The optical version of a crystal, namely the photonic crystal, is a periodic distribution of a dielectric structure with a period on the order of an optical wavelength. According to Maxwell's equations, under certain circumstances a photonic band gap can appear, and therefore the propagation of light with particular frequencies is completely forbidden . Photonic crystal fibres (PCF) composed of silica and air, have become very attractive for many new applications due to their special features such as large nonlinearity and adjustable dispersion and wave guidance by the photonic bandgap effect. If in such fibre the photonic band gap expands above the air line, k = beta, guiding of light in an air core can be possible. Dispersion and polarization properties of solid-core square photonic band gap fibres have been extensively studied . However, the possibility of air guiding in square lattice photonic crystals fibres, to the best of our knowledge, has not been studied. The basic square structure, a square arrangement of circular holes, presents very narrow gaps crossing the air line. Here, we study an arrangement of octagonal holes in a square pattern, see Fig.1. This structure presents wider bandgaps than the basic square lattice since isolated high-index regions a! re con nected by very narrow veins. PBG regions extending above the air line begin to appear for air filling factors around 65%. For low air filling factors, 70 to 80%, the relative width of the gap crossing the air line is between 17 and 28% while triangular structures present gaps with relative widths of less than 13% for the same range of air filling factors. Numerical results demonstrate that such band gaps can be used to guide light in a properly chosen air core design.
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