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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. Key words: Gravitational-wave detector, mode cleaner, Fabry-Perot cavity, interferometer.

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 [[micro]meter] long is [approximately] 1.3 X [10.sup.-16] m/[square root of Hz] at [approximately] 1 kHz, which was obtained without a frequency-stabilization system. It was, however, large compared with the shot noise.

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 X 10 to the -12, expressed in terms of the root Allan variance (Tau = 35 s).

Waseda University Nasu telescope array is a spatial fast Fourier transform interferometer consisting of eight linearly aligned antennas with 20 m spherical dishes. This type of interferometer was developed to survey transient radio sources with an angular resolution as high as that of a 160 m dish and a field of view as wide as that of a 20 m dish. We have been performing drift-scan-mode observations, in which the telescope scans the sky around a selected declination as the Earth rotates. The black hole X-ray binary V404 Cygni underwent a new outburst in 2015 June after a quiescent period of 26 yr. Because of the interest in black hole binaries, a considerable amount of data on this outburst at all wavelengths was accumulated. Using the above telescope, we had been monitoring V404 Cygni daily from one month before the X-ray outburst, and two radio flares at 1.4 GHz were detected on 2015 June 21.73 and June 26.71. The flux density and timescale of the flares were 313 ± 30 mJy and 1.50 ± 0.49 d, 364 ± 30 mJy and 1.70 ± 0.16 d, respectively. We also confirmed the extreme variation of the radio spectra within a short period by collecting other radio data observed with several radio telescopes. Such spectral behavior is considered to reflect the change in the opacity of the ejected blobs associated with these extreme activities in radio and X-ray. Our 1.4 GHz radio data are expected to be helpful for studying the physics of the accretion and ejection phenomena around black holes. [ABSTRACT FROM AUTHOR]

DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is a future Japanese space gravitational-wave antenna. The most important objective of DECIGO, among various sciences to be aimed at, is to detect gravitational waves coming from the inflation of the universe. DECIGO consists of four clusters of spacecraft, and each cluster consists of three spacecraft with three Fabry–Perot Michelson interferometers. As a pathfinder mission of DECIGO, B-DECIGO will be launched, hopefully in the 2020s, to demonstrate technologies necessary for DECIGO as well as to lead to fruitful multimessenger astronomy. B-DECIGO is a small-scale or simpler version of DECIGO with the sensitivity slightly worse than that of DECIGO, yet good enough to provide frequent detection of gravitational waves. [ABSTRACT FROM AUTHOR]

We have developed a light source suitable for laser interferometric gravitational-wave detectors. The developed light source has high power, TEM[sub 00] 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. [ABSTRACT FROM AUTHOR]

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 intefferometer incorporated with a mirror mounted on the vibration isolation system. [ABSTRACT FROM AUTHOR]

A wideband (dc to 2 kHz) highly sensitive seismometer using a Michelson laser interferometer has been developed. The horizontal acceleration of the ground is sensed by a suspended mirror; its position is then detected as the fringe signal of the light in the interferometer. The minimum measurable displacement with this seismometer is 3 × 10[sup -11] m/ √Hz at 1 Hz (limited by the thermal fluctuation of air and the frequency noise of the laser) and 2 × 10[sup -15] m/ √Hz above 2 kHz (limited by the shot noise of the light). One application of this seismometer is to use it for very sensitive vibration measurements, such as in seismic noise investigations of possible sites for a large-scale laser-interferometric gravitational wave detector. [ABSTRACT FROM AUTHOR]

A search for Lorentz violation in electrodynamics was performed by measuring the resonant frequency difference between two counterpropagating directions of an optical ring cavity. Our cavity contains a dielectric element, which makes our cavity sensitive to the violation. The laser frequency is stabilized to the counterclockwise resonance of the cavity, and the transmitted light is reflected back into the cavity for resonant frequency comparison with the clockwise resonance. This double-pass configuration enables a null experiment and gives high common mode rejection of environmental disturbances. We found no evidence for odd-parity anisotropy at the level of δc/c≲10-14. Within the framework of the standard model extension, our result put more than 5 times better limits on three odd-parity parameters κ˜o+JK and a 12 times better limit on the scalar parameter κ˜tr compared with the previous best limits. [ABSTRACT FROM AUTHOR]

We systematically measured and compared the mechanical losses of various kinds of bulk fused silica. Their quality factors ranged widely from 7×105 to 4×107, the latter being one of the highest reported among bulk fused silica. We observed frequency-dependent losses and a decrease in the losses upon annealing. [Copyright &y& Elsevier]

Our recent theoretical research [Phys. Lett. A 305 (2002) 18] showed that the actual thermal noise of mirrors with an inhomogeneous loss in interferometric gravitational wave detectors is largely different from the traditional evaluation by modal expansion. We have tested this prediction experimentally using an oscillator similar to a mirror. The experimental result was consistent with our previous Letter. [Copyright &y& Elsevier]

We have evaluated the thermal noise of mirrors with inhomogeneous loss in interferometric gravitational wave detectors using a reliable estimation method. Our calculation showed that the traditional estimation by modal expansion is very different from the actual thermal noise. In a typical model case, when the loss is localized on a surface illuminated by a laser beam, the thermal noise is about three times larger than the estimation of the modal expansion. When the dissipation is concentrated at points far from the beam spot, the thermal noise is about fifteen times smaller. [Copyright &y& Elsevier]

The preparation of a mechanical oscillator driven by quantum back-action is a fundamental requirement to reach the standard quantum limit (SQL) for force measurement, in optomechanical systems. However, thermal fluctuating force generally dominates a disturbance on the oscillator. In the macroscopic scale, an optical linear cavity including a suspended mirror has been used for the weak force measurement, such as gravitational-wave detectors. This configuration has the advantages of reducing the dissipation of the pendulum (i.e., suspension thermal noise) due to a gravitational dilution by using a thin wire, and of increasing the circulating laser power. However, the use of the thin wire is weak for an optical torsional anti-spring effect in the cavity, due to the low mechanical restoring force of the wire. Thus, there is the trade-off between the stability of the system and the sensitivity. Here, we describe using a triangular optical cavity to overcome this limitation for reaching the SQL. The triangular cavity can provide a sensitive and stable system, because it can optically trap the mirror's motion of the yaw, through an optical positive torsional spring effect. To show this, we demonstrate a measurement of the torsional spring effect caused by radiation pressure forces.

We present the first upper limit on gravitational wave (GW) backgrounds at an unexplored frequency of 0.2 Hz using a torsion-bar antenna (TOBA). A TOBA was proposed to search for low-frequency GWs. We have developed a small-scaled TOBA and successfully found Ω(gw)(f)<4.3×10(17) at 0.2 Hz as demonstration of the TOBA's capabilities, where Ω(gw)(f) is the GW energy density per logarithmic frequency interval in units of the closure density. Our result is the first nonintegrated limit to bridge the gap between the LIGO band (around 100 Hz) and the Cassini band (10(-6)-10(-4)  Hz).

We propose a novel type of gravitational-wave antenna, formed by two bar-shaped test masses and laser-interferometric sensors to monitor their differential angular fluctuations. This antenna has a fundamental sensitivity to low-frequency signals below 1 Hz, even with a ground-based configuration. In addition, it is possible to expand the observation band to a lower limit determined by the observation time, by using modulation and up-conversion of gravitational-wave signals by rotation of the antenna. The potential sensitivity of this antenna is superior to those of current detectors in a 1 mHz-10 Hz frequency band and is sufficient for observations of gravitational waves radiated from in-spiral and merger events of intermediate-mass black holes.

We directly measured mechanical thermal fluctuations in mirrors over three decades of frequency range using a short-length Fabry-Perot interferometer. This is the first such measurement at wide off-resonant frequency band that is much lower than mechanical resonant frequencies. Theoretically, the mechanical fluctuation in mirrors had been thought to become the principal noise in precise interferometry, such as in gravitational wave detection. We identified the thermally induced noises in the interferometer, the so-called substrate Brownian noise, substrate thermoelastic noise, and coating Brownian noise.