Investigation of the nonlinearity in few mode fibers

Nonlinear effects in few-mode fibers (FMF) have been explored in the early years of optical communication. Due to the development of space-division multiplexing (SDM) technology for fiber transmission in FMF, the nonlinearity in FMF has attracted significant research interests recently. Analogous to single-mode fiber, nonlinear effects may eventually limit the information capacity in FMF transmission systems. In this paper, we first reviewed the past and current research in this area. The paper then concentrates on the theoretical and numerical investigation of inter-modal four-wave mixing (IM-FWM) among up to four spatial modes in FMFs. Different forms of multi-mode non-linear Schrödinger equation (MM-NLSE) for two, three and four spatial modes are derived, along with up to twelve types of wave/mode distributions and their corresponding phase matching conditions. We analyze the mixed effects of differential mode group delay (DMGD), chromatic dispersion (CD), random mode coupling (RMC) and wavelength separation upon IM-FWM efficiency in FMF. In addition, intermodal nonlinear effects in FMFs may potentially provide a variety of innovative optical signal processing functionality for fiber network applications through this additional spatial dimension. For example, we show simultaneous mode and wavelength conversion based on IM-FWM, and explore the effects of group velocity dispersions in order to achieve higher conversion efficiency.