Suppressing parametric instabilities in LIGO using low-noise acoustic mode dampers

TL;DR

This paper demonstrates the successful use of tuned passive dampers on LIGO test masses to significantly reduce parametric instabilities at high laser power, with minimal impact on thermal noise, enabling higher sensitivity in gravitational wave detection.

Contribution

Introduction of tuned passive dampers applied directly to LIGO test masses to effectively suppress parametric instabilities without increasing thermal noise.

Findings

Mode quality factors reduced by at least a factor of ten.

No visible increase in interferometer's thermal noise.

Dampers projected to eliminate most parametric instabilities at full power.

Abstract

Interferometric gravitational-wave detectors like LIGO need to be able to measure changes in their arm lengths of order $10^{-18}~$m or smaller. This requires very high laser power in order to raise the signal above shot noise. One significant limitation to increased laser power is an opto-mechanical interaction between the laser field and the detector's test masses that can form an unstable feedback loop. Such parametric instabilities have long been studied as a limiting effect at high power, and were first observed to occur in LIGO in 2014. Since then, passive and active means have been used to avoid these instabilities, though at power levels well below the final design value. Here we report on the successful implementation of tuned, passive dampers to tame parametric instabilities in LIGO. These dampers are applied directly to all interferometer test masses to reduce the quality factors of their internal vibrational modes, while adding a negligible amount of noise to the gravitational-wave output. In accordance with our model, the measured mode quality factors have been reduced by at least a factor of ten with no visible increase in the interferometer's thermal noise level. We project that these dampers should remove most of the parametric instabilities in LIGO when operating at full power, while limiting the concomitant increase in thermal noise to approximately 1%.