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arxiv: 2504.12657 · v2 · pith:HSMOITMWnew · submitted 2025-04-17 · 🌌 astro-ph.IM

Photon Calibration Performance of KAGRA during the 4th Joint Observing Run (O4)

Dan Chen , Shingo Hido , Darkhan Tuyenbayev , Dripta Bhattacharjee , Nobuyuki Kanda , Richard Savage , Rishabh Bajpai , Sadakazu Haino
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Takahiro Sawada Takahiro Yamamoto Takayuki Tomaru Yoshiki Moriwaki
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Pith reviewed 2026-05-25 08:30 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords photon calibrationKAGRAgravitational wave detectoruncertainty estimationcryogenic detectorlaser power sensorsO4 observing runbeam position
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The pith

KAGRA's photon calibration system reaches 0.79% uncertainty during O4

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that the photon calibration systems on KAGRA delivered an overall uncertainty of 0.79 percent in the O4 run, three times lower than the level reached in O3. Calibration injects a known reference signal from a laser to convert the detector output into physical units of strain. The improvement stems from tighter control of the two largest error sources: laser power sensors and the position of the beam on the main mirror. Because the systems must operate without disturbing the cryogenic cooling, they use special placement and telephoto cameras that view the mirror at nearly normal incidence. This performance supplies a working template for any future kilometer-scale cryogenic gravitational-wave detector.

Core claim

The photon calibration systems employed during KAGRA O4 have an estimated system uncertainty of 0.79 percent. This figure is obtained by combining contributions from all identified error sources, with laser power sensors contributing the largest share and beam positions on the main mirror the second-largest share. The systems are the first fully functional calibration systems built for a cryogenic gravitational-wave telescope and incorporate placement choices plus telephoto cameras to avoid thermal interference with the cryogenics.

What carries the argument

The photon calibration (Pcal) system, a laser-based reference signal injector whose output power and beam location on the mirror are measured to convert detector signals into calibrated strain.

If this is right

  • Lower calibration uncertainty directly reduces the systematic floor on extracted gravitational-wave parameters such as distance and sky location.
  • The design features that permit Pcal operation inside the cryogenic vacuum can be copied by any future detector that adopts similar cooling.
  • Continued monitoring of power-sensor and beam-position drifts offers a clear route to push the total uncertainty below the current 0.79 percent value.
  • In runs where Pcal uncertainty becomes the dominant term, the reported number sets the minimum achievable accuracy for strain amplitude.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the same error-budget methods are applied in the upcoming O4 segment, any further reduction would tighten the overall error budget for joint LIGO-Virgo-KAGRA analyses.
  • The telephoto-camera technique for non-invasive beam monitoring could be tested on other optical systems that must remain at cryogenic temperatures.
  • A direct comparison of the KAGRA Pcal uncertainty budget with the budgets published by LIGO and Virgo would reveal whether the cryogenic environment introduces unique error terms.
  • If power-sensor stability proves to be the limiting factor, targeted hardware upgrades could be evaluated before the next observing run.

Load-bearing premise

The uncertainty estimation methods correctly identify and quantify every significant error source without missing systematic biases from the cryogenic environment or sensor interactions.

What would settle it

A side-by-side comparison of Pcal-calibrated KAGRA data against an independent absolute reference (such as a known injected displacement or a signal cross-checked with LIGO or Virgo) that shows a discrepancy larger than 0.79 percent.

Figures

Figures reproduced from arXiv: 2504.12657 by Dan Chen, Darkhan Tuyenbayev, Dripta Bhattacharjee, Nobuyuki Kanda, Richard Savage, Rishabh Bajpai, Sadakazu Haino, Shingo Hido, Takahiro Sawada, Takahiro Yamamoto, Takayuki Tomaru, Yoshiki Moriwaki.

Figure 1
Figure 1. Figure 1: Schematic diagram of the differential arm length (DARM) control loop [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Operating concept of the photon calibration (Pcal) systems in KAGRA. To [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Optical efficiencies and laser power estimation on the end test mass (ETM) [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Calibration factor transfer. NIST provided the absolute calibration of a [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Measurement setup for a comparison measurement between GSK and WSK [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Comparison of WSK and GSK calibration results at University of Toyama [PITH_FULL_IMAGE:figures/full_fig_p013_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Measured responsivity ratio between Pcal-X RxPD and WSK ( [PITH_FULL_IMAGE:figures/full_fig_p013_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Measured responsivity ratios of TxPDs based on WSK. [PITH_FULL_IMAGE:figures/full_fig_p013_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Measured optical efficiencies of the photon calibration (Pcal)-X laser beam [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Measured normalized separation ratios of Pcal-X. As the optical efficiencies [PITH_FULL_IMAGE:figures/full_fig_p014_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Tcam picture. The mirror face shape and the end test mass mirror (ETM) [PITH_FULL_IMAGE:figures/full_fig_p016_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Left: Variation of the main interferometer beam position during O4a. Middle: [PITH_FULL_IMAGE:figures/full_fig_p017_12.png] view at source ↗
read the original abstract

KAGRA is a kilometer-scale cryogenic gravitational-wave (GW) detector in Japan. It joined the 4th joint observing run (O4) in May 2023 in collaboration with the Laser Interferometer GW Observatory (LIGO) in the USA, and Virgo in Italy. After one month of observations, KAGRA entered a break period to enhance its sensitivity to GWs, and it is planned to rejoin O4 before its scheduled end in October 2025. To accurately recover the information encoded in the GW signals, it is essential to properly calibrate the observed signals. We employ a photon calibration (Pcal) system as a reference signal injector to calibrate the output signals obtained from the telescope. In ideal future conditions, the uncertainty in Pcal could dominate the uncertainty in the observed data. In this paper, we present the methods used to estimate the uncertainty in the Pcal systems employed during KAGRA O4 and report an estimated system uncertainty of 0.79%, which is three times lower than the uncertainty achieved in the previous 3rd joint observing run (O3) in 2020. Additionally, we investigate the uncertainty in the Pcal laser power sensors, which had the highest impact on the Pcal uncertainty, and estimate the beam positions on the KAGRA main mirror, which had the second highest impact. The Pcal systems in KAGRA are the first fully functional calibration systems for a cryogenic GW telescope. To avoid interference with the KAGRA cryogenic systems, the Pcal systems incorporate unique features regarding their placement and the use of telephoto cameras, which can capture images of the mirror surface at almost normal incidence. As future GW telescopes, such as the Einstein Telescope, are expected to adopt cryogenic techniques, the performance of the KAGRA Pcal systems can serve as a valuable reference.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper describes the photon calibration (Pcal) systems deployed in the cryogenic KAGRA detector during the O4 run. It presents methods for estimating the total system uncertainty, reports a value of 0.79% (three times lower than the O3 result), identifies the dominant contributions from laser power sensor calibration and beam-position measurements on the main mirror, and highlights unique design features (telephoto-camera placement at near-normal incidence) to avoid interference with the cryogenic environment. The work positions the KAGRA Pcal as the first fully functional calibration system for a cryogenic GW telescope and a reference for future instruments.

Significance. If the reported uncertainty budget holds after independent validation, the result constitutes a meaningful technical advance: a factor-of-three reduction in calibration uncertainty for a cryogenic interferometer, achieved under realistic observing conditions. This supplies a concrete performance benchmark and design precedent for next-generation cryogenic detectors such as the Einstein Telescope. The manuscript supplies numerical uncertainty components and describes the error-propagation approach, which are useful even if further cross-checks are required.

major comments (2)
  1. [Uncertainty estimation and error propagation] Uncertainty budget section: the 0.79% total uncertainty is stated to result from quadrature combination of contributions dominated by laser-power-sensor calibration and beam-position measurements, with error propagation described and telephoto-camera placement noted to mitigate cryogenic effects. However, no external cross-validation (e.g., comparison against interferometer response, another actuator, or independent power measurement) is provided to test whether unaccounted thermal, alignment, or cryo-specific systematics remain. This modeling assumption is load-bearing for the central numerical claim.
  2. [Introduction and results summary] Comparison with O3: the factor-of-three improvement is asserted without an explicit citation or tabulation of the O3 uncertainty value, the sensors or methods used in O3, or a side-by-side breakdown showing which error sources were reduced. Without this, the improvement factor cannot be independently verified.
minor comments (2)
  1. Figure captions and axis labels for beam-position and power-sensor data should explicitly state the number of independent measurements and the time period over which the data were collected.
  2. Notation for the individual uncertainty terms (e.g., power-sensor vs. beam-position contributions) should be defined once in a table or equation before being used in the propagation formula.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and constructive comments on our manuscript. We respond to each major comment below.

read point-by-point responses

  1. Referee: [Uncertainty estimation and error propagation] Uncertainty budget section: the 0.79% total uncertainty is stated to result from quadrature combination of contributions dominated by laser-power-sensor calibration and beam-position measurements, with error propagation described and telephoto-camera placement noted to mitigate cryogenic effects. However, no external cross-validation (e.g., comparison against interferometer response, another actuator, or independent power measurement) is provided to test whether unaccounted thermal, alignment, or cryo-specific systematics remain. This modeling assumption is load-bearing for the central numerical claim.

    Authors: The reported 0.79% uncertainty is obtained from the quadrature sum of the individually quantified components together with the error-propagation formalism described in the text. We agree that an external cross-check against the interferometer response or an independent power measurement would provide additional confidence that unaccounted cryo-specific or alignment systematics are absent. No such dedicated validation campaign was executed during O4, as the observing schedule prioritized data collection. We will revise the manuscript to state explicitly that the quoted figure is the estimated uncertainty from the present budget and to note the lack of external validation as a limitation to be addressed in future work. revision: partial

  2. Referee: [Introduction and results summary] Comparison with O3: the factor-of-three improvement is asserted without an explicit citation or tabulation of the O3 uncertainty value, the sensors or methods used in O3, or a side-by-side breakdown showing which error sources were reduced. Without this, the improvement factor cannot be independently verified.

    Authors: We will add a citation to the O3 Pcal reference and insert a concise comparison (either in the text or as a short table) that lists the O3 uncertainty value, the principal sensors and methods employed then, and the specific reductions achieved in the dominant O4 error sources. revision: yes

Circularity Check

0 steps flagged

No circularity: reported measurement outcome from component calibrations

full rationale

The paper is an experimental report on measured uncertainty in KAGRA photon calibration systems. The 0.79% total is obtained by standard error propagation over quantified contributions (laser power sensors, beam positions) using telephoto-camera placement to mitigate cryo effects. No load-bearing step reduces by construction to fitted inputs, self-citations, or ansatzes; the result is a direct empirical budget, not a derivation. Self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The reported uncertainty rests on the unverified assumption that the Pcal reference injection functions without cryogenic interference and that the listed components fully capture the error budget.

axioms (1)
  • domain assumption The photon calibration system provides a reliable reference signal whose uncertainty can be estimated from laser power sensors and beam position measurements without significant unaccounted systematics.
    Abstract states these are the highest-impact sources and that unique placement avoids cryogenic interference.

pith-pipeline@v0.9.0 · 5932 in / 1160 out tokens · 21823 ms · 2026-05-25T08:30:25.877832+00:00 · methodology

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. GW240925 and GW250207: Astrophysical Calibration of Gravitational-wave Detectors

    gr-qc 2026-05 unverdicted novelty 8.0

    The first informative astrophysical calibration of gravitational-wave detectors is reported using GW240925 and GW250207.

Reference graph

Works this paper leans on

18 extracted references · 18 canonical work pages · cited by 1 Pith paper

  1. [1]

    B. P. Abbott, R. Abbott, T. D. Abbott, M. R. Abernathy, F. Acernese, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, C. Affeldt, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, B. Allen, A. Allocca, P. A. Altin, S. B. Anderson, W. G. Anderson, K. Arai, M. A. Arain, M. C. Araya, C. C. Arceneaux, J. S....

    work page 2016

  2. [2]

    Abbott, T

    R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams, N. Adhikari, R. X. Adhikari, V. B. Adya, C. Affeldt, D. Agarwal, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, S. Akcay, T. Akutsu, S. Albanesi, A. Allocca, P. A. Altin, A. Amato, C. Anand, S. Anand, A. Ananyeva, S. B. Anderson, W. G. Anderson, M. Ando, T. Andrad...

    work page 2023

  3. [3]

    Overview of KAGRA: Calibration, detector characterization, physical environmental monitors, and the geophysics interferometer

    T Akutsu, M Ando, K Arai, Y Arai, S Araki, A Araya, N Aritomi, H Asada, Y Aso, S Bae, Y Bae, L Baiotti, R Bajpai, M A Barton, K Cannon, Z Cao, E Capocasa, M Chan, C Chen, K Chen, Y Chen, C Y Chiang, H Chu, Y K Chu, S Eguchi, Y Enomoto, R Flaminio, Y Fujii, Y Fujikawa, M Fukunaga, M Fukushima, D Gao, G Ge, S Ha, A Hagiwara, S Haino, W B Han, K Hasegawa, K ...

    work page 2021

  4. [4]

    The virgo newtonian calibration system for the o4 observing run

    F Aubin, E Dangelser, D Estevez, A Masserot, B Mours, T Pradier, A Syx, and P Van Hove. The virgo newtonian calibration system for the o4 observing run. Classical and Quantum Gravity , 41(23):235003, oct 2024

    work page 2024

  5. [5]

    Calibration of the gravitational wave telescope KAGRA

    Dan Chen. Calibration of the gravitational wave telescope KAGRA. PoS, ICRC2023:1549, 2023

    work page 2023

  6. [6]

    Calibration of the glasgow 10 m prototype laser interferometric gravitational wave detector using photon pressure

    D.A Clubley, G.P Newton, K.D Skeldon, and J Hough. Calibration of the glasgow 10 m prototype laser interferometric gravitational wave detector using photon pressure. Physics Letters A , 283(1):85–88, 2001

    work page 2001

  7. [7]

    R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward. Laser phase and frequency stabilization using an optical resonator. Appl. Phys. B, 31(2):97–105, 1983

    work page 1983

  8. [8]

    The advanced virgo photon calibrators

    D Estevez, P Lagabbe, A Masserot, L Rolland, M Seglar-Arroyo, and D Verkindt. The advanced virgo photon calibrators. Classical and Quantum Gravity , 38(7):075007, feb 2021

    work page 2021

  9. [9]

    Sensitivity studies for third-generation gravitational wave observatories

    S Hild, M Abernathy, F Acernese, P Amaro-Seoane, N Andersson, K Arun, F Barone, B Barr, M Barsuglia, M Beker, N Beveridge, S Birindelli, S Bose, L Bosi, S Braccini, C Bradaschia, T Bulik, E Calloni, G Cella, E Chassande Mottin, S Chelkowski, A Chincarini, J Clark, E Coccia, C Colacino, J Colas, A Cumming, L Cunningham, E Cuoco, S Danilishin, K Danzmann, R...

    work page 2011

  10. [10]

    Inoue, B

    Y. Inoue, B. H. Hsieh, K. H. Chen, Y. K. Chu, K. Ito, C. Kozakai, T. Shishido, Y. Tomigami, T. Akutsu, S. Haino, K. Izumi, T. Kajita, N. Kanda, C. S. Lin, F. K. Lin, Y. Moriwaki, W. Ogaki, H. F. Pang, T. Sawada, T. Tomaru, T. Suzuki, S. Tsuchida, T. Ushiba, T. Washimi, T. Yamamoto, and T. Yokozawa. Development of advanced photon calibrator for Kamioka gra...

    work page 2023

  11. [11]

    Karki, D

    S. Karki, D. Tuyenbayev, S. Kandhasamy, B. P. Abbott, T. D. Abbott, E. H. Anders, J. Berliner, J. Betzwieser, C. Cahillane, L. Canete, C. Conley, H. P. Daveloza, N. De Lillo, J. R. Gleason, E. Goetz, K. Izumi, J. S. Kissel, G. Mendell, V. Quetschke, M. Rodruck, S. Sachdev, T. Sadecki, P. B. Schwinberg, A. Sottile, M. Wade, A. J. Weinstein, M. West, and R....

    work page 2016

  12. [12]

    Mossavi, M

    K. Mossavi, M. Hewitson, S. Hild, F. Seifert, U. Weiland, J.R. Smith, H. L¨ uck, H. Grote, B. Willke, and K. Danzmann. A photon pressure calibrator for the geo 600 gravitational wave detector. Physics Letters A , 353(1):1–3, 2006

    work page 2006

  13. [13]

    The einstein telescope: a third-generation gravitational wave observatory

    M Punturo, M Abernathy, F Acernese, B Allen, N Andersson, K Arun, F Barone, B Barr, M Barsuglia, M Beker, N Beveridge, S Birindelli, S Bose, L Bosi, S Braccini, C Bradaschia, T Bulik, E Calloni, G Cella, E Chassande Mottin, S Chelkowski, A Chincarini, J Clark, E Coccia, C Colacino, J Colas, A Cumming, L Cunningham, E Cuoco, S Danilishin, K Danzmann, G De ...

    work page 2010

  14. [14]

    M. W. Regehr, F. J. Raab, and S. E. Whitcomb. Demonstration of a power-recycled michelson interferometer with fabry–perot arms by frontal modulation. Opt. Lett., 20(13):1507–1509, Jul 1995

    work page 1995

  15. [15]

    Brown, Yanbei Chen, Dennis Coyne, Robert Eisenstein, Matthew Evans, Peter Fritschel, Evan D

    David Reitze, Rana X Adhikari, Stefan Ballmer, Barry Barish, Lisa Barsotti, GariLynn Billingsley, Duncan A. Brown, Yanbei Chen, Dennis Coyne, Robert Eisenstein, Matthew Evans, Peter Fritschel, Evan D. Hall, Albert Lazzarini, Geoffrey Lovelace, Jocelyn Read, B. S. Sathyaprakash, David Shoemaker, Joshua Smith, Calum Torrie, Salvatore Vitale, Rainer Weiss, C...

    work page 2019

  16. [16]

    Taylor and Chris E

    Barry N. Taylor and Chris E. Kuyatt. Guidelines for evaluating and expressing the uncertainty of nist measurement results. NIST Technical Note, 1297, 1994

    work page 1994

  17. [17]

    Status of a cryogenic mirror suspension for KAGRA gravitational wave detector, pages 1606–1611

    Takafumi Ushiba. Status of a cryogenic mirror suspension for KAGRA gravitational wave detector, pages 1606–1611. 2022

    work page 2022

  18. [18]

    Reconstructing the calibrated strain signal in the advanced Photon Calibration Performance of KAGRA during the 4th Joint Observing Run (O4) 31 ligo detectors

    A D Viets, M Wade, A L Urban, S Kandhasamy, J Betzwieser, Duncan A Brown, J Burguet- Castell, C Cahillane, E Goetz, K Izumi, S Karki, J S Kissel, G Mendell, R L Savage, X Siemens, D Tuyenbayev, and A J Weinstein. Reconstructing the calibrated strain signal in the advanced Photon Calibration Performance of KAGRA during the 4th Joint Observing Run (O4) 31 l...

    work page 2018