Construction of the Japanese second-generation gravitational-wave detector KAGRA has been started. In the next 6 \sim 7 years, we will be able to observe the space-time ripple from faraway galaxies. KAGRA is equipped with the latest advanced technologies. The entire 3-km long detector is located in the underground to be isolated from the seismic motion, the core optics are cooled down to 20 K to reduce thermal fluctuations, and quantum non-demolition techniques are used to decrease quantum noise. In this paper, we introduce the detector configuration of KAGRA; its design, strategy, and downselection of parameters.
KAGRA uses cryogenics to cool its sapphire test masses down to 20 K to reduce the thermal noise. However, cryocooler vibration and structural resonances of the cryostat couple to test mass and can contaminate the detector sensitivity. We performed vibration analysis of the cooling system at cryogenic temperature to study its impact on detector sensitivity. Our measurement show shield vibration below 1 Hz is not impacted by cryocooler operation or structural resonances and follows ground motion. The noise floor of the shield in 1-100 Hz was observed to be 2-3 order of magnitude larger than seis…
Upgrades to improve the sensitivity of gravitational wave detectors enable more frequent detections and more precise source parameter estimation. Unlike other advanced interferometric detectors such as Advanced LIGO and Advanced Virgo, KAGRA requires different approach for the upgrade since it is the only detector which employs cryogenic cooling of the test masses. In this paper, we describe possible KAGRA upgrades with technologies focusing on different detector bands, and compare the impacts on the detection of compact binary coalescences. We show that either fivefold improvement in the $100…
KAGRA is the first km-scale gravitational wave detector to be constructed underground and employ cryogenics to cool down its test masses. While the underground location provides a quiet site with low seismic noise, the cooling infrastructure is known to generate large mechanical vibrations due to cryocooler operation and structural resonances of the cryostat. As cooling system components are relatively heavy and in close proximity to the test masses, oscillation of gravity force induced by their vibration, so-called Newtonian noise, could contaminate the detector sensitivity. In this paper, we…
KAGRA is a 3-km interferometric gravitational wave telescope located in the Kamioka mine in Japan. It is the first km-class gravitational wave telescope constructed underground to reduce seismic noise, and the first km-class telescope to use cryogenic cooling of test masses to reduce thermal noise. The construction of the infrastructure to house the interferometer in the tunnel, and the initial phase operation of the interferometer with a simple 3-km Michelson configuration have been completed. The first cryogenic operation is expected in 2018, and the observing runs with a full interferometer…
KAGRA is a second-generation interferometric gravitational-wave detector with 3-km arms constructed at Kamioka, Gifu in Japan. It is now in its final installation phase, which we call bKAGRA (baseline KAGRA), with scientific observations expected to begin in late 2019. One of the advantages of KAGRA is its underground location of at least 200 m below the ground surface, which brings small seismic motion at low frequencies and high stability of the detector. Another advantage is that it cools down the sapphire test mass mirrors to cryogenic temperatures to reduce thermal noise. In April-May 201…
Ground-based gravitational-wave detectors are based on high precision laser interferometry. One promising technique to improve the detector's sensitivity is the detuning of an optical cavity, which enhances the signal at around certain frequencies for target astronomical sources. The detuning, however, involves technical noise due to an asymmetry of the control sidebands, which includes photo-detector noise and oscillator-phase noise. Here, we introduce a solution to reduce the two kinds of excess noise using an amplitude-modulation sideband that compensates the asymmetry. The solution is plan…
KAGRA is a cryogenic interferometric gravitational wave detector being constructed at the underground site of Kamioka mine in Gifu prefecture, Japan. We performed an optimization of the interferomter design, to achieve the best sensitivity and a stable operation, with boundary conditions of classical noises and under various practical constraints, such as the size of the tunnel or the mirror cooling capacity. Length and alignment sensing schemes for the robust control of the interferometer are developed. In this paper, we describe the detailed design of the KAGRA interferometer as well as the …
Ringdown gravitational waves of compact binary mergers are an important target to test general relativity. The main components of the ringdown waveform after merger are black hole quasinormal modes. In general relativity, all multipolar quasinormal modes of a black hole should give the same values of black hole parameters. Although the observed binary black hole events so far are not significant enough to perform the test with ringdown gravitational waves, it is expected that the test will be achieved in third generation detectors. The Japanese gravitational wave detector KAGRA, called bKAGRA …
KAGRA is a new gravitational wave detector which aims to begin joint observation with Advanced LIGO and Advanced Virgo from late 2019. Here, we present KAGRA's possible upgrade plans to improve the sensitivity in the decade ahead. Unlike other state-of-the-art detectors, KAGRA requires different investigations for the upgrade since it is the only detector which employs cryogenic cooling of the test mass mirrors. In this paper, investigations on the upgrade plans which can be realized by changing the input laser power, increasing the mirror mass, and injecting frequency dependent squeezed vacuu…
Most second-generation gravitational-wave detectors employ an optical resonator called an output mode-cleaner (OMC), which filters out junk light from the signal and the reference light, before it reaches the detection photodiode located at the asymmetric port of the large-scale interferometer. The optical parameters of the OMC should be carefully chosen to satisfy the requirements to filter out unwanted light whilst transmitting the gravitational wave signal and reference light. The Japanese gravitational-wave detector KAGRA plans to use a very small amount of reference light to minimize the …
Cryogenic mirrors have been introduced to the KAGRA gravitational wave telescope in Japan, and are also planned to be used in next-generation gravitational wave telescopes to further improve their sensitivity. Molecular gases inside vacuum chambers adhere to cold mirror surfaces because they lose their kinetic energy when they hit cryogenic surfaces. Finally, a number of adsorbed molecules form an adlayer, which will grow with time. The growing adlayer functions as an optical coating and changes the properties of the underlying mirror, such as reflectance, transmittance, and absorption, which …
KAGRA is a 3-km cryogenic interferometric gravitational wave telescope located at an underground site in Japan. In order to achieve its target sensitivity, the relative positions of the mirrors of the interferometer must be finely adjusted with attached actuators. We have developed a model to simulate the length control loops of the KAGRA interferometer with realistic suspension responses and various noises for mirror actuation. Using our model, we have designed the actuation parameters to have sufficient force range to acquire lock as well as to control all the length degrees of freedom witho…
The test mass suspensions of cryogenic gravitational-wave detectors such as the KAGRA project are tasked with extracting the heat deposited on the optics. Thus these suspensions have a non-uniform temperature, requiring the calculation of thermal noise in non-equilibrium conditions. While it is not possible to describe the whole suspension system with one temperature, the local temperature anywhere in the system is still well defined. We therefore generalize the application of the fluctuation-dissipation theorem to mechanical systems, pioneered by Saulson and Levin, to non-equilibrium conditio…
CLIO (Cryogenic Laser Interferometer Observatory) is a Japanese gravitational wave detector project. One of the main purposes of CLIO is to demonstrate thermal-noise suppression by cooling mirrors for a future Japanese project, LCGT (Large-scale Cryogenic Gravitational Telescope). The CLIO site is in Kamioka mine, as is LCGT. The progress of CLIO between 2005 and 2007 (room- and cryogenic-temperature experiments) is introduced in this article. In a room-temperature experiment, we made efforts to improve the sensitivity. The current best sensitivity at 300 K is about $6 \times 10^{-21} /\sqrt{\…
Current status of TAMA and CLIO detectors in Japan is reported in this article. These two interferometric gravitational-wave detectors are being developed for the large cryogenic gravitational wave telescope (LCGT) which is a future plan for detecting gravitational wave signals at least once per year. TAMA300 is being upgraded to improve the sensitivity in low frequency region after the last observation experiment in 2004. To reduce the seismic noises, we are installing new seismic isolation system, which is called TAMA Seismic Attenuation System, for the four test masses. We confirmed stable …
The recent detections of gravitational waves (GWs) reported by LIGO/Virgo collaborations have made significant impact on physics and astronomy. A global network of GW detectors will play a key role to solve the unknown nature of the sources in coordinated observations with astronomical telescopes and detectors. Here we introduce KAGRA (former name LCGT; Large-scale Cryogenic Gravitational wave Telescope), a new GW detector with two 3-km baseline arms arranged in the shape of an "L", located inside the Mt. Ikenoyama, Kamioka, Gifu, Japan. KAGRA's design is similar to those of the second generat…
Cryogenic cooling of the test masses of interferometric gravitational wave detectors is a promising way to reduce thermal noise. However, cryogenic cooling limits the incident power to the test masses, which limits the freedom of shaping the quantum noise. Cryogenic cooling also requires short and thick suspension fibers to extract heat, which could result in the worsening of thermal noise. Therefore, careful tuning of multiple parameters is necessary in designing the sensitivity of cryogenic gravitational wave detectors. Here, we propose the use of particle swarm optimization to optimize the …
Construction of a large-scale cryogenic gravitational-wave telescope KAGRA has been completed and the four sapphire test masses have been installed in cryostat vacuum chambers. It recently turned out that a sapphire substrate used for one of the input test masses shows a characteristic strcuture in its transmission map due to non-uniformity of the crystal. We performed an interferometer simulation to see the influence of the non-uniformity using measured transmission/reflection maps.
KAGRA is a newly built gravitational-wave telescope, a laser interferometer comprising arms with a length of 3\,km, located in Kamioka, Gifu, Japan. KAGRA was constructed under the ground and it is operated using cryogenic mirrors that help in reducing the seismic and thermal noise. Both technologies are expected to provide directions for the future of gravitational-wave telescopes. In 2019, KAGRA finished all installations with the designed configuration, which we call the baseline KAGRA. In this occasion, we present an overview of the baseline KAGRA from various viewpoints in a series of of …
We report on the current status of CLIO (Cryogenic Laser Interferometer Observatory), which is a prototype interferometer for LCGT (Large Scale Cryogenic Gravitational-Wave Telescope). LCGT is a Japanese next-generation interferometric gravitational wave detector featuring the use of cryogenic mirrors and a quiet underground site. The main purpose of CLIO is to demonstrate a reduction of the mirror thermal noise by cooling the sapphire mirrors. CLIO is located in an underground site of the Kamioka mine, 1000 m deep from the mountain top, to verify its advantages. After a few years of commissio…
KAGRA, the underground and cryogenic gravitational-wave detector, was operated for its solo observation from February 25th to March 10th, 2020, and its first joint observation with the GEO 600 detector from April 7th -- 21st, 2020 (O3GK). This study presents an overview of the input optics systems of the KAGRA detector, which consist of various optical systems, such as a laser source, its intensity and frequency stabilization systems, modulators, a Faraday isolator, mode-matching telescopes, and a high-power beam dump. These optics were successfully delivered to the KAGRA interferometer and op…
Recently, a considerable amount of attention has been given to the search for ultralight dark matter by measuring the oscillating length changes in the arm cavities of gravitational wave detectors. Although gravitational wave detectors are extremely sensitive for measuring the differential arm length changes, the sensitivity to dark matter is largely attenuated, as the effect of dark matter is mostly common to arm cavity test masses. Here, we propose to use auxiliary length channels, which measure the changes in the power and signal recycling cavity lengths and the differential Michelson inter…
A leading candidate source of detectable gravitational waves is the inspiral and merger of pairs of stellar-mass compact objects. The advanced LIGO and advanced Virgo detectors will allow scientists to detect inspiral signals from more massive systems and at earlier times in the detector band, than with first generation detectors. The signal from a coalescence of two neutron stars is expected to stay in the sensitive band of advanced detectors for several minutes, thus allowing detection before the final coalescence of the system. In this work, the prospects of detecting inspiral signals prior…
A Polarization-Decomposed Method for Simulating Inhomogeneous Birefringence in Laser-Interferometric Gravitational-Wave Detectors
Institute of Science Tokyo（东京科学大学）；University of Tokyo（东京大学）；Cardiff University（卡迪夫大学）；University of Adelaide（阿德莱德大学）；The University of Tokyo（东京大学）；National Astronomical Observatory of Japan（日本国立天文台）；Università di Trento（特伦托大学）；Japan Aerospace Exploration Agency（日本宇宙航空研究开发机构）
An evolving Japanese gravitational-wave (GW) mission in the deci-Hz band: B-DECIGO (DECihertz laser Interferometer Gravitational wave Observatory) will enable us to detect GW150914-like binary black holes, GW170817-like binary neutron stars, and intermediate-mass binary black holes out to cosmological distances. The B-DECIGO band slots in between the aLIGO-Virgo-KAGRA-IndIGO (hecto-Hz) and LISA (milli-Hz) bands for broader bandwidth; the sources described emit GWs for weeks to years across the multiband to accumulate high signal-to-noise ratios. This suggests the possibility that joint detecti…
For the benefit of the readers of this journal, the editors requested that we prepare a brief review of the history of the development of the theory, the experimental attempts to detect them, and the recent direct observations of gravitational waves (GWs). The theoretical ideas and disputes beginning with Einstein in 1916 regarding the existence and nature of GWs and the extent to which one can rely on the electromagnetic analogy, especially the controversies regarding the quadrupole formula and whether GWs carry energy, are discussed. The theoretical conclusions eventually received strong obs…
We report the results of the first joint observation of the KAGRA detector with GEO600. KAGRA is a cryogenic and underground gravitational-wave detector consisting of a laser interferometer with three-kilometer arms, and located in Kamioka, Gifu, Japan. GEO600 is a British--German laser interferometer with 600 m arms, and located near Hannover, Germany. GEO600 and KAGRA performed a joint observing run from April 7 to 20, 2020. We present the results of the joint analysis of the GEO--KAGRA data for transient gravitational-wave signals, including the coalescence of neutron-star binaries and gene…
