Effective dissipation and nonlocality induced by nonparaxiality

We investigate beam diffraction and spatial modulation instability of coherent light beams propagating in the nonparaxial regime in a nonlinear Kerr medium. We study the instability of plane-wave solutions in terms of the degree of nonparaxiality, beyond the regime of validity of the nonlinear Schroedinger equation. We also numerically analyze the way nonparaxial terms break the integrability and affect the periodical evolution of higher-order soliton solutions. We discuss nonlocality and effective dissipation introduced by the nonlinear coupling with evanescent waves