Attached are spectrograms (1500-18:00 UTC vs 20-22 Hz) of the EY optical lever power sum over a 3-hour period today containing the laser swap and of a witness magnetometer channel that appeared to indicate on March 14 that a change in laser power strengthened the 0.25-Hz-offset 1-Hz comb at EY. Today's spectrograms, however, don't appear to support that correlation. During the 11-minute period when the optical lever laser is off, the magnetometer spectrogram shows steady lines at 20.25 and 21.25 Hz.

For reference, corresponding 3-hour spectrograms are attached from March 14 that do appear to show the 20.25-Hz and 21.25-Hz teeth appear right after a power change in the laser at about 17:11 UTC.

Similarly, 3-hour spectrograms are attached from March 14 that show the same lines turning on at EX at about 16:07 UTC. Additional EX power sum and magnetometer spectrograms are also attached, to show that those two lines persist during a number of power level changes over an additional 8 hours. In my earlier correlation check, I noted the gross changes in magnetometer spectra, but did not appreciate that the 0.25-Hz lines were relatively steady.

In summary, those lines strengthened at distinct times on March 14 (roughly 16:07 UTC at EX and 17:11 at EY) that coincide (at least roughly) with power level changes in the optical lever lasers, but the connection is more obscure than I had appreciated and could be chance coincidence with other maintenance work going on that day. Sigh. 

Can anyone recall some part of the operation of increasing the optical lever laser powers that day that could have increased coupling of combs into DARM, e.g., tidying up a rack by connecting previously unconnected cables? A shot in the dark, admittedly, but it's quite a coincidence that these lines started up at separate times at EX and EY right after those lasers were turned off (or blocked from shining on the power sum photodiodes) and back on again. 


Spectrograms of optical level power sum and magnetometer channels

Fig 1: EY power - April 4 - 15:00-18:00 UTC
Fig 2: EY witness magnetometer - Ditto

Fig 3: EY power - March 14 - 15:00-18:00 UTC
Fig 4: EY magnetometer - Ditto

Fig 5: EX power - March 14 - 14:00-17:00 UTC
Fig 6: EX witness magnetometer - Ditto

Fig 7: EX power - March 14 - 17:00-22:00 UTC
Fig 8: EX witness magnetometer - Ditto

Fig 9: EX power - March 15 - 00:00-04:00 UTC
Fig 10: EX witness magnetometer - Ditto

Laser continued to glitch after the swap; see attachment from 4/5/2017 ETMy oplev summary page.  My suspicion is that the VEA temp was just different enough from the Pcal lab (where we stabilize the lasers before install) that the operating point of the laser once installed was just outside the stable range set in the lab.  So during today's commissioning window I went to End Y and slightly increased the laser power to hopefully return the operating point to within the stable range.  Using the Current Mon port on the laser to monitor the power increase:

• Old value: 0.860 V
• New value: 0.865 V

Preliminary results look promising, so I will let it run overnight and evaluate in the morning whether or not further tweaks to the laser power are necessary.