Parametric instability in GEO 600 interferometer

We present analysis of undesirable effect of parametric instability in signal recycled GEO 600 interferometer. The basis for this effect is provided by excitation of additional (Stokes) optical mode, having frequency ω1, and mirror elastic mode, having frequency ωm, when the optical energy stored in the main FP cavity mode, having frequency ω0, exceeds a certain threshold and detuning Δ=ω0−ω1−ωm is small. We discuss the potential of observing parametric instability and its precursors in GEO 600 interferometer. This approach provides the best option to get familiar with this phenomenon, to develop experimental methods to depress it and to test the effectiveness of these methods in situ.