Modulation instability in high-order coupled nonlinear Schrödinger equations with saturable nonlinearities

Abstract: The influence of a saturable nonlinearity on the modulation instability in oppositely directed coupler in the presence of high-order effects is investigated. By using the standard linear stability analysis, we obtain the instability gain that exhibits a significant change in the bands of instability due to the effects of a saturable nonlinearity. We also show that even in the presence of saturation there is no change in instability gain when we compare the results obtained for both channels not influenced by self-steepening effect or for both channels influenced by self-steepening effect but opposite in sign. Regarding the Raman effect, there is reflection symmetry (asymmetry) to the gain at zero perturbation frequency when the values of the Raman coefficients in each directional coupler are equal and with same (opposite) sign. For the anomalous group velocity dispersion regime we observe that the growth of the saturation parameter is followed by an increase in the null gain region near {\Omega} = 0, enlarging the separation of the instability bands close to this point. Finally, we show that an efficient control of the modulation instability can be realized by adjusting self-steepening effect and intrapulse Raman scattering, even in the presence of a saturable nonlinearity.