Surface modes in air-core photonic band-gap fibers
Hollow-core photonic band-gap fibers (PBGFs) are typically made up of a periodic array of air holes in glass, surrounding an air core. These fibers have attracted considerable interest because they hold the promise of exhibiting lower attenuation and lower nonlinearity than conventional glass-core fibers. The potential for lower attenuation derives from the lower absorption and lower Rayleigh scattering of air relative to glass. In practice however, the attenuation of small-core PBGFs is still far from the losses typical in standard single-mode optical fiber (~0.2 dB/km), with a low value of 13 dB/km reported in 2002 [1,2]. Photonic band-gap fibers rely on the interference of light reflected from the periodic structures in the cladding. Among the most commonly suggested loss mechanisms in such fibers is tunneling or confinement loss due to the exponential tail of the mode field extending beyond the finite extent of the cladding. Tunneling loss is not unique to PBGFs and is present in all conventional optical fibers with high-index polymer coatings. Recent work has examined the impact of air-filling fraction and the number of rows of air holes on tunneling loss in PBGFs [3,4]. These calculations suggest that tunneling losses of <0.1 dB/km should be achievable in 10-row, high air-fill structures (>94%) with single-mode cores, thus reported attenuations do not appear to be limited by this mechanism. Consequently, as is assumed in the study of conventional optical fiber, we will treat the theoretically “leaky” core modes as true guided modes. Experimental confirmation of the present tunneling calculations will require the fabrication of fibers with significantly lower losses or fewer rows of air holes.
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