Photonic-crystal fiber (PCF) is a class of optical fiber based on the properties of photonic crystals. It was first explored in 1996 at University of Bath, UK. 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. 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). Photonic crystal fibers may be considered a subgroup of a more general class of microstructured optical fibers, where light is guided by structural modifications, and not only by refractive index differences. Description Optical fibers have evolved into many forms since the practical breakthroughs that saw their wider introduction in the 1970s as conventional step index fibers and later as single material fibers where propagation was defined by an effective air cladding structure. In general, regular structured fibers such as photonic crystal fibers, have a cross-section (normally uniform along the fiber length) consisting of one, two or more materials, most commonly arranged periodically over much of the cross-section. This zone is known as the "cladding" and surrounds a core (or several cores) where light is confined. For example, the fibers first demonstrated by Philip Russell consisted of a hexagonal lattice of air holes in a silica fiber, with a solid or hollow core at the center where light is guided. Other arrangements include concentric rings of two or more materials, first proposed as "Bragg fibers" by Yeh and Yariv, bow-tie, panda, and elliptical hole structures (used to achieve higher birefringence due to irregularity in the relative refractive index), spiral designs which allow for better control over optical properties as individual parameters can be changed. (Note: PCFs and, in particular, Bragg fibers, should not be confused with fiber Bragg gratings, which consist of a periodic refractive index or structural variation along the fiber axis, as opposed to variations in the transverse directions as in PCF. Both PCFs and fiber Bragg gratings employ Bragg diffraction phenomena, albeit in different directions.) The lowest reported attenuation of solid core photonic crystal fiber is