Global control using a laser strainmeter for KAGRA

Abstract

Abstract The KAGRA is a large-scale laser interferometric gravitational wave (GW) telescope that is has been constructed in Japan’s underground. Underground telescopes such as KAGRA and the proposed Einstein Telescope have the advantage of substantially reduced seismic noise above 1 Hz relative to GW telescopes on the surface. However, even in underground, seismic motion at low frequencies such as microseisms (< 1Hz) and earthquakes (< 10mHz), could severely degrade GW telescopes’ stability. In the current design of the KAGRA, these disturbances are suppressed by feedback controls using inertial sensors such as seismometers and geophones. Nevertheless, suppression with this control method is limited by the insufficient sensitivity and tilt-horizontal coupling of the sensors at low frequencies. In this paper, we have developed a new control method based on a laser strainmeter named the Geophysics Interferometer (GIF) to resolve this problem. This was installed in the KAGRA tunnel and has a superior sensitivity of less than 100 mHz and no problems with tilt coupling. This new method will achieve improved stability for the operation of an interferometer. Also presented in the paper is the design of the control system and an experimental result.