Classification of the Core Modes of Hollow-Core Photonic-Bandgap Fibers

Using a new full-vectorial finite-difference mode solver utilizing a hexagonal Yee´s cell, we calculated the dispersion diagram of a slightly multimode (16 modes) air-core photonic-bandgap fiber (PBF) and the electric-field profiles of all of its core modes. Careful comparison shows striking similarities between these properties and those of the hybrid modes of a conventional step-index fiber, in terms of the modes´ field profiles, the modes´ degeneracy, the order in which the modes mode cut off in the wavelength space, and the maximum number of modes. Based on these similarities, we propose for the first time a systematic nomenclature for the modes of a PBF, namely hybrid HE and EH modes and of quasi-TE and quasi-TM modes. Other small but relevant similarities and differences between the modes of these two types of fibers are also discussed.