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Entry  Fri Jun 19 17:06:50 2020, gautam, Update, LSC, What should the short-term commissioning goals be? CARM_superBoost.pdf
    Reply  Wed Jun 24 17:20:16 2020, gautam, Update, LSC, What should the short-term commissioning goals be? 
       Reply  Tue Jun 30 10:59:16 2020, gautam, Update, LSC, Three sensing matrices PRMI_1f_20200625sensMat.pdfPRFPMI_20200629sensMat.pdfPRFPMI_20200629sensMat_wOffset.pdf
          Reply  Mon Jul 6 17:41:19 2020, gautam, Update, LSC, An LSC puzzler LSCsignals.jpg
Message ID: 15442     Entry time: Tue Jun 30 10:59:16 2020     In reply to: 15427     Reply to this: 15457
Author: gautam 
Type: Update 
Category: LSC 
Subject: Three sensing matrices 

Summary:

I injected some sensing lines and measured their responses in the various photodiodes, with the interferometer in a few different configurations. The results are summarized in Attachments #1 - #3. Even with the PRMI (no arm cavities) locked on 1f error signals, the MICH and PRCL signals show up in nearly the same quadrature in the REFL port photodiodes, except REFL165. I am now thinking if the output (actuation) matrix has something to do with this - part of the MICH control signal is fed back to the PRM in order to minimize the appearance of the MICH dither in the PRCL error signal, but maybe this matrix element is somehow horribly mistuned?

Details:

Attachment #1:

  • ETMs were misaligned and the PRMI was locked with the carrier resonant in the cavity (i.e. sidebands reflected).
  • The locking scheme was AS55_Q --> MICH and REFL11_I --> PRCL.

Attachment #2:

  • The PRFPMI was locked. The vertex DoFs were still under control using 3f error signals (REFL165_I for PRCL and REFL165_Q for MICH).
  • Still, the MICH/PRCL degeneracy in all photodiodes except REFL165 persists.

Attachment #3:

  • Nearly identical configuration to Attachment #2.
  • The main difference here is that I applied some offsets to the MICH and PRCL error points.
  • The offsets were chosen so that the appearance of a ~300 Hz dither in the length of MICH/PRCL was nulled in the AS110_Q / POP22_I signals respectively.
  • For the latter, the appearance of this peak in the POP110_I signal was also nulled, as it should be if our macroscopic PRC length is set correctly.
  • The offsets that best nulled the peak were 110 cts for PRCL, 25 cts for MICH. The measured sensing response is 1e12 cts/m for PRCL in REFL165_I and 9.2e11 cts/m for MICH in REFL165_Q. So these offsets, in physical units, are: 110 pm for PRCL and 27 pm for MICH. They seem like reasonable numbers to me - the PRC linewidth is ~7.5 nm, so the detuning without any digital offset applied is only 1.5% of the linewidth.
  • Note that I changed the POP22/POP110 demod phases to maximize the signal in the I quadrature. The final numbers were -124 degrees / -10 degrees respectively.
  • Yet another piece of evidence suggesting these were the correct offsets is that the DC value of POX and POY were zero on average after these offsets were applied.
  • However, the MICH/PRCL responses in the 1f REFL port photodiodes remain nearly degenerate.

Some other mysteries that I will investigate further:

  1. While POP22 indicated stable buildup of 11 MHz power in the PRC, I couldn't make any sense of the AS110 signals at the dark port - there was large variation of the signal content in the two quadratures, so unlike the POP signals, I couldn't find a digital demod phase that consistently had all the signal in one of the two quadratures. This is all due to angular fluctuations?
  2. My ASC simulations suggest that the POP QPD is a poor sensor of PRM motion when the PRFPMI is locked. However, I find that turning on a feedback loop with the POP QPD as a sensor and the PRM as the actuator dramatically reduces the low-frequency fluctuations of the arm cavity carrier buildup. 🤔

I blew the long lock last night because I forgot to not clear the ASS offsets when trying to find the right settings for running the ASS system at high power. Will try again tonight...

Quote:

Lock the PRMI on carrier and measure the sensing matrix, see if the MICH and PRCL signals look sensible in 1f and 3f photodiodes.

Attachment 1: PRMI_1f_20200625sensMat.pdf  120 kB  Uploaded Tue Jun 30 12:21:24 2020  | Hide | Hide all
PRMI_1f_20200625sensMat.pdf
Attachment 2: PRFPMI_20200629sensMat.pdf  128 kB  Uploaded Tue Jun 30 12:21:33 2020  | Hide | Hide all
PRFPMI_20200629sensMat.pdf
Attachment 3: PRFPMI_20200629sensMat_wOffset.pdf  129 kB  Uploaded Tue Jun 30 12:21:43 2020  | Hide | Hide all
PRFPMI_20200629sensMat_wOffset.pdf
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