The quest of direct detection of gravitational waves (GWs) has reached the most interesting stage. During GW observation using LIGO and partially VIRGO and GEO600 for three years, no GW events were observed within a ∼15 Mpc radius from the Earth. To detect several GW events per year, enhancement of strain sensitivity, about ten times as high as for these GW detectors, is in progress. In Japan, Large-scale Cryogenic Gravitational wave Telescope (LCGT) project has also started in 2010 to detect GWs and to form a GW detection network with other GW detectors. LCGT will be constructed at the Kamioka mine, Hida-city, Gifu-prefecture. It is located about 200 meters under the ground surface for stable operation. LCGT is also designed to utilize cryogenic mirrors and cryogenic mirror suspension wires to reduce thermal noises and to reach the quantum noise limiting sensitivity of 10⁻²² in rms strain.

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  In order to constantly detect gravitational waves from astronomical sources, the required sensitivity for a detector is better than 10⁻²³ in strain. To achieve such a high sensitivity, variants of Michelson interferometer with additional mirrors are used as the second generation interferometric gravitational wave detectors. The extra-mirrors form a total of four Fabry-Perot cavities in an interferometer to enhance the optical power and increase the observation bandwidth. While this complex optical configuration enables us to tailor the quantum noise shape to maximize the astronomical output and provides ways to cope with technical constraints, a sophisticated control scheme is required to keep the interferometer at its optimal operating point. We will give a brief review of the evolution of the interferometer configuration from a simple Michelson interferometer as well as the signal extraction techniques to control the advanced interferometers.

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  In order to detect a gravitational wave, a kilometer-scale interferometer with a high-power, single-frequency laser with extremely low noises is required. In the Japanese gravitational wave detector named Large-scale Cryogenic Gravitational wave Telescope (LCGT), a single-frequency laser with an output power of more than 100 W will be used. The laser system used in LCGT consists of a master laser, an amplifier, a pre-modecleaner, a modulation system, and a modecleaner. The system also includes stabilization systems for laser intensity and frequency. We have already developed most of them. This article describes the characteristics of the laser system used in LCGT and the current status of our developments.

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  Interferometer gravitational wave detectors consist of high quality mirrors, which should be isolated from ground vibration strongly. The vibration isolation system needs not only attenuation more than 10⁹ at 100 Hz but also reduction of root mean square motion of the mirrors. Many kinds of mechanisms for isolation at low frequencies have been suggested for gravitational wave detectors. We employed an inverted pendulum and geometric anti-spring filters as the isolator in Large-scale Cryogenic Gravitational wave Telescope (LCGT).

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  The Cryogenic Laser Interferometer Observatory (CLIO) is a laser interferometric gravitational wave (GW) detector. CLIO is a prototype of the Large-Scale Cryogenic Gravitational Wave Telescope (LCGT). Both CLIO and LCGT use the same cryogenic mirror technique as well as the underground site of Kamioka mine. The cryogenic mirror technique is a method to reduce thermal noises that are fundamental in GW detectors. The most important purpose of CLIO is to demonstrate sensitivity improvement by using the cryogenic mirror technique. During the operation of CLIO with cryogenic mirrors, several unique behaviors in the interferometer systems were found. One of the serious issues that we found was low effectiveness of the magnet damping system for a mirror suspension system at cryogenic temperatures. Thus, we have developed a new damping system, the Local Suspension Point Interferometer (LSPI). In this article, we give a brief introduction to CLIO and recent results of the LSPI experiments in CLIO.

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  The LCGT (Large-scale Cryogenic Gravitational wave Telescope) is an interferometer of 3-km long baseline, using a laser beam of 1064-nm wavelength. The entire system of the interferometer is to be operated in a vacuum for avoiding any noises caused by in-space molecules and adsorbed molecules on the mirrors. The beam tubes of 3 km long and 800 mm in diameter are to be kept in the order of 10⁻⁷ Pa so as to reduce scattering-effects due to residual gas molecules. In April 2011, production of 484 of unit tubes of 12 m long and 800 mm in diameter, having a flexible bellows and flanges, were started. In this report, the production process of the unit tubes is described concerning surface treatment for outgas reduction.

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