism for this correlation to DARM? The bandwidth of the optical lever damping loops is quite narrow around 0.3-0.5 [Hz], and control is applied only at the penultimate stage, which means it'll get a factor of 1/f^2 above ~5 Hz. Further, it's angular control as well, so you'd need a pretty high angle-to-length coupling coefficient... Did you see any improvement in the combs seen in DARM after Sheila installed a better cutoff filter (see LHO aLOG 34988)?
Following up on Jeff's question, below are spectral comparisons of pre-March 14, the period between March 14 and March 21, and the period afterward. I don't see evidence of a significant change in the comb structure in DARM after the damping loop rolloff adjustment on March 21. Included are zooms around four teeth in the 1-Hz comb on 0.25-Hz offsets from integer frequencies. Fig 1: 20-60 Hz Fig 2: Zoom around 20.25 Hz Fig 3: Zoom around 21.25 Hz Fig 4: Zoom around 22.25 Hz Fig 5: Zoom around 23.25 Hz
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