Exact Expressions for Vector Mode Cutoff in Three-Layer Step-Index Fibers

Two-layer step-index fibers have well-known cutoff solutions, while cutoff solutions for three-layer step-index fiber exist for only one geometry (ring fiber). We derive exact expressions for cutoff frequencies of both vector and scalar modes for all geometries of three-layer step-index fibers. While vector modes have been solved for these fibers, to the best of our knowledge, this is the first time cutoff expressions are reported. Expressions for cutoff help predict the number of supported modes for a given fiber profile. Using these cutoff expressions, we deduce characteristics of fiber modal content as a function of fiber profile.