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Symplectic and Multi-Symplectic Methods For Coupled Nonlinear Schrödinger Equations With Periodic Solutions

Oluşturulma Tarihi: 07-09-2015

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Konu(lar): BİLİM, Matematik,

Yazar(lar): Aydin , Ayhan (Yazar), Karasözen, Bülent (Yazar),

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We consider for the integration of coupled nonlinear Schrödinger equations with periodic plane wave solutions a splitting method from the class of symplectic integrators and the multi-symplectic six-point scheme which is equivalent to the Preissman scheme. The numerical experiments show that both methods preserve very well the mass, energy and momentum in long-time evolution. The local errors in the energy are computed according to the discretizations in time and space for both methods. Due to its local nature, the multi-symplectic six-point scheme preserves the local invariants more accurately than the symplectic splitting method, but the global errors for conservation laws are almost the same.

İçindekiler

## 1. Introduction

The nonlinear Schrödinger equation (NLS) arises as model equation with second-order dispersion and cubic nonlinearity for describing the dynamics of slowly varying wave packets in nonlinear optics and fluid dynamics. If there are two or more modes the coupled nonlinear Schrödinger (CNLS) system would be the relevant model. The two coupled nonlinear Schrödinger (CNLS) equations are given by

equation(1)  where ψ1(x,t) and ψ2(x,t) are complex amplitudes or ‘envelopes’ of two wave packets, i is the imaginary number, x and t are the space and time variables, respectively. The CNLS system has many applications including nonlinear optics and  and geophysical fluid dynamics  and . The parameters αj are the dispersion coefficients, σj the Landau constants which describe the self-modulation of the wave packets, and v12 and v21 are the wave–wave interaction coefficients which describe the cross-modulations of the wave packets  and .

Analytical solutions can be obtained only for a few special integrable cases, like the Manakov model  where the self-modulation and wave–wave interaction coefficients are equal, i.e. α12=1σ12=v12=v21. Other integrable cases are: α12,σ12=v12=v21 and α1=−α2σ12=−v12=−v21 which can be solved by the inverse scattering method . For nonintegrable cases, where the parameters are different, numerical methods have to be used in order to understand different nonlinear phenomena that arise by the interaction of stable and unstable wave packets in the CNLS system.

There has been a lot work done for the solitary wave solutions of (1). Symplectic and multi-symplectic methods are also used for the numerical solution of soliton collision and . Little attention has been paid to plane wave solutions under periodic boundary conditions. In this work we will consider the plane wave solutions of (1) under periodic boundary conditions with period L with the dispersion relations where the amplitudes ψ10 and ψ20 are assumed to be real  and . We takev12=v21=v because this choice of the parameters allows a multi-symplectic formulation of the CNLS system.

Besides the Manakov's case which corresponds to elliptical polarization we consider two other polarizations  and :

linear polarization: v12=v21=2/3,

circular polarization: v12=v21=2

with α12=1σ12=1.

Several numerical studies have been carried out in recent years in order to understand the behavior of the periodic plane wave solutions of the CNLS system. In  a pseudospectral discretization was used in the space variables and the resulting system of ordinary differential equations (ODEs) were integrated using the standard fourth-order Runge–Kutta method. Because the method used in  is not conservative, no indication is given about the preservation of the conserved quantities of the CNLS system. In  the CNLS system with periodic boundary conditions was discretized in space using second-order finite differences and the resulting system of ordinary differential equations (ODEs) are integrated in time using the so-called Hopscotch method, a mixture of an explicit and an implicit method. The accuracy of the solutions were checked only using the norm conservation of each amplitude of the CNLS system. The choice of particular initial conditions may affect the preservation of some invariants. As mentioned in  in the context of multi-symplectic integrators usually symmetric initial conditions are used in the literature. Unsymmetric initial conditions may lead to an increase of errors in preserving some invariants like momentum over time .

The numerical solution of nonlinear wave equations using symplectic and multi-symplectic geometric integrators has been the subject of many studies in recent years (see for a recent review ). The NLS and CNLS systems represent an infinite-dimensional Hamiltonian system. After semi-discretization in the space variables one usually obtains a system of Hamiltonian ordinary differential equations, for which various symplectic integrators can be applied  and . In Section 2 we give a symplectic integrator based on the splitting of the semi-discretized Hamiltonian system in linear and nonlinear parts. Another way is to use the multi-symplectic structure of the NLS and CNLS equations and apply multi-symplectic integrators  and . A brief review of multi-symplectic integrators and their application to the CNLS system given in Section3. In Section 4 numerical results obtained with the symplectic splitting method and multi-symplectic six-point Preissman scheme are compared with results found in the literature for symmetric and unsymmetric initial conditions. Section 5 is devoted to concluding remarks.

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