Photonic crystal fiber (PCF) has been studied intensively in the past decade owing to its potential device applications in optical communication systems. PCFs have many advantages compared with the conventional fibers. Firstly, they have flexibly-tailored optical properties. Due to the microstructure dependence of their modal properties, the degree of freedom is increased dramatically in the design. Many properties such as the cutoff conditions, birefringence, dispersion and nonlinearity can be tailored beyond the possibilities in conventional fibers. Moreover, because of the single material in most PCFs, their improved temperature stability makes such novel fiber a suitable transmission medium. Secondly, the unique holey structure makes many applications possible. Due to the special stack-and-draw fabrication technique, the arrangement of air-holes in the microstructure can be customized. Multiple layers of claddings can be easily obtained by controlling the air-hole dimensions of capillary tubes in different layers. Multi-core structures are achieved by simply introducing multiple solid rods during the stacking process. The existence of air-holes enables the possibilities of injecting gases/liquids, so that PCF can be used as sensors.

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