Reducing suspension control noise with interferometric sensors -- an experimental concept

TL;DR

This paper articulates an experimental concept to reduce control-induced displacement noise in ground-based gravitational-wave detectors by employing interferometric local displacement sensors, specifically the Compact Balanced Readout Interferometer (COBRI) based on deep frequency modulation. It sketches a two-HRTS suspension setup housed in a actively isolated vacuum environment to compare COBRI against shadow-sensing BOSEM readouts, using an optical cavity and Pound-Drever-Hall locking to quantify improvements in length stability and six-DOF control performance. A detailed noise-budget model incorporating seismic, thermal, quantum, and laser-frequency noise predicts notable gains for COBRI, particularly in the longitudinal axis, and highlights the importance of enhanced seismic pre-isolation to maximize observable benefits. The authors outline practical upgrades to suspension mechanics and seismic isolation, with the goal of a robust demonstration of interferometric sensing advantages for current and next-generation detectors, with results expected around 2026. ($6$-DOF framework, multiple noise channels, and control-system considerations are integral to the analysis.)

Abstract

One of the limiting noise sources of ground-based gravitational wave detectors at frequencies below 30 Hz is control-induced displacement noise. Compact laser interferometric sensors are a prime candidate for improved local displacement sensing. In this paper we present the design of an experiment that aims to demonstrate the advantages of interferometric sensors over shadow sensors. We focus on the compact balanced readout interferometer (COBRI) - a sensor currently in development that is based on deep frequency modulation. We mount COBRIs on two HAM Relay Triple Suspension (HRTS) systems that suspend two mirrors forming an optical cavity. By measuring the length stability of this cavity relative to a stable reference we aim to probe the direct motion reduction when using COBRIs for active damping and we aim to investigate their behavior and auxiliary functions, such as absolute ranging, in the context of the 6 degree-of-freedom controls of the suspensions. Here we describe the design of the experiment and simulations of the achievable noise levels that were obtained using mechanical models of the HRTS suspensions. We discuss all relevant noise sources, the modeled influence of the interferometric sensor damping and the current limitations and necessary improvements of our testing facility in terms of seismic pre-isolation to achieve a shadow sensor limited noise at around 5 Hz, where, according to our simulations, we can demonstrate superior performance for COBRIs in the longitudinal degree of freedom.

Figures (12)

• Figure 1: Sketch of the planned experimental setup. Two HRTS facing each other on a passively isolated optical table, inside VATIGrav, an actively isolated vacuum chamber. 6 DOF sensing and control at the top mass with both BOSEMs and COBRIs.
• Figure 2: Proposed optical setup for the suspension cavity readout. The laser frequency stabilization is realized by locking the laser to a reference using PDH. The amplitude is stabilized via dc-offset locking with a power pick-off after the mode-cleaner in vacuum. The mode-cleaner is locked to the laser-frequency with PDH and the suspended cavity is kept on resonance with a digital control system, which also takes advantage of the PDH sidebands from locking to the reference cavity.
• Figure 3: Amplitude spectral density models of the BOSEM- and expected COBRI sensing noise. The COBRI noise is dominated by laser freqeuncy noise at low frequencies and electronic readout noise at high frequencies.
• Figure 4: ASD of the seismic measurements in translation on the optical table with active and passive pre-isolation.
• Figure 5: ASD of the seismic measurements in rotation on the optical table with active and passive pre-isolation.