Due to the ever increasing capacity demands for optical communication links, single-mode fibers will soon not be able to support the exponential growth of data rates. A strong candidate to overcome the upcoming capacity crunch is spacedivision multiplexing (SDM) where different parallel paths in one fiber are used to transmit independent data streams. Suitable candidates for SDM are e.g. multi-mode fibers (MMF), where different orthogonal modes are independently addressed. In this paper, we discuss nonlinear interaction between signals that propagate in different fiber modes. Based on the nonlinear effect of four-wave mixing, phase-matching between spectral components that propagate in different fiber modes is studied. With the knowledge of intra- and intermodal four-wave mixing, all Kerr-effect based nonlinearities can be expressed in an analytical way. We propose a method to calculate the maximal achievable Optical Signal to Noise Ratio (OSNR) for each mode, based on the assumption that the nonlinear interference can be taken into account for by an additive noise term.

We analyzed a sample transmission link that uses a 55-mode graded-index multi-mode fiber, aiming to minimize the group-delay differences between signals that travel in different fiber modes. It is shown that each of the 55 modes can achieve an equal or higher OSNR compared to a standard single-mode fiber.