Optical fiber modes

Summary

The modes of an optical fiber are the field patterns that travel along the fiber without changing shape. In a step-index fiber, light is confined in a core of higher refractive index by total internal reflection at the cladding. A guided mode has a propagation constant between the wavenumbers of the cladding and of the core, and decays exponentially in the cladding. The number of guided modes is governed by the normalized frequency $V$. For $V < 2.405$ only one mode is guided, which is the basis of single-mode fibers for long-distance communication.