# Modulational Instability in Linearly Coupled Asymmetric Dual-Core Fibers

## TL;DR

This paper analyzes modulational instability in asymmetric dual-core fibers, revealing how asymmetries and coupling influence MI spectra and nonlinear evolution, including soliton formation.

## Contribution

It provides a comprehensive analysis of MI in asymmetric dual-core fibers, including conditions, spectra, and nonlinear development, extending understanding beyond symmetric systems.

## Key findings

- Increased inter-core coupling reduces MI gain spectrum.
- Coupling induces and controls MI in normal-GVD regime.
- Nonlinear simulations show formation of soliton-like localized peaks.

## Abstract

We investigate modulational instability (MI) in asymmetric dual-core nonlinear directional couplers incorporating the effects of the differences in effective mode areas and group velocity dispersions, as well as phase- and group-velocity mismatches. Using coupled-mode equations for this system, we identify MI conditions from the linearization with respect to small perturbations. First, we compare the MI spectra of the asymmetric system and its symmetric counterpart in the case of the anomalous group-velocity dispersion (GVD). In particular, it is demonstrated that the increase of the inter-core linear-coupling coefficient leads to a reduction of the MI gain spectrum in the asymmetric coupler. The analysis is extended for the asymmetric system in the normal-GVD regime, where the coupling induces and controls the MI, as well as for the system with opposite GVD signs in the two cores. Following the analytical consideration of the MI, numerical simulations are carried out to explore nonlinear development of the MI, revealing the generation of periodic chains of localized peaks with growing amplitudes, which may transform into arrays of solitons.