Optimizing active seismic isolation systems in gravitational-wave detectors

Abstract Gravitational wave detectors such as KAGRA, a 3-km long underground laser interferometer in Japan, require elaborate passive and active seismic isolation of their mirrors. With the aim of detecting passing gravitational waves that create a relative mirror displacement of less than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>22</mml:mn> </mml:mrow> </mml:msup> <mml:mtext> </mml:mtext> <mml:mrow> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> </mml:mrow> </mml:math> at frequencies of tens to hundreds of Hz, all environmental couplings must be stringently suppressed. This paper presents the result of applying the H-infinity optimization method to the active seismic isolation of a gravitational-wave detector for the first time. The so-called sensor correction and sensor fusion schemes of the seismic attenuation system of KAGRA’s signal recycling mirror are used as a test bed. We designed and implemented optimal sensor correction and sensor fusion filters, resulting in a sevenfold attenuation of seismic noise coupling to the signal recycling mirror in the 0.1–0.5 Hz band, with the downstream effect of an 88.2% noise performance improvement in the same frequency band. When combined with other hardware upgrades, the implementation of sensor correction and sensor fusion contributed to an increase in KAGRA’s duty cycle from 53% in the joint GEO600-KAGRA observing run (O3GK) (7th April 2020 to 21st April 2020) observation run to 80% in the first segment of the forth observing run (O4a) (KAGRA joined from 24th May 2023 to 21st June 2023), demonstrating the effectiveness of the H-infinity optimization approach.