Absolute instability of the interaction between optical and acoustic waves

The conditions for the development of a photoelastic instability are investigated. It is shown that the threshold of this instability in a resonator may be considerably higher if the associated Stokes wave is not in resonance with the resonator. A mechanism is suggested and conditions are determined for the development of an absolute instability in the absence of an external feedback. It is shown that the absorption of an excitation wave leads to an absolute instability of the photoelastic interaction.