Introduction.
In the 1970's researchers demonstrated air-glass fiber profiles as a possible approach to low-loss optical fiber. A few years ago, interest in air-glass fiber profiles was rekindled by the idea of creating two-dimensional photonic band gaps in optical fibers. Typically, these optical fibers have a collection of air holes that run along the length of the fiber. The discovery of a number of interesting physical effects, some not necessarily associated with photonic band gaps, has blossomed into the field of Photonic Crystal Fiber. Along with that, a potentially confusing series of monikers has developed to distinguish between the various embodiments. Fortunately, the physics describing the fibers conveniently separates them into two distinct classes, those employing photonic band gaps for guidance and those that use a type of total internal reflection for guidance. The terms holey fiber, hole-assisted fiber, microstructured fiber, and effective-index fiber refer to fibers that employ total internal reflection as the guidance mechanism. Photonic band-gap fiber, Bragg fiber, and omnidirectional waveguide refer to fibers that use photonic band gaps as the guidance mechanism. We have used the term photonic crystal fiber (PCF), the highlight of this Focus Issue, to represent all these types of fibers. Though not all of the fiber profiles contain extended periodic structures normally associated with crystal structures, most all do possess a degree of regularity in the fiber profile that imparts mechanical and optical advantage.
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