The basis of undesirable e.ect of parametric oscillatory instability in Fabry-Perot cavity is excitation of the
additional (Stokes) optical mode having frequency ω<sub>1</sub>, and the mirror elastic mode, having frequency ω<sub>m</sub>. It
appears when optical energy stored in the main mode, with frequency ω<sub>0</sub>, exceeds a certain threshold and the
frequencies are related as ω<sub>1</sub> approximately equal to ω<sub>0</sub> - ω<sub>m</sub>. The presence of anti-Stokes with frequency ω<sub>2</sub> approximately equal to ω<sub>0</sub> + ω<sub>m</sub> may depress parametric instability. We analyze the time evolution of parametric instability with account of both
Stokes and anti-Stokes mode. It allows predicting the characteristics of precursors of parametric instability. Observation of such precursors may help to avoid parametric instability.