Modulational instability in oppositely doubled-doped directed couplers with a non-kerr-like nonlinear refractive-index change

We theoretically investigate modulation instability (MI) in a two-core nonlinear oppositely directed coupler with a negative-index material channel in the presence of a non-kerr-like nonlinear refractive-index change. Using linear stability analysis, we obtain an expression for the instability gain. It is found that the MI in the nonlinear two-core oppositely directed coupler is significantly influenced by the ratio of the forward- to backward-propagating wave power, nonlinearity, power and saturation. In the normal dispersion regime, the septic nonlinearity is found to suppress the MI by reducing both the gain and width of the instability nonlinearity. But in anomalous dispersion regime, the situation is different because septic nonlinearity enhances the MI by increasing both the gain and width of MI. The increase of septic nonlinearity both in normal and anomalous dispersion regimes increases the gain of the instability nonlinearity. Thus, we report new ways to generate and manipulate the MI and solitons in two-core oppositely directed couplers with a particular emphasis on a negative-index material (NIM) channel in presence of septic nonlinearity.