Parametric Oscillatory Instability in Fabry-Perot (FP) Interferometer

We present an approximate analysis of a nonlinear effect of parametric oscillatory instability in FP interferometer. The basis for this effect is the excitation of the additional (Stokes) optical mode and of the mirror's elastic mode when the optical energy stored in the FP resonator main mode exceeds the certain threshold. This effect is undesirable in laser gravitational wave antennae because it may create a specific upper limit for the value of energy stored in FP resonator. In order to avoid it the detailed analysis of the mirror's elastic modes and FP resonator optical modes structure is necessary.