Mode Coupling Resulting From Forward Rayleigh Scattering in Multimode Fibers

We investigate the contribution of forward Rayleigh scattering to the linear mode coupling in multimode fibers for mode-division multiplexing. The prediction of mode coupling is of special importance in the case of a reduced MIMO approach, where just coupling within mode groups is mitigated. Therefore, we investigate fibers with weak coupling between mode groups and a moderate number of about 10 spatial modes. While other linear mode coupling effects such as geometric perturbations of the refractive index profile might be reduced by improved fiber manufacturing, Rayleigh scattering remains an inherent effect in fibers consisting of amorphous silica. We find that the power coupling resulting from forward Rayleigh scattering increases roughly linearly with the fiber length in fibers in the weak coupling regime due to the incoherent nature of the scattering process. Therefore, forward Rayleigh scattering might limit the applicability of a reduced MIMO approach to transmission lengths of a few hundred kilometers. Comparative analyses among different fiber types underscore the significance of mode overlap and the numerical aperture in determining the strength of forward Rayleigh scattering. Additionally, we investigate the influence of the spatially resolved Rayleigh scattering coefficient due to the refractive index profile on the cross talk. Thereby, we find that this effect is most pronounced in graded-index fibers but results in only minor differences between the results of the spatially resolved and the non-spatially resolved calculations for a fiber length of 48 km.