I. Introduction
The dispersion-managed (DM) nonlinear Schrödinger equation (NLSE) is a constant-coefficient partial differential equation describing the propagation of a polarized electric field along an ultralong DM periodic optical link [1]– [3]. It is derived from the standard NLSE using the method of multiple scales and represents an alternative mathematical derivation of the propagation equation previously obtained by the method of averaging for long-haul systems with periodic amplification [4], [5] or periodic amplification and dispersion [6]. A fundamental role in the DM-NLSE is played by the kernel, whose properties substantially determine the distortions induced on the propagating signals [2]. Since in the DM-NLSE each span represents a differential [4], its applicability is in principle restricted to links with a very large number of spans, such as the ultralong submarine links with DM solitons for which it was initially developed.