  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.
