One of the most novel and unusual new fiber optics is the photonic crystal or photonic bandgap (PBG) fiber. Photonic crystal fibers (PCFs) were first formed in pure fused silica in 1995 by Russell and his group at the University of Bath. PCFs are composed of a series of regularly spaced holes oriented along the fiber's optic axis. A schematic representation of a PCF is shown in Fig. 11.1(a). The air holes, with a refractive index of nearly 1, are surrounded by a honeycomb silica matrix. The defect site for the structure in Fig. 11.1(a) is the central air hole; but this defect could also be a solid core, which would be somewhat analogous to the solid core of a conventional SM fiber. Figure 11.1(b) shows a microphotograph of a silica PCF with its honeycomb structure of regularly spaced holes surrounding an air core. These micro structured PCFs are today receiving enormous interest, as they have some very attractive properties that differ from those for solid-core/clad fibers.
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