A study on the coupling coefficients for multi-core fibers

The complete analytical expressions of mode coupling dynamics for homogeneous 7-core multi-core fibers (MCFs) are obtained by using orthogonal coupled-mode theory (CMT). All the coupling coefficients between the adjacent cores, nonadjacent cores and self-coupling coefficients are taken into account, and proposed to be calculated by using their definition formulas rather than the frequently-used twin core fiber (TCF) model utilized in most publications before. Simulation results show that the coupling coefficients calculated by using the TCF model are smaller than that calculated by using definition formulas and consequently are not accurate, especially for strong coupling MCFs. Also the coupling coefficients between nonadjacent cores and self-coupling coefficients can be comparable to the coupling coefficients between adjacent cores, and consequently cannot be ignored in the analysis of strong coupling MCFs.