Photonic-crystal fiber (PCF) is a class of

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair Hair is a protein filament that grows ...

based on the properties of

photonic crystals A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic ...

. 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-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pre ...

, 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefri ...

due to irregularity in the relative refractive index),

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:fiber Bragg gratings, which consist of a periodic

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, ...

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 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 (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. Air holes are most commonly created by gathering hollow rods into a bundle, and heating the bundle to fuse it into a single rod with ordered holes before drawing, 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 Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...

, but other glasses have also been used to obtain particular optical properties (such as high optical non-linearity). 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 Chalcogenide glass (pronounced hard ''ch'' as in ''chemistry'') is a glass containing one or more chalcogens (sulfur, selenium and tellurium, but excluding oxygen). Such glasses are covalently bonded materials and may be classified as covalent netw ...

was used by Temelkuran ''et al.''Temelkuran, B., Hart, S., Benoit, G. ''et al.'' Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission. ''Nature'' 420, 650–653 (2002). https://doi.org/10.1038/nature01275 in 2002 for 10.6

μm The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...

wavelengths (where silica is not transparent).

Modes of operation

Photonic Crystal Fibers Diagram (index guide and photonic bandgap)

Photonic crystal fibers can be divided into two modes of operation, according to their mechanism for confinement: index guiding and photonic bandgap. Index guiding photonic crystal fibers are characterized by a core with a higher average refractive index than that of the cladding. The simplest way to accomplish this is to maintain a solid core, surrounded by a cladding region of the same material but interspersed with air holes, as the refractive index of the air will necessarily lower the average refractive index of the cladding. These photonic crystal fibers 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, they can also be made with much ''lower'' effective index contrast. Alternatively, one can create a photonic bandgap photonic crystal 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. PCF can also be modified by coating the holes with sol-gels of similar or different index material to enhance the transmittance of light.

History

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).

References

External links

* Centre for Photonics and Photonic Materials (CPPM), University of Bat

* Group of Prof. Philip St. John Russell at the

Max Planck Institute for the Science of Light The Max Planck Institute for the Science of Light (MPL) performs basic research in optical metrology, optical communication, new optical materials, plasmonics and nanophotonics and optical applications in biology and medicine. It is part of the M ...

in Erlange

with some introductory material, reviews and information about current research.
Encyclopedia of Laser Physics and Technology on photonic crystal fibers
with many references * Steven G. Johnson
Photonic-crystal and microstructured fiber tutorials
(2005).
Philip Russell: Photonic Crystal Fibers, Historical account in: IEEE LEOS Newsletter, October 2007
* John D. Joannopoulos, Steven G. Johnson, Joshua N. Winn, and Robert D. Meade,
Photonic Crystals: Molding the Flow of Light
', second edition (Princeton, 2008), chapter 9. (Readable online.)
Philip Russell plenary presentation: Emerging Applications of Photonic Crystal Fibers
SPIE Newsroom {{DEFAULTSORT:Photonic-Crystal Fiber Photonics Optical fiber