Theory of modulational interaction of two coupled waves. Part 1. General formulae for a nonlinear medium

The modulational interaction of two waves propagating in a cubically nonlinear medium is considered. It is shown that the correct theory operates with ‘renormalized’ equations for the linear dielectric function and for an effective third-order medium response. This renormalization is due to so-called interference terms. The appearance of interference terms is a specific feature of the two-mode modulational instability in comparison with the well-known instability of a single mode.