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 The term "photonic-crystal fiber" was coined by Phillip Russell in 1995-1997 (he states (2003) that the idea dates to unpublished work in 1991), although other terms such as microstructured fiber and photonic-bandgap fiber are also used and the nomenclature in the field is not entirely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. Optical fibers An optical fiber (or fibre) is a transparent thin fiber, usually made of glass or plastic, for transmitting light. ... The opal in this bracelet contains a natural periodic microstructure responsible for its iridescent color. ... Optical communication is any form of telecommunication that uses light as the transmission medium. ... The range of sizes in which lasers exist is immense, extending from microscopic diode lasers (top) to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion. ... A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ...

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 (1996) 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 (1976, 1978), a variant of which was recently fabricated by Temelkuran et al. (2002). The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ...

(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 refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ... Bragg diffraction was first proposed by William Lawrence Bragg in 1912 as a means of analyzing the structure of crystals. ...

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. cm redirects here, alternate uses: cm (disambiguation) A centimetre (symbol cm; American spelling: centimeter) is an SI unit of length. ... Hair with a round cross-section will fall straight, as opposed to curly hair, which has a flat cross-section Hair is a filamentous outgrowth of the skin found only in mammals. ... A kilometre (American spelling: kilometer) (symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ...

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 can 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, etcetera (or they can also be made with much lower effective index contrast). 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. The larger the angle to the normal, the smaller is the fraction of light transmitted, until the angle when total internal reflection occurs. ... This article treats polarization in electrodynamics. ...

References

• P. St. J. Russell, "Photonic-crystal fibers," Science 299, 358-362 (2003). (Review article.)
• Burak Temelkuran, Shandon D. Hart, Gilles Benoit, John D. Joannopoulos, and Yoel Fink, "Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission", Nature 420, 650-653 (2002).
• J. C. Knight, J. Broeng, T. A. Birks and P. St. J. Russell, “Photonic band gap guidance in optical fibers,” Science 282, 1476-1478 (1998).
• J. C. Knight, T. A. Birks, P. St. J. Russell and D. M. Atkin, “All-silica single-mode fiber with photonic crystal cladding,” Opt. Lett. 21, 1547-1549 (1996). Erratum, ibid 22, 484-485 (1997).
• P. Yeh, A. Yariv, and E. Marom, “Theory of Bragg fiber,” J. Opt. Soc. Am. 68, 1196–1201 (1978).
• P. Yeh and A. Yariv, "Bragg reflection waveguides," Optics Communications 19, 427-430 (1976).

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).