Modeling of nonlinear propagation in space-division multiplexed fiber-optic transmission

In recent years, space-division multiplexed (SDM) transmission in multimode and multicore fiber structures has been attracting growing interest as a mean of scaling the capacity of the optical transport network. As in the case of standard systems based on the single-mode fibers, the ultimate limit to the achievable transmission rate is set by the nonlinearity of the fiber, and hence, the accurate modeling of nonlinear propagation in SDM fibers is a critical task. A key feature of long multimode fiber structures that are relevant for communications is the existence of random coupling between modes. This coupling has a major effect on the properties of nonlinear propagation, and in its presence, the coupled nonlinear Schrödinger equations, which are characterized by a very large number of propagation constants, reduce to the much simpler form of the coupled generalized Manakov equations. These equations shed light on the relevant aspects of signal propagation dynamics, and facilitate the establishment of an intuitive physical picture. Another key feature of SDM fibers is the existence of modal dispersion that introduces frequency dependence into the mode mixing process and modifies the effects of nonlinear propagation. In this paper, we review all of the above mentioned phenomena, and in addition, we assess the way in which the information capacity of SDM fibers is expected to scale with the number of propagation modes. Finally, we extend the Manakov formalism so as to account for the noninstantaneous Raman contribution to the nonlinear response of silica.