Inter-Modal Nonlinear Interactions Between Well Separated Channels in Spatially-Multiplexed Fiber Transmission

We demonstrate a new manifestation of Kerr nonlinear interactions in multimode fibers. We show that nonlinear distortions between spatial modes of multimode fibers can be larger for channels that are well separated in wavelength than for channels in close proximity. Introduction The maximum reach and the quantity of information that can be transmitted over single-mode fibers (SMFs) is determined by Shannon's theory and by the instantaneous Kerr nonlinearity [1]. Nonlinear propagation in SMFs can be decomposed in elementary nonlinear interactions that can help understand the main limitation of a system and develop techniques to increase system performance. It has become clear in the last few years [2] that increasing the number of spatial modes in fibers is an important path to increasing fiber capacity. Understanding nonlinear interactions in multimode, coupled multicore and microstructure fibers is critical to determine the performance of spatial multiplexing for long-haul transmission. There has been a limited number of studies of nonlinear propagation in multimode fibers. This includes the derivation of a set of nonlinear equations of propagation [3], [4], the numerical studies of supercontinuum generation [5] and early experimental demonstration of four-wave mixing (FWM) between modes in a few tens of meters of multimode fibers [6], [7]. In this paper, we present experimental evidences that nonlinear interactions in a 25-km long multimode fibers can be stronger for signals well separated in wavelength (5 nm) than for signals closely spaced (0.4 nm) if they propagate in different spatial modes.