Predicting Mode Properties of Porous-Core Honeycomb Bandgap THz Fibers by Semi-Analytical Theory

We describe a semi-analytical approach for modeling fibers which guide THz radiation in a porous core by the photonic bandgap effect due to a honeycomb cladding of air holes. These porous-core honeycomb bandgap THz fibers (PCHBTFs) are modeled as equivalent step-index fibers (SIFs). The effective core index of the SIFs can be solved analytically as that of the fundamental space-filling mode of the triangular lattice comprising the porous core. The cladding index is taken as that of the bandgap edge with the lower index and is computed numerically. The validity of the analogy between PCHBTFs with SIFs is established by comparing the mode profiles, mode number, mode indices, and the fraction of mode power in the core obtained for a 19-cell core multimode THz fiber by the semi-analytical theory with results by finite element method. The subtle difference in mode degeneracy of the PCHBTFs and SIFs is briefly discussed. The semi-analytical theory also predicts that a seven-cell core PCHBTF is effectively single mode with higher-order modes pushed close to the bandgap edge.