Abstract

We numerically and theoretically investigate the core modes of two-dimensional solid-core photonic bandgap (PBG) fibers based on hexagonal arrays of high-index circular rods. Such fibers guide light in discrete bandgaps, and the number of core-guided modes depends on the order of the bandgap as well as the position within the bandgap. We first classify the different core-guided modes in such fibers and we discuss the links among band structure, losses, and number and type of modes. We demonstrate that, similar to the case of bandgapless Kagome and ring-based fibers, solid-core bandgap fibers can have core-guided modes that are within photonic bands of the cladding. We discuss the classification of core modes in such fibers, and highlight analogies and differences with that of index-guiding fibers. Through an asymptotic expansion of an analytic model of a fiber’s photonic bands, we show that, in the limit of higher-order gaps (i.e., short wavelengths), the number of modes in the middle of gaps tends to a constant that is independent of refractive index contrast, as is the case for index-guiding photonic crystal fibers. We also discuss the evolution of the effectively single-mode propagation regime with geometrical parameters of structures having constant or variable band diagrams. For small- and large-core PBG fibers, we compute the exact number of core-guided modes within the center of the transmission band. We discuss their evolution with gap orders and coupling strength between high-index inclusions in the cladding. We find good agreement of the core-guided mode number in the center of the gaps computed with our theoretical model and with a numerical method for short wavelengths.

© 2012 Optical Society of America

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