Compact integrated optical sensors and electromagnetic actuators for vibration isolation systems in the gravitational-wave detector KAGRA.

This paper reports on the design and characteristics of a compact module integrating an optical displacement sensor and an electromagnetic actuator for use with vibration-isolation systems installed in KAGRA, the 3-km baseline gravitational-wave detector in Japan. In the technical concept, the module belongs to a family tree of similar modules used in other interferometric gravitational-wave detector projects. After the initial test run of KAGRA in 2016, the sensor part, which is a type of slot sensor, was modified by increasing the spacing of the slot from 5 mm to 15 mm to avoid the risk of mechanical interference with the sensor flag. We confirm that the sensor performance is comparable to that of the previous design despite the modification. We also confirm that the sensor noise is consistent with the theoretical noise budget. The noise level is 0.5 nm/Hz1/2 at 1 Hz and 0.1 nm/Hz1/2 at 10 Hz, and the linear range of the sensor is 0.7 mm or more. We measured the response of the actuator to be 1 N/A and also measured the resistances and inductances of coils of the actuators to confirm consistency with theory. Coupling coefficients among the different degrees of freedom were also measured and shown to be negligible, varying little between designs. A potential concern about thermal noise contribution due to eddy current loss is discussed. As of 2020, 42 of the modules are in operation at the site.

Measuring scattering light distributions on high-absorptive surfaces for stray-light reduction in gravitational-wave detectors.

In order to establish scattering measurements in material investigations for gravitational-wave detectors, we have built-up devices for measuring the hemispherical scattering distribution of materials which are planned to be used in those detectors as suppressors of scattered light. The measurement benches we have built, a hemispherical goniometer and a direct back-scatterometer, have a maximum background noise of ∼10 -4sr -1 BRDF at 1.064 µm wavelength which is the wavelength of the laser-light for our large interferometer for detecting gravitational waves, KAGRA. With these instruments, we have characterized the surface scattering of, e.g., NiP platings, metals, and different carbonaceous coatings, which are supposed to minimize the amount of scattered light in interferometers. The three most important materials for KAGRA's construction (SiC, "Solblack", and "VantaBlack") are presented in this paper. Furthermore, we will try to explain the scattering distributions with the generalized Harvey-Shack model (smooth-surface approximation) which is a common method for surface-scattering calculations. At the end, we give also some valuations about the vacuum compatibility of the materials, which is important for instruments like KAGRA that work under ultra-high vacuum conditions.

Calculation method for light scattering caused by multilayer coated mirrors in gravitational wave detectors.

Scattered light in inteferometric gravitational wave detectors needs to be reduced so that it will not harm the actual signals coming from a gravitational wave. In this paper, we report on the application of the theory of light scattering from mirrors in interferometric detectors having multilayer coatings on their surfaces and compared the results with single-surface scattering theories, which are traditionally used in the field of gravitational wave detectors. For the first time in this field, we have calculated the scattering distributions of the power-recycling, the signal-recycling, and the beam-splitter mirrors in KAGRA (a cryogenic interferometric gravitational wave detector currently under construction in the Kamioka mine in Japan) by using models of their multilayer coatings. Furthermore, we have performed simulations to show the differences between multilayer scattering and single-surface scattering models in the back-scattering of mechanical structures close to the mirrors and the impact on the sensitivity of the KAGRA detector. We show that the back-scattering by using those coatings can be larger by up to almost two orders of magnitude and they also give rise to additional scattering features that should be taken into account for all optical applications in gravitational wave detectors.

Characterization of the room temperature payload prototype for the cryogenic interferometric gravitational wave detector KAGRA.

KAGRA is a cryogenic interferometric gravitational wave detector currently under construction in the Kamioka mine in Japan. Besides the cryogenic test masses, KAGRA will also rely on room temperature optics which will hang at the bottom of vibration isolation chains. The payload of each chain comprises an optic, a system to align it, and an active feedback system to damp the resonant motion of the suspension itself. This article describes the performance of a payload prototype that was assembled and tested in vacuum at the TAMA300 site at the NAOJ in Mitaka, Tokyo. We describe the mechanical components of the payload prototype and their functionality. A description of the active components of the feedback system and their capabilities is also given. The performance of the active system is illustrated by measuring the quality factors of some of the resonances of the suspension. Finally, the alignment capabilities offered by the payload are reported.

Search for a stochastic background of 100-MHz gravitational waves with laser interferometers.

This Letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of approximately 10;{-16} Hz;{-1/2} at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found h{100};{2}Omega_{gw}<6 x 10;{25} to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed.