This study focuses on the performance of the KAGRA detector in O3GK, taking into account the limiting factors of sensitivity. The interferometer mirrors are suspended using vibration isolation systems (VIS) to mitigate unwanted mirror movements caused by ground motion. The optical configuration of the GEO 600 detector differs from the KAGRA detector and can be found in Ref.
In the Fabry–Pérot cavities, the phase change of the laser field is accumulated for the cavity storage. In total there are four longitudinal degrees of freedom to control: the differential length of the arm cavities corresponding to the GW degree of freedom (DARM), the common length of the arm cavities (CARM), differential movement between the BS and each input test mass (MICH) , and the length of the power recovery cavity (PRCL). In addition, the modulation (13.78 MHz) for the longitudinal sensing of the input mode cleaner (IMC) in Fig.
The sets of optical property parameters of the KAGRA interferometer are summarized in Table 1. The lock acquisition scheme of the KAGRA detector is similar to the advanced LIGO scheme [26] but with two differences. The main interferometer optics are first aligned prior to the lock-in acquisition procedure, as follows.
The common arm signals of the ALS (ALS CARM) and the differential arm signals (ALS DARM) are fed back to the frequency of the main laser to which the green laser is phase locked and the differential motion of the final test masses (ETMX and ETMY), respectively.
![Fig. 1. Schematic of the KAGRA interferometer. A pre-stabilized laser (PSL) acquires phase modulation sidebands at 16.87 and 45.00 MHz in the super-clean PSL room for signal extraction techniques (such as the Pound–Drever–Hall method [15])](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10489630.0/6.918.167.748.92.427/schematic-interferometer-stabilized-acquires-modulation-sidebands-extraction-techniques.webp)
Noise budget
The other noise contributions (seismic motion through the Type-A tower and vibration through the heat connections) and the resonance peaks of the Type-A suspension are also discussed. Before O3GK, a transfer function of the feedback signal from the MN stage to the GUT was measured. They isolate the vibrations of the mirrors in the horizontal (along the laser beam) and vertical directions of the mirrors.
The seismic noise at O3GK was dominated by the vertical vibration of the ETMY Type-A tower. The resonances of the A-type suspension eigenmodes appeared in the observed sensitivity as different peaks. As depicted in the bottom plot of Fig.4, five peaks at approximately and 55 Hz constitute the eigenmodes of the A-type suspensions.
Local damping control noise in Type-Bp suspensions between 50 and 100 Hz was dominant in sensitivity. The transfer functions from the mirror phase feedback signals in the DARM were measured before O3GK to account for the noise contribution. The acoustic noise power spectral density in DARM Gacoustic(f) is estimated using
R(f, f)×Gmic(f)df, (1) where R(f, f) denotes a response function and Gmic(f) represents the power spectral density of the microphone signal. 8. Fit results for the violin mode (top: first, middle: second, bottom: third modes) peaks in the power spectral density (PSD) of the DARM sensitivity. Values PDC and Hint depend on the operating condition of the interferometer (amount of the local oscillator field and the contrast defect).
The noise power spectral density of the radiation pressure at the mirror displacement is expressed as. The amplitude spectral density of the CARM error signal was detected and the transfer function from the CARM to DARM error signal was measured to estimate the contribution of frequency noise. The laser frequency noise contribution to DARM is represented by the product of the amplitude spectral density and the transfer function.
The product of the transfer function and the fluctuation is the intensity noise contribution to DARM. For the former, the interferometer asymmetry must be as low as possible, as in the case of the laser frequency noise [53].
Future prospects
MICH and PRCL were excited at 66.6 and 63.1 Hz, respectively, to monitor the time dependence of their transfer function in DARM. However, calibration lines cannot be removed at the hardware level due to their functions; however, they may be discounted during software processing, such as during DARM reconstruction or some offline analyses, because activation powers are also recorded. Asymmetries between the two arms can account for couplings from auxiliary degrees of freedom (as presented in Section 3.2.2).
Unaccounted for noise sources from this study, such as jet jitter, acoustic coupling at other locations (acoustic coupling around the IMC and PRC is explained in Sect. 3.3.1), coupling from magnetic fields, the electric charge of test masses, and residual gas [44,60 ] will contribute more significantly to the sensitivity when the sensitivity is improved in the upgraded detector and will be taken into account in the noise budget. According to the experiences in Advanced LIGO and Advanced Virgo, scattering light noise and angle-to-length coupling noise can occur in a broad frequency range between 10 and 100 Hz, respectively. Additional optical baffles and beam spills are prepared for various locations in the vacuum enclosure and on the optical tables in the sky to mitigate the scattered light.
The problem of angle-to-length coupling noise will be solved by a new global angle detection and control scheme for the KAGRA detector. Even for the noise identified in O3GK, coupling mechanisms may change and need to be re-evaluated after the detector upgrade towards O4. The KAGRA noise was reduced by 4.5 orders of magnitude in O3GK [8] compared to the first cryogenic operation in 2018 [10].
The measured sensitivity can be explained by adding the effect of each noise, and noise reduction strategies are discussed. Based on these studies, KAGRA sensitivity will be improved for the next observation run, intended to contribute to GW astrophysics. This work was supported by MEXT, the JSPS Leading-edge Research Infrastructure Program, a JSPS Grant-in-Aid for Specially Promoted Research 26000005, JSPS Grants-in-Aid for Scientific Research on Innovative Areas 2905: JP17H06358, JP17H06361 and JP17H06364, the JSPS Core-to-Core Program A.
Advanced Research Networks, JSPS Grants-in-Aid for Scientific Research (S) 17H06133 and 20H05639, JSPS Grants-in-Aid for Transformative Research Areas (A) 20A203: JP20H05854, the joint research program of the Research Institute for Cosmic Ray, the University of Tokyo, the National Research Foundation (NRF), the Mitsubishi Foundation, the Computer Infrastructure Project of KISTI-GSDC in Korea, Academia Sinica (AS), the AS Grid Center (ASGC) and the Ministry of Science and Technology (MoST) in Taiwan under grants including AS-CDA-105-M06, the Advanced Technology Center (ATC) of NAOJ, the Mechanical Engineering Center of KEK, the LIGO project, the Virgo project and Terri Pearce. Finally, we would like to thank all the essential workers in the KAGRA observatory; we could not have completed this work without them. ALS CARM common length of the arm pits of arm length stabilization ALS DARM differential length of the arm pits of arm length stabilization.
![Fig. 5. Schematic of Type-A suspension [19–21]. The suspension is a multiple-stage pendulum with a total height of 13 m](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10489630.0/14.918.286.622.99.421/schematic-type-suspension-suspension-multiple-stage-pendulum-height.webp)
