Investigation of Mode Coupling Introduced by Forward Rayleigh Scattering

In this paper, we use analytical calculations to estimate the cross talk introduced by Rayleigh scattering in multi-mode fibers with 10 modes. Unlike linear mode coupling, which is caused by perturbations of the fiber geometry, Rayleigh scattering cannot completely be avoided in amorphous SiO2 and potentially results in a lower limit for mode coupling. We show that due to the incoherent nature of the scattering process, the cross talk increases linearly with the length of the transmission link. For the investigated step-index and graded-index fibers we observe cross talk >20 dB even after a moderate link length of 120 km.