Photonic-crystal fiber

Photonic-crystal fiber (PCF), also spelled fibre, 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 optical communications, fiber lasers, nonlinear devices, high-power transmission, highly sensitive gas (etc.) sensors, and other areas. The term "photonic-crystal fiber" was coined by Phillip Russell in 1995, although other terms such as microstructured fiber and photonic-bandgap fiber are also used and the nomenclature in the field is not entirely consistent. More specific categories of PCF include holey fiber (and hole-assisted fiber) and Bragg fiber.

In general, such fibers have a cross-section (normally uniform along the fiber length) microstructured from two or more materials, most commonly arranged periodically over much of the cross-section, usually as a "cladding" surrounding a core (or several cores) where light is confined. For example, the fibers first demonstrated by Russell consisted of a hexagonal lattice of air holes in a silica fiber, with a solid (1995) or hollow (1998) core at the center where light is guided. Other arrangements include concentric rings of two or more materials, first proposed as "Bragg fibers" by Yariv and Yeh in 1979.

(Note: PCFs and, in particular, Bragg fibers, should not be confused with fiber Bragg gratings , which consist of a periodic index or structural variation along the fiber axis, as opposed to variations in the transverse directions as in PCF.)

Generally, such fibers are constructed by the same general principles as other optical fibers: first, one constructs a "preform" on the scale of centimeters in size, and then heats the preform and draws it down to a much smaller diameter (often nearly as small as a human hair), shrinking the preform cross section but (usually) maintaining the same features. In this way, kilometers of fiber can be produced from a single preform.

Such fibers can be divided into two modes of operation, according to their mechanism for confinement. Those with a solid core, or a core with a higher average index than the microstructured cladding, can operate on the same index-guiding principle as conventional optical fiber — however, they have a much higher effective-index contrast between core and cladding, and therefore can have much stronger confinement for applications in nonlinear optical devices, polarization-maintaining fibers, etc. Alternatively, one can create a "photonic bandgap" fiber, in which the light is confined by a photonic bandgap created by the microstructured cladding — such a bandgap, properly designed, can confine light in a lower-index core and even a hollow (air) core. Bandgap fibers with hollow cores can potentially circumvent limits imposed by available materials, for example to create fibers that guide light in wavelengths for which transparent materials are not available (because the light is primarily in the air, not in the solid materials). Another potential advantage of a hollow core is that one can dynamically introduce materials into the core, such as a gas that is to be analyzed for the presence of some substance.

External links

• Crystal Fibre, manufacturer of PCFs (primarily via air holes in silica fiber).
• OmniGuide Communications, Inc., manufacturer of concentric-ring bandgap fibers (a variant of Bragg fibers).
• Steven G. Johnson, Photonic-crystal and microstructured fiber tutorials (2005).