Classification of wave instabilities produced by interaction of grating modes

We present the criteria and the main results of the wave instability studies in a phase grating containing space harmonic set. It is now generally accepted that all instabilities may be reduced to those of Bragg´s kind. The Bragg´s correlations pertain, however, only to the special case periodical system. The existence of instabilities is due to interaction between electromagnetic wave and phase grating modes. The periodicity of the grating is likely to be unimportant. By combining our analytical studies with model calculations may lead to a better understanding of wave instabilities