Optimizing Active Seismic Isolation Systems in Gravitational-Wave Detectors

TL;DR

This paper demonstrates the first application of H-infinity optimization to active seismic isolation in gravitational-wave detectors, significantly reducing seismic noise and improving detector duty cycle.

Contribution

It introduces the use of H-infinity optimization for seismic isolation in gravitational-wave detectors, achieving substantial noise attenuation and operational improvements.

Findings

Sevenfold seismic noise attenuation in 0.1-0.5 Hz band

88.2% noise performance improvement in targeted frequency range

Increased detector duty cycle from 53% to 80%

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 $10^{-22}\ \mathrm{m}$ 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 \textit{sensor correction} and \textit{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 O3GK observation run to 80% in O4a, demonstrating the effectiveness of the H-infinity optimization approach.