Suppressing parametric instabilities in LIGO using low-noise acoustic mode dampers
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 optomechanical 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 10 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%.
© 2019 American Physical Society. Received 18 September 2019; published 6 December 2019. The authors acknowledge the entire LIGO Scientific Collaboration for their wide-ranging expertise and contributions. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation, and it operates under Cooperative Agreement No. PHY-1764464. Advanced LIGO was built under Grant No. PHY-0823459. This paper carries LIGO Document No. LIGO-P1900243.
Submitted - 1909.07805.pdf
Published - PhysRevD.100.122003.pdf