Photonic Crystal Fiber (PCF)
The original PCF or ‘holey’ fiber has relatively small air holes surrounding a solid silica core. Although the design does not produce a band gap the fiber does have the unusual property of supporting only a single transverse mode7, leading to the name endlessly single-mode (ESM) fiber. This behavior can be explained in terms of modified total internal reflection (TIR) in which the effective core-cladding index contrast is a very strong function of wavelength. In standard fibers this index contrast is only weakly dependent on wavelength and higher-order modes appear as the wavelength decreases. But in a correctly designed PCF, the index contrast decreases at shorter wavelengths preventing the onset of any higherorder modes. Because of the relationship between wavelength and core size, the ESM property can also lead to a theoretically unlimited modal area. However, for practical applications, both the endless single mode and large effective area are severely limited by bend loss just as in comparable low-contrast step-index fibers. Examples of the various air/silica micro-structured fibers. a) The original PCF design that guides using modified TIR. b) An air-clad core that, in the limit of large air holes, reduces to a silica rod suspended in air. c) The air-core PBGF that guides using a true band-gap mechanism.