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.