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

**Authors:** S. Biscans, S. Gras, C. D. Blair, J. Driggers, M. Evans, P. Fritschel, T. Hardwick, G. Mansell  
**arXiv:** [1909.07805](https://arxiv.org/abs/1909.07805)  
**Date:** 2019-09-17  
**Summary page:** https://www.summarizepaper.com/en/paper/1909.07805v1/

## Key points

- Interferometric gravitational-wave detectors like LIGO measure changes in arm lengths of order $10^{-18}~$m or smaller
- Achieving this requires high laser power to raise the signal above shot noise
- Parametric instabilities are a limiting effect at high power, caused by an opto-mechanical interaction between the laser field and the detector's test masses
- A recent study led by S. Biscans and colleagues successfully implemented tuned, passive dampers to tame parametric instabilities in LIGO
- These dampers reduce the quality factors of internal vibrational modes while adding negligible noise to the gravitational-wave output
- Measured mode quality factors have been reduced by at least a factor of ten with no visible increase in thermal noise level
- The dampers should remove most of the parametric instabilities when operating at full power while limiting the concomitant increase in thermal noise to approximately 1%
- This breakthrough could pave the way for future gravitational wave observations with even greater sensitivity than previously thought possible

## Summary

Interferometric gravitational-wave detectors like LIGO are designed to measure changes in their arm lengths of order $10^{-18}~$m or smaller. However, achieving this requires very high laser power 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, known as parametric instabilities. These 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, but only at power levels well below the final design value. In a recent study led by S. Biscans and colleagues, published in Physical Review D, they 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. The researchers found that 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. They 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%. This breakthrough could pave the way for future gravitational wave observations with even greater sensitivity than previously thought possible. Overall, this study highlights how innovative solutions can be developed and implemented effectively to overcome technical limitations and improve our understanding of fundamental physics phenomena such as gravitational waves.