Parametric amplification and modulational instabilities in dispersive nonlinear directional couplers with relaxing nonlinearity

We investigate the effects of a nonlinearity with a finite response time, combined with group-velocity dispersion and self-phase modulation, on the parametric amplification and modulational instability exhibited by light propagating in a nonlinear directional coupler. A linear-stability analysis of the nonlinear coupled-mode equations describing propagation in the coupler furnishes simple general expressions for the bandwidth of the spatial growth rate of an initially weak modulation for the case in which the carrier propagates in either the symmetric or the antisymmetric mode of the coupler. New types of scalar and vectorial modulational instabilities of the nonlinear Schrodinger equation are predicted, in both the normal- and the anomalous-dispersion regimes. The physical origin of these instabilities is the interplay among linear coupling, parametric four-photon mixing, and Raman scattering.