Standing and Traveling Waves in a Model of Periodically Modulated One-dimensional Waveguide Arrays

TL;DR

This paper investigates how periodic modulation in a one-dimensional nonlinear waveguide array influences the formation of standing and traveling waves, revealing power-dependent behaviors and conditions for stable, exact traveling solutions.

Contribution

It introduces a simplified step-function coupling model to analyze and construct stable traveling and stationary coherent structures in modulated waveguide arrays.

Findings

Low power leads to unidirectional energy transport.

High power results in stationary solutions.

Stable, exact traveling solutions are identified under specific parameters.

Abstract

In the present work, we study coherent structures in a one-dimensional discrete nonlinear Schr\"odinger lattice in which the coupling between waveguides is periodically modulated. Numerical experiments with single-site initial conditions show that, depending on the power, the system exhibits two fundamentally different behaviors. At low power, initial conditions with intensity concentrated in a single site give rise to transport, with the energy moving unidirectionally along the lattice, whereas high power initial conditions yield stationary solutions. We explain these two behaviors, as well as the nature of the transition between the two regimes, by analyzing a simpler model where the couplings between waveguides are given by step functions. In this case, we numerically construct both stationary and moving coherent structures, which are solutions reproducing themselves exactly after an integer multiple of the coupling period. For the stationary solutions, which are true periodic orbits, we use Floquet analysis to determine the parameter regime for which they are spectrally stable. Typically, the traveling solutions are characterized by having small-amplitude, oscillatory tails, although we identify a set of parameters for which these tails disappear. These parameters turn out to be independent of the lattice size, and our simulations suggest that for these parameters, numerically exact traveling solutions are stable.