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

Three-mode parametric-instabilities risk stable operation of gravitational-wave detectors. Instabilities occur through time 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 ~0.4W of TEM01 mode and ~1kW of TEM00 mode circulated inside the cavity, an ~181.6kHz excitation was observed with amplitude ~5E-13m. The results verify the driving force term in Braginsky's parametric instability feedback model. The interaction parametric gain was (3.8+/-0.5)E-3 and mass-ratio scaled opto-acoustic overlap 2.7+/-0.4.