Mode Coupling in Optical Fibers

Multimode and multicore optical fibers are pivotal for spatial division multiplexing, a key technology for future high-capacity optical communication systems. A critical transmission characteristic of these fibers is modal dispersion, which is made random by unpredictable perturbations and manufacturing imperfections. While the statistical properties of modal dispersion are well-explored, the underlying local phenomenon of mode coupling—its primary cause—has received comparatively less attention. Accurate modeling of mode coupling is essential for a deeper understanding of both the linear and nonlinear propagation dynamics in these fibers and for advancing their practical implementation. This paper provides a comprehensive review of mode coupling in multimode and multicore fibers, highlighting aspects of general validity and conducting an in-depth analysis of bending and twisting—the two most common perturbations affecting deployed fibers. The results reveal significant similarities between the mode coupling behaviors of multimode and multicore fibers, along with some remarkable differences.