S. Susmithan

Radiation pressure excitation of test mass ultrasonic modes via three mode opto-acoustic interactions in a suspended Fabry-Perot cavity

Abstract Three-mode parametric instabilities may compromise stable operation of gravitational wave detectors. Instabilities manifest as varying radiation pressure distributions, derived from beating between two optical modes, exciting mirror acoustic modes in Fabry–Perot cavities. Here we report the first demonstration of radiation pressure driving of ultrasonic acoustic modes via pairs of optical modes in gravitational wave type optical cavities. In this experiment ∼ 0.4 W of TEM 01 mode and ∼ 1 kW of TEM 00 mode circulated inside the cavity, an ∼ 181.6 kHz excitation was observed with amplitude ∼ 5 × 10 − 13 m . The results verify the driving force term in the parametric instability feedback model (Braginsky et al., 2001) [1] . The interaction parametric gain was ( 3.8 ± 0.5 ) × 10 − 3 and mass-ratio scaled opto-acoustic overlap 2.7 ± 0.4 .

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.

Thermal tuning the optical cavity for 3 mode interaction studies using a CO2 laser

Three mode interactions could induce parametric instability in advanced gravitational wave detectors with high optical power circulating in the cavities. One of the conditions for parametric instability to occur is when the cavity frequency difference between fundamental mode and the high order mode matches the test mass acoustic mode frequency. The optical mode spacing is a function of cavity g-factor (radius of curvature). At the Gingin High Optical Power Facility, we have an 80 meter optical cavity particularly designed for studying high optical power effects in advanced gravitational wave detectors such as parametric instabilities. Here we present the recent results of thermal tuning the cavity g-factor by heating the test mass surface with a CO2 laser to investigate the 3-mode interactions. Observation of test mass thermal noise peaks above 160 kHz enhanced by 3 mode interaction is presented.