Photonic-crystal fiber 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, holey fiber, hole-assisted fiber, and Bragg fiber. 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 microstructured from one, two or more materials, most commonly arranged periodically over much of the cross-section, usually as a "cladding" surrounding a core 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, a variant of which was recently fabricated by Temelkuran et al., bow-tie, panda and elliptical holes structures used to achieve higher Birefringence due to irregularity in the relative refractive index, spiral designs for higher degrees of freedom in controlling the optical properties due to the flexibility in changing different parameters and other types. The lowest reported attenuation of solid core photonic crystal fiber is 0.37 dB/km, and for hollow core is 1.2 dB/km
Construction
Generally, such fibers are constructed by the same methods 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, shrinking the preform cross section but maintaining the same features. In this way, kilometers of fiber can be produced from a single preform. The most common method involves stacking, although drilling/milling was used to produce the first aperiodic designs. This formed the subsequent basis for producing the first soft glass and polymer structured fibers. Most photonic crystal fibers have been fabricated in silica glass, but other glasses have also been used to obtain particular optical properties. There is also a growing interest in making them from polymer, where a wide variety of structures have been explored, including graded index structures, ring structured fibers and hollow core fibers. These polymer fibers have been termed "MPOF", short for microstructured polymer optical fibers. A combination of a polymer and a chalcogenide glass was used by Temelkuran et al. for 10.6 μm wavelengths.
Modes of operation
Photonic crystal 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- refractive index contrast between core and cladding, and therefore can have much stronger confinement for applications in nonlinear optical devices, polarization-maintaining fibers,. 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 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. 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. PCF can also be modified by coating the holes with sol-gels of similar or different index material to enhance its transmittance of light.
History
The term "photonic-crystal fiber" was coined by Philip Russell in 1995–1997.
