External Review for interferometer alignment controls (August 2021)

Purpose

From February 25 to April 21, 2020, KAGRA performed its first observing run for 4 weeks with power-recycled Fabry-Perot-Michelson (PRFPMI) congifuration. The sensitivity during the April run with GEO600, dubbed O3GK, was at the binary neutron star range of 0.5-0.7 Mpc. From July 13 to October 13, we had an intense commissioning period to try locking the full resonant sideband extraction (RSE) interferometer. We had many achievements during that period, but we have never achieved the full RSE lock yet.

During the O3GK run and the RSE trial, we faced several issues related to the interferometer alignment. The sensitivity of KAGRA depended very much on the alignment, and the best sensitivity could be achieved only when the expert aligned the interferometer manually. Almost no alignment sensing and controls (ASC) loops where closed, except for a few dither alignment loops in the power recycling cavity (PRC).

The purpose of this External Review is to receive comments from LIGO/Virgo interferometer experts on our plans to improve such situations. We would like to know if the issues we are trying to solve are reasonable, and if our plans are sound. We would also like to find out what are the issues we haven't identified yet. Finally, we would like to prioritize the works in the order of importance for the full RSE lock with ASC.

Agenda

1. Daily alignment of the interferometer
   • summary of O3-RSE trial situation and improvement plans
2. Commissioning and simulations for alignment sensing and control
   • summary of current situation and plans, focus on global controls using wave front sensors (WFS) and QPDs
3. Input mode cleaner alignment sensing and control report
   • report from current on-site works

Basics of KAGRA interferometer

• f1=16.88 MHz, f2= 45.02 MHz, f2-f1=28.13 MHz

• List of optical parameters

• PRD 88, 043007 (2013)

1. Daily alignment of the interferometer

1-1. Summary of the current status

• Sumary of Alignment Scheme of KAGRA in O3GK JGW-T2112594

1-2. Known issues and our plans (highly uncertain)

• Issue 1
  • Plans ...
• Issue 2
  • Plans ...

1-3. Known issues and our plans (less uncertain)

2. Commissioning and simulations for alignment sensing and control

2-1. Commissioning status

So far, WFS and QPD loops for IMC, PRMI and FPMI are mostly closed. For PRFPMI, we didn't have much time for ASC commissioning. Also, we had ugly beam shape at POP and POP WFS were not commissioned at all so far. TRX QPDs had strange pitch and yaw coupling which needs further investigations. Achievements for different interferometer congifurations are summarized below.

• IMC
  • IMC REFL WFS to control IP1 and MCe
    • UGF ~0.1 Hz
  • IMC TRANS DC QPD to control MCo
    • UGF ~0.05 Hz

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=13916

  • IMMT1T were not used because of scattering lights when PRFPMI is RF locked
  • IMC ASC loops were turned off during O3GK due to excess noise
• WFS and QPDs for the main interferometer
  • REFL and AS are commissioned relatively well and WFS DC centering loops are closed.
  • POP beam was too dirty and WFS are taken out

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=11506

  • TRX QPDs had strange pitch and yaw coupling

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=12019

• Xarm
  • REFL RF17 to control SOFT/HARD closed
    • UGF ~50 mHz for SOFT and ~20 mHz for HARD in yaw, ~60 mHz for both HARD and SOFT in pitch

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=11543

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=11537

    • https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=10996

  • Sensing matrix measurement with POP, AS, TRX not done
• Yarm
  • TRY QPD to control SOFT closed

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=12325

• PRMI
  • REFL RF45 to control PRM and PR3 closed
    • UGF ~0.1 Hz

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=12488

  • AS RF28 to control BS closed

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=12445

  • PRMI ASC sensing matrix

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=14019

• DRMI
  • BS and PR3 dither loops using POP90 closed.

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=15165

• FPMI
  • REFL RF17 to control Differential ETMs, Common ETMs closed

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=11885

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=11997

  • AS RF28 Q to control BS closed
    • UGF ~30 mHz for yaw, ~80mHz for pitch

    • https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=11061

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=12156

• PRFPMI
  • AS RF to control DHARD (DETM) almost closed

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=14027

  • Sensing matrix measurements for PRM and PR3

    • https://klog.icrr.u-tokyo.ac.jp/osl/?r=12802

2-2. Simulation status

• Optickle

  • FPMI, PRFPMI, SRFPMI, DRFPMI (BRSE) sensing matrix simulations with imperfections done JGW-T1910359

  • PRMI, DRMI simulations not yet
• FINESSE

  • For LSC without mirror maps, GUI for simulations with any interferometer configurations done JGW-T2012132

  • Working on ASC integration
  • Also input mode cleaner (IMC) ASC simulations and analytical calculations on going independently

2-3. Strategies for commissioning

Commissioning towards O3 and RSE trial mostly focused on length sensing and control, and little time was occasionally allocated for ASC, when there was some spare time. For the commissioning towards O4, we should spend more time for ASC to achieve a stable lock.

Our planned timeline and goals for each step are summarized below (timeline based on JGW-G2113055).

• IMC (May - August 2021)
• Xarm (mid-August - mid-November 2021; ~12 weeks)
  • Find out the reasons for ugly POP beam
  • Investigate TRX pitch and yaw coupling issue
  • Check if REFL WFS, POP WFS, AS WFS and TRX QPD give consistent sensing matrices with simulations
    • Check the polarization content at each port when Xarm is locked/unlocked, cross check with FINESSE birefringence model
  • Hold the lock of Xarm in IR with ASC for a duration of longer than 2 hours continuously
    • SOFT and HARD controlled with REFL/AS and TRX
• Yarm (November 2021 - mid-January 2022; ~9 weeks)
  • Check if REFL WFS, POP WFS, AS WFS and TRY QPD give consistent sensing matrices with simulations
  • Hold the lock of Yarm in IR with ASC for a duration of longer than 2 hours continuously
    • SOFT and HARD controlled with REFL/AS and TRY
• FPMI (January - February 2022; ~8 weeks)
  • Check the polarization content at each port, mode content at AS, cross check with FINESSE birefringence model
    • Compare them with a simple Michelson configuration to see Lawrence effect
  • OMC alignment commissioning
  • Hold the lock of FPMI in IR with ASC for a duration of longer than 2 hours continuously
    • CHARD controlled with REFL, DHARD controlled with AS
    • CSOFT and CHARD controlled with TRX and TRY
    • BS controlled with AS RF28
• Center area pumping (mid-February - mid-March 2022; ~4weeks)
• PRMI (March - April 2022; ~7 weeks)
  • Check PRC Gouy phase and length (inconsistent results so far)
  • Hold the lock of PRMI in 1f or 3f signals with ASC for a duration of longer than 2 hours continuously
    • PR mirrors controlled with POP
    • BS controlled with AS RF28
• PRFPMI (May - June 2022; ~8 weeks)
  • Hold the lock of PRFPMI with ASC for a duration of longer than 2 hours continuously
    • CHARD controlled with REFL, DHARD controlled with AS
    • CSOFT and CHARD controlled with TRX and TRY
    • PR mirrors controlled with POP
    • BS controlled with AS RF28
• Towards DR
  • Check SRC Gouy phase and length (inconsistent results so far)
  • Examine the mode-hop criteria
    • Which mirror affects most and how much angular motions are tolerable

3. Input mode cleaner alignment sensing and control report