Coating thermal noise investigation for KAGRA

Gravitational wave astronomy is an emerging field in astronomy and astrophysics. For proper investigation and research in this field, the noise in the interferometers used as gravitational wave detectors must be reduced. Thermal noise caused by mechanical loss in the reflective coating on the mirrors is a fundamental source of noise. The cryogenic technique is a noise reduction method adopted by KAGRA (Kamioka, Japan) and the Einstein Telescope (to be constructed in Europe). However, further thermal noise reduction is required. For this purpose, an apparatus for measuring the mechanical loss of the coating at cryogenic temperatures must be developed. Such an apparatus was prepared at University of Toyama, Japan. The apparatus was cooled using liquid nitrogen and helium. The losses of the sample disks with and without the coating were measured. The mechanical loss in the coating and the thermal noise were derived from the difference between the losses of the sample disks with and without the coating. This apparatus was then used to investigate conventional coatings; and this apparatus is expected to be used in coatings in the future. The outline and important details of the apparatus are provided in this article.