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Interferometric gravitational-wave detectors like LIGO need to be able tomeasure changes in their arm lengths of order $10^{-18}~$m or smaller. Thisrequires very high laser power in order to raise the signal above shot noise.One significant limitation to increased laser power is an opto-mechanicalinteraction between the laser field and the detector's test masses that canform an unstable feedback loop. Such parametric instabilities have long beenstudied as a limiting effect at high power, and were first observed to occur inLIGO in 2014. Since then, passive and active means have been used to avoidthese instabilities, though at power levels well below the final design value.Here we report on the successful implementation of tuned, passive dampers totame parametric instabilities in LIGO. These dampers are applied directly toall interferometer test masses to reduce the quality factors of their internalvibrational modes, while adding a negligible amount of noise to thegravitational-wave output. In accordance with our model, the measured modequality factors have been reduced by at least a factor of ten with no visibleincrease in the interferometer's thermal noise level. We project that thesedampers should remove most of the parametric instabilities in LIGO whenoperating at full power, while limiting the concomitant increase in thermalnoise to approximately 1%.

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