Shigenori Moriwaki

LARGE-SCALE CRYOGENIC GRAVITATIONAL WAVE TELESCOPE

We present here the Large-scale Cryogenic Gravitational wave Telescope (LCGT) project which is aimed to improve the sensitivity of the existing gravitational wave projects by ten times. LCGT is the project constructing the km-scale gravitational wave detector in Japan succeeding the TAMA project, which adopts cryogenic mirrors with a higher power laser. We are planing to build it in an underground site in Kamioka mine. If its target sensitivity is attained, we will be able to catch a few events per month.

Thermal effects in high-power CW second harmonic generation in Mg-doped stoichiometric lithium tantalate.

We investigated thermal behaviors of single-pass second-harmonic generation of continuous wave green radiation with high efficiency by quasi-phase matching in periodically poled Mg-doped stoichiometric lithium tantalate (PPMgSLT). Heat generation turned out to be directly related to the green light absorption in the material. Strong relation between an upper limit of the second harmonic power and confocal parameter was found. Single-pass second-harmonic generation of 16.1 W green power was achieved with 17.6% efficiency in Mg:SLT at room temperature.

Present status of large-scale cryogenic gravitational wave telescope

The large-scale cryogenic gravitational wave telescope (LCGT) is the future project of the Japanese gravitational wave group. Two sets of 3 km arm length laser interferometric gravitational wave detectors will be built in a tunnel of Kamioka mine in Japan. LCGT will detect chirp waves from binary neutron star coalescence at 240 Mpc away with a S/N of 10. The expected number of detectable events in a year is two or three. To achieve the required sensitivity, several advanced techniques will be employed such as a low-frequency vibration-isolation system, a suspension point interferometer, cryogenic mirrors, a resonant side band extraction method, a high-power laser system and so on. We hope that the beginning of the project will be in 2005 and the observations will start in 2009.

100 W, single-frequency operation of an injection-locked Nd:YAG laser

We have built a single-frequency Nd:YAG laser capable of producing an output power of 101 W by injection locking a slave laser that can emit an output power of 121 W in the free-running state to a 2-W master laser. We confirmed that the output mode was diffraction limited and linearly polarized.

Absolute-length determination of a long-baseline Fabry-Perot cavity by means of resonating modulation sidebands.

A new method has been demonstrated for absolute-length measurements of a long-baseline Fabry-Perot cavity by use of phase-modulated light. This method is based on determination of a free spectral range (FSR) of the cavity from the frequency difference between a carrier and phase-modulation sidebands, both of which resonate in the cavity. Sensitive response of the Fabry-Perot cavity near resonant frequencies ensures accurate determination of the FSR and thus of the absolute length of the cavity. This method was applied to a 300-m Fabry-Perot cavity of the TAMA gravitational wave detector that is being developed at the National Astronomical Observatory, Tokyo. With a modulation frequency of approximately 12 MHz, we successfully determined the absolute cavity length with resolution of 1 microm (3 x 10(-9) in strain) and observed local ground strain variations of 6 x 10(-8).

Japanese large-scale interferometers

The objective of the TAMA 300 interferometer was to develop advanced technologies for kilometre scale interferometers and to observe gravitational wave events in nearby galaxies. It was designed as a power-recycled Fabry–Perot–Michelson interferometer and was intended as a step towards a final interferometer in Japan. The present successful status of TAMA is presented. TAMA forms a basis for LCGT (large-scale cryogenic gravitational wave telescope), a 3 km scale cryogenic interferometer to be built in the Kamioka mine in Japan, implementing cryogenic mirror techniques. The plan of LCGT is schematically described along with its associated R&D.

Iodine-stabilized Nd:YAG laser applied to a long-baseline interferometer for wideband earth strain observations

We stabilized a frequency-doubled Nd:YAG laser (λ=532 nm) with reference to an iodine absorption line, and applied it to a long-baseline interferometer for earth strain observations. To obtain unmodulated light, saturated absorption signals of an external iodine cell were detected by the modulation transfer technique using an acousto-optic modulator working as both amplitude and frequency modulators. Two feedback loops, which could control the laser frequency by a piezo-electric actuator and a thermal actuator, realized fast and wide-range frequency stabilization, and ensured long-term stable operation. From a beat-note measurement between two identical systems, we obtained a frequency stability (in Allan variance) of ≲2×10−13 for time intervals of 10 to 1000 s. By applying the stabilized Nd:YAG laser to a light source of a 10 m interferometer, we successfully observed earth tides and earthquakes in strain variations. Other geophysical signals, detectable by this strainmeter, are also discussed.

Triple‐pendulum vibration isolation system for a laser interferometer

We have developed a triple‐pendulum vibration isolation system for suspended mirrors used in a laser interferometer for gravitational wave detection. Owing to the passive damping method using high‐performance permanent magnets, the system has a very simple, compact structure. We measured the isolation ratio for vibration below 40 Hz and confirmed that the observed and calculated values are in good agreement. The stability of the system has been demonstrated by operating a Michelson laser interferometer incorporated with a mirror mounted on the vibration isolation system.

Shot-noise limited low-frequency intensity noise of a Nd:YAG laser

We have reduced the intensity fluctuation of a laser-diode-pumped Nd:YAG laser to the shot-noise level by controlling the diode current. The obtained relative-intensity noise is less than 10-7 in Hz-1/2 within the 100 Hz to 10 kHz range.

Development of a light source with an injection-locked Nd:YAG laser and a ring-mode cleaner for the TAMA 300 gravitational-wave detector

We have developed a light source suitable for laser interferometric gravitational-wave detectors. The developed light source has high power, TEM00 mode, linear polarization, high frequency stability, and low intensity noise. The light source with the quality is essential for attaining the goal sensitivity in the TAMA 300 and was found to be available for a observation run of a gravitational-wave detector.