Robust Bilinear-Noise-Optimal Control for Gravitational-Wave Detectors: A Mixed LQG/$H_\infty$ Approach

TL;DR

This paper introduces a novel control approach combining LQG and $H_$ methods to minimize bilinear noise in gravitational-wave detectors, enhancing their sensitivity by optimizing feedback control systems.

Contribution

It develops a benchmark framework and a mixed LQG/$H_$ control method to achieve robust, optimal feedback control, reducing noise in LIGO's low-frequency measurements.

Findings

Established lower bounds on bilinear control noise.

Demonstrated fast computation of globally optimal robust feedback.

Showed potential for significant noise reduction in existing and future detectors.

Abstract

At its lowest frequencies, LIGO is limited by noise in its many degrees of freedom of suspended optics, which, in turn, introduce noise in the interferometer through their feedback control systems. Nonlinear interactions are a dominant source of low-frequency noise, mixing noise from multiple degrees of freedom. The lowest-order form is bilinear noise, in which the noise from two feedback-controlled subsystems multiplies to mask gravitational waves. Bilinear couplings require control trade-offs that simultaneously balance high- and low-frequency noise. Currently, there is no known lower limit to bilinear control noise. Here, we develop benchmark cost functions for bilinear noise and associated figures of merit. Linear-quadratic-Gaussian control then establishes aggressive feedback that saturates the lower bounds on the cost functions. We then develop a mixed LQG and $H_\infty$ approach to directly compute stable, robust, and optimal feedback, using the LIGO's alignment control system as an example. Direct computations are fast while ensuring a global optimum. By calculating optimal robust control, it is possible to construct the lower bound on controls noise along the Pareto front of practical controllers for LIGO. This method can be used to drastically improve controls noise in existing observatories as well as to set subsystem control noise requirements for next-generation detectors with parameterized design.