Observation of three-mode parametric interactions in long optical cavities

We report the observation of three-mode optoacoustic parametric interactions of the type predicted to cause parametric instabilities in a $77\text{\ensuremath{-}}\mathrm{m}$-long, high-optical-power cavity that uses suspended sapphire mirrors. Resonant interaction occurs between two distinct optical modes and an acoustic mode of one mirror when the difference in frequency between the two optical cavity modes is close to the frequency of the acoustic mode. Experimental results validate the theory of parametric instability in high-power optical cavities, and demonstrate tunable parametric gain $\ensuremath{\sim}{10}^{\ensuremath{-}2}$ and more than $20\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$ amplification of a high-order optical mode power generated by an applied acoustic signal.