Stable operation of a 300-m laser interferometer with sufficient sensitivity to detect gravitational-wave events within our galaxy.

TAMA300, an interferometric gravitational-wave detector with 300-m baseline length, has been developed and operated with sufficient sensitivity to detect gravitational-wave events within our galaxy and sufficient stability for observations; the interferometer was operated for over 10 hours stably and continuously. With a strain-equivalent noise level of h approximately 5x10(-21)/sqrt[Hz], a signal-to-noise ratio of 30 is expected for gravitational waves generated by a coalescence of 1.4M-1.4M binary neutron stars at 10 kpc distance. We evaluated the stability of the detector sensitivity with a 2-week data-taking run, collecting 160 hours of data to be analyzed in the search for gravitational waves.

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).

Vibration transducer using an optical cavity comprising birefringent mirrors.

We have developed an optical-cavity system comprising birefringent mirrors for a vibration transducer. To obtain a dispersive-shape signal necessary for reading out the information of mirror vibration, we measure the polarization change of the cavity transmission light caused by the natural birefringence appearing on interferential mirrors; this effect is enhanced by the cavity resonance. Since there are no additional polarization-changing elements inside the cavity, we can achieve the high finesse that is indispensable for a highly sensitive vibration measurement. The principle and an experimental demonstration of the system are reported here.

Optical mode cleaner with suspended mirrors.

We report on the development of a new type of mode cleaner that reduces any geometric noise of the laser beam in an interferometric gravitational-wave detector. The mode cleaner is a Fabry-Perot cavity that comprises independently suspended mirrors and works as a frequency-stabilization reference as well as a mode selector; the length of the cavity is 1 m. Stand-alone tests have shown at least a 30-dB reduction in the geometric fluctuation of the beam and a 60-dB reduction of the frequency noise of the laser. We have also succeeded in operating a 20-m Fabry-Perot prototype detector (at the National Astronomical Observatory, Tokyo, Japan) by using this mode cleaner.

Vibration transducer using an ultrashort Fabry-Perot cavity.

We report here on the development of an optical transducer for mechanical vibration, making use of an ultrashort Fabry-Perot cavity. The noise level measured by a cavity 150 µm long is ~1.3 × 10(-16) m/√Hz at ~1 kHz, which was obtained without a frequency-stabilization system. It was, however, large compared with the shot noise.

Automatic alignment-control system for a suspended Fabry-Perot cavity.

In interferometric gravitational-wave detectors, most of the optical components are suspended by wires so that they are isolated from all kinds of forces except gravity. The requirement for the alignment of optical components to the laser beam is crucial. We have demonstrated a servo system developed for a Fabry-Perot cavity whose mirrors are suspended independently. We use mechanical modulation and a lock-in detection method to detect any misalignment. This system directly detects the relation between the axis of the laser beam and the axis of the cavity and automatically aligns the cavity to the laser beam. We confirmed that the intensity of the reflected light from the suspended Fabry-Perot cavity can be minimized with this system. Automated control of the alignment of the large-scale detectors is also discussed.

Design of a stabilized He-Ne laser by using a thin-film heater.

Details concerning the design of the stabilization system for a He-Ne laser based on a two-mode method that uses a thin-film heater are discussed. The stability is evaluated by measuring the beat signal between two stabilized lasers; it is 1.1 × 10(-12), expressed in terms of the root Allan variance (τ = 35 s).