External Review for interferometer alignment controls (August 2021)


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 and the stability 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 a stable full RSE lock with ASC.


  1. Daily alignment of the interferometer (Yuta Michimura, JGW-G2113198)

    • summary of O3-RSE trial situation and our improvement plans
  2. Commissioning and simulations for alignment sensing and control (Yuta Michimura, JGW-G2113198)

    • summary of current situation and plans for commissioning towards O4, focusing on global controls using wave front sensors (WFS) and QPDs
  3. Input mode cleaner alignment sensing and control report (Kenta Tanaka JGW-G2113205, Chiaki Hirose JGW-G2113211)

    • status report from the site works and simulation activities

Basics of KAGRA interferometer


1. Daily alignment of the interferometer

1-1. Summary of the current status

The alignment scheme used for O3GK and RSE trial is summarized graphically in JGW-T2112594.

We started the daily alignment from the Xarm using the green beam. The Xarm is locked with green, and ITMX and ETMX alignments were tuned to center the beam on them, by monitoring camera (Tcam) images of the HR surfaces by our eyes. PR3 dither alignment was also done at this step to maximize the green transmission. The green injection from the back of PR2 was also tuned manually using a steering mirror (POP POM) at the back of PR2 occasionally.

We then proceeded to align IR beam to the Xarm by locking the Xarm with IR, and maximizing the IR transmission with IMMT2 and PR2 dither alignment.

Next, we aligned PRMI by maximizing POP90 with PRM and BS dither alignment. Somehow, when we do the Yarm alignment before PRMI, MICH fringe was not good.

We then aligned Yarm with ETMY dither alignment. By monitoring camera (Tcam) images of the HR surface of ETMY by our eyes, ITMY alignment was also tuned occasionally to center the beam on ETMY. When ITMY was moved, we went back to PRMI alignment again. Note that beam position on ITMY is not checked in this scheme.

Then, Yarm green beam is aligned by maximizing green transmission with SR3 dither alignment. The green injection from the back of SR2 was also tuned manually using a steering mirror (POS POM) at the back of SR2 occasionally.

And Finally, the single bounce beam from ITMY is aligned to OMC by maximizing OMC transmission with OMMT2 and OMC dither alignment.

Such alignment scheme was not fully automated and the interferometer alignment was not reproducible. Towards fully automated reproducible alignment, we are planninng following modifications.

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

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 configurations are summarized below.

Note that most of the loops indicated "closed" were "once closed" and were not stable enough to implement them in the guardian. Even if the loop was once closed, it was not reproducible. We suspect that linear ranges are too small and/or zero-crossing points are dependent on the interferometer alignment (for example, simulated WFS signals are distorted if we include inhomogeneity of ITMs PhysRevD.100.082005). We hope we can improve the situation once we establish reproducible daily alignment procedure.

2-2. Simulation status

2-3. Strategies for commissioning

Commissioning towards O3 and RSE trial mostly focused on length sensing and control, and noise hunting towards achieving the 1 Mpc goal. Stability of the lock was not considered seriously 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.

We also require the suspension systems to provide user friendly alignment actuators with clear and calibrated signal paths. The alignment fluctuation of each suspension needs to be settled down to respective requirements (summarized below) with local controls. These efforts are coordinated together with the suspension commissioning team, and suspension experts are expected to be present at the control room during the interferometer commissioning period.

Our planned timeline and targets for each step are summarized below (timeline based on JGW-E1809209). Full automation of the lock acquisition and daily alignment process should also be established at each stage.

3. Input mode cleaner alignment sensing and control report

Status of the site work and simulation activities towards achieving the IMC ASC goals discussed above will be reported by Kenta Tanaka and Chiaki Hirose.

KAGRA/Subgroups/MIF/ExternalReview2021 (last edited 2021-08-26 00:30:15 by haoyu.wang)