craig.cahillane@LIGO.ORG - posted 04:35, Thursday 17 January 2019 - last comment - 11:37, Friday 18 January 2019(46495)
Locking tonight
Koji, Craig - We ran the RF9 modulation depth change test Sheila asked us to do. Koji will post an alog about those results. - Koji had me lock the OMC with PZT2 at only 2 volts, rather than the usual ~40 V, for +9MHz 9th order HOM considerations. - We measured the PRCL OLG after full thermalization at high power, and got a 30 Hz UGF. This seems much lower than what was reported by Jenne a week ago. - I carried the laptop in with me as I made my way down to the PSL racks. This time, I was able to make it without causing a lockloss, but when I got there the SR785 GPIB was gone, and when going to get it from the squeezer bay, while walking between HAM2 and HAM3 I killed the lock. The area directly in front of the door in the LVEA, between HAM2 and HAM3, is the most sensitive to walking around, according to IMC_F. Stepping on the lower tier floor directly inside of the LVEA door seemed to produce the largest response in IMC_F, including during my first excursion to the PSL racks. - We reached nominal low noise again, but lost it within ten minutes, probably due to some CHARD, DHARD pitch ringup. - We reach NLN a third time, and this time held it for two hours, during which I was able to get to the PSL racks and measure all CARM spectra. We eventually lost lock due to a super slowly growing ASC 0.9 Hz oscillation (lost lock about an hour after the ringup started). - Had to readjust TMSY alignment in pitch significantly to get the guardian to continue locking in FIND_IR.
We started the modulation of the modulation depth from 4:48 UTC. I'd say the data from 5:43 UTC is clean. (Attachment 1 Left Bottom Plot). The modulation power for 9MHz was switched between 20.4dBm (LOW) and 23.4dBm (HIGH). We could clearly see that the DARM noise power in each monitored frequency band (Other plots in Attachment 1, RLP1: 10-20Hz, RLP2 20-29Hz, RLP3 38-60Hz, RLP4 60-100Hz, RLP5 100-450Hz). Interestingly, the change of the noise power is visible even at the lower frequency (10-20Hz) although the change is small (~4%) and difficult to confirm with the power spectrum.
The coherence between DCPDSUM and OMC QPDs (as well as the coherence between the QPDs) were measured both in the HIGH and LOW states.
Attachment 2:
High modulation depth: 17/1/2019 6:02:00 UTC~ 1HzBW 500AVG
Low modulation depth: 17/1/2019 6:15:00 UTC~ 1HzBW 500AVG
The comparison of the OMC DCPD spectra (=DARM) is shown in Attachment 3. Here we added another plot with the low modulation and OMC PZT Voltage around 0 (i.e. One FSR away from the nominal locking point). This gave us ~85Mpc, probably because of slightly better rejection of 9MHz SB by the OMC (not so certain).
We took the relatively glitch free data for the high modulation state between 17/1/2019 7:01:45UTC~7:13:45UTC.
As the OMC QPD signal contains the carrier TEM00 too, the coherence between DCPD and QPDs can not be completely zero. I have not yet estimated the quantitative limit what the minimum coherence we can realize by eliminating the 9MHz noise.
Here's a comparison of two times when the 9 MHz modulation depth was increased to have 23dBm at the driver, with different OMC PZT offsets.
The first time is Jan 16th at 6:40:30 UTC, with an OMC PZT2 monitor readback of 77.6 V, the second time is Jan 17th at 4:48:51 UTC with PZT2 at 44.8 Volts.
The coherence between the OMC QPD and the DCPD's is lower for the lower PZT voltage, which fits with Koji's model that the 9 MHz is reaching the DCPD's through the higher order mode which has an OMC resonance close to the carrier resonance.