Three mode interactions as a precision monitoring tool for advanced laser interferometers

Many thousands of three mode opto–acoustic interactions are expected to be observable in the advanced laser interferometer gravitational wave detectors now under construction. Each interaction represents a high-Q acoustic resonance interacting with high order optical modes inside the interferometer. This paper shows that this huge set of signals between 10–100 kHz have high sensitivity to changes in the optical wavefronts within the interferometer and can be used to create a powerful probe of the entire interferometer. We show that 3MI signals can be used to monitor thermal distortions corresponding to wavefront changes ∼3 × 10−12 m. Observations can be used at low optical power to predict parametric instabilities that could occur at higher power. In addition, the observed mode amplitudes could be used to control the interferometer operating point against slow environmental perturbations. Data on 80 m cavities and modelling results are used to demonstrate the sensitivity of 3MI monitoring. Experimental observations on advanced interferometers are suggested as a means to turn 3MI monitoring into an effective tool. (paper)