The role of septic saturation on the cross-phase instability in two-core birefringent oppositely directed PIM-NIM coupler

Abstract

We examine the cross-phase modulational instability in a two-core birefringent coupler made of a positive and negative index material. The modified form of generalized nonlinear Schrödinger equation contains a nonlinear septic term, cross-phase modulation, intrapulse Raman scattering, self-steepening, and conventional type of saturation nonlinearity. We investigate normal and anomalous group-velocity dispersion regimes and cases of birefringent effects such as zero-birefringence, linear birefringence, and circular birefringence. We observe that the MI gain spectrum shows significant changes in the presence of cross-phase modulation with a nonlinear saturation effect and non-Kerr nonlinearity. In addition, the self-steepening brings out new sidebands and shifts the existing sidebands to bring them together, providing a new pathway to generate solitons or ultrashort pulses in the presence of nonlinear septic saturation. The intrapulse Raman scattering effect causes conventional and Raman bands, extending the modulational instability (MI) domain range. The saturable nonlinearity suppresses the MI gain and reduces the bandwidth of the spectrum for both normal and anomalous dispersion regimes. But, in the case of nonlinear saturation with non-Kerr nonlinearity, the MI gain spectra exhibit efficient modification in a two-core birefringent oppositely directed coupler (ODCs) with adjustable effects of SS, IRS, and cross-phase modulation. Therefore, manipulating MI and solitons in a two-core ODCs is better performed when the birefringence is linear rather than circular. Here we present how to control and generate the MI and solitons in birefringent ODCs, emphasizing the negative index material (NIM) channel.