Photonic-crystal Fiber

Photonic-crystal fiber (PCF) is a new class of optical fiber based on the properties of photonic crystals. Because of its ability to confine light in hollow cores or with confinement characteristics not possible in conventional optical fiber, PCF is now finding applications in fiber-optic communications, fiber lasers, nonlinear devices, high-power transmission, highly sensitive gas sensors, and other areas. The term "photonic-crystal fiber" was coined by Philip Russell in 1995–1997 (he states (2003) that the idea dates to unpublished work in 1991), although other terms such as microstructured fiber are also used and the nomenclature in the field is not entirely consistent. More specific categories of PCF include photonic-bandgap fiber (PCFs that confine light by band gap effects), holey fiber (PCFs using air holes in their cross-sections), hole-assisted fiber (PCFs guiding light by a conventional higher-index core modified by the presence of air holes), and Bragg fiber (photonic-bandgap fiber formed by concentric rings of multilayer film).

Structured optical fibers, those based on channels running along their entire length go back to Kaiser and Co in 1974. These include air-clad optical fibers, microstructured optical fibers sometimes called photonic crystal fiber when the arrays of holes are periodic and look like a crystal, and many other subclasses. Martelli and Canning realized that the crystal structures that have identical interstitial regions are actually not the most ideal structure for practical applications and pointed out aperiodic structured fibers, such as Fractal fibers, are a better option for low bend losses. Aperiodic fibers are a subclass of Fresnel fibers which describe optical propagation in analogous terms to diffraction free beams. These too can be made by using air channels appropriately positioned on the virtual zones of the optical fiber.