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ScienceWeek
APPLIED PHYSICS: ON PHOTONIC CRYSTAL FIBERS
The term "photonic crystal" refers in general to a crystal lattice in which diffraction of electromagnetic radiation occurs with visible light. An example of such a lattice is a material consisting of a periodic array of drilled microscopic holes.
The following points are made by Jonathan C. Knight (Nature 2003 424:847):
1) Could optical fibers as we now know them become obsolete? It seems an unlikely proposition, and yet a new generation of optical fibers currently being developed could outperform conventional fibers for many applications. Modern optical fibers were one of the major technological successes of the 20th century, and they have become the pre-eminent method of communicating information. This technology has developed at an incredible pace, from the first low-loss (less than 20 dB/km) single-mode waveguides in 1970 to being key components of our sophisticated global telecommunication network.
2) Modern optical fibres, which transmit information in the form of short optical pulses over long distances at exceptionally high speeds, have become an integral part of life in the information age. There are non-telecom applications for fibres, too, in beam delivery for medicine, machining and diagnostics, sensing and a host of other fields. And yet, amazingly, the basic physics of optical fibres has remained unchanged since the 19th century. This, in part, may be a direct result of the fast initial implementation of fibre optic technology. The system was established early on, and all future developments were incremental improvements to individual components.
3) State-of-the-art optical fibres represent a careful trade-off between optical losses, optical nonlinearity, group–velocity dispersion, and polarization effects. Some of these effects (e.g. optical losses) are inherent in the raw material used to make the fibres, which is usually synthetically produced silica, SiO2. Others (nonlinearity and dispersion) are strongly affected by the material's properties but can be influenced by fibre design. Other properties such as polarization-mode dispersion result from imperfections in the fabrication processes. After 30 years of intensive research, incremental steps have refined the capabilities of the system and the fabrication technology nearly as far as they can go.
4) Researchers and engineers in several laboratories around the world are working on ways to revolutionize fibre-optic design and performance. Since the 1980s, optical physicists have recognized that the ability to structure materials on the scale of the optical wavelength, a fraction of a micrometre or less, will allow the development of new optical materials known as "photonic crystals". Photonic crystals rely on regular morphological microstructure incorporated into the material to radically alter its optical properties. Many research groups are investigating these materials in two and three dimensional and planar geometries, and a few researchers are using them to form new types of optical fibre. Such fibres are known as "photonic crystal fibres" (PCFs), as they rely on the unusual properties of photonic crystals to deliver previously unimaginable performance from an optical fibre waveguide.
5) In summary: Photonic crystal fibers have wavelength-scale morphological microstructure running down their length. This structure enables light to be controlled within the fibre in ways not previously possible or even imaginable. Our understanding of what an optical fibre is and what it does is changing because of the development of this new technology, and a broad range of applications based on these principles is being developed.(1-5)
References (abridged):
1. Birks, T. A., Knight, J. C. and Russell, P. St J. Endlessly single-mode photonic crystal fiber. Opt. Lett. 22, 961-963 (1997)
2. Birks, T. A., Roberts, P. J., Russell, P. St.J., Atkin, D. M. & Shepherd, T. J. Full 2-D photonic bandgaps in silica/air structures. Elect. Lett. 31, 1941-1942 (1995)
3. Yeh, P., Yariv, A. & Marom, E. Theory of Bragg fiber. J. Opt. Soc. Am. 68, 1196-1201 (1978)
4. Knight, J. C., Birks, T. A., Russell, P. St J. & Atkin, D. M. All-silica single-mode optical fiber with photonic crystal cladding Opt. Lett. 21, 1547-1549 (1996)
5. Kaiser, P. & Astle, H. W. Low loss single material fibers made from pure fused silica. Bell Syst. Tech. J. 53, 1021-1039 (1974)
Nature http://www.nature.com/nature
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ON PHOTONIC CRYSTALS AND BAND GAPS
The following points are made by John D. Joannopoulos (Nature 2001 414:257):
1) Photonic crystals have emerged as a potentially powerful platform for making light do things previously impossible. For example, optical light could be guided through air rather than through optical fibers, which would reduce losses. Photonic devices, from high-speed switches to low-power microlasers, would be useful ingredients in fiber-optics communications and in future approaches to high-speed optical computing. The essential idea is to design periodic structures that affect the behavior of photons in much same way that crystalline semiconductors affect the properties of electrons.
2) Central to this idea is the formation of a photonic "band gap" -- a range of frequencies for which light is forbidden to exist within the bulk of the photonic crystal. The presence of a band gap depends on a particular periodic structure within the crystal, but whereas the periodic arrangement of atoms occurs naturally in semiconductors, photonic crystals need to be fabricated artificially.
3) The challenge of fabricating photonic band-gap structures is enormous, since the lattice constant of the photonic crystal (the size of the periodic unit cell) must be comparable to the wavelength of the light passing through the crystal. Optical communications systems, for example, operate in the near-infrared, which means that the photonic crystal-lattice constant must have dimensions of approximately a micron. Although this is some 2000 times larger than the lattice constant of atomic crystals, it is still over 100 times smaller than the average diameter of a human hair. At such micron length scales, controlling the fabrication of intricate structures is far from straightforward.
Nature http://www.nature.com/nature
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PHOTONIC CRYSTALS AND OPTOELECTRONICS
The following points are made by Eli Yablonovitch (Scientific American 2001 December):
1) The microelectronics and information revolution is based on the elaborate control of electric currents achieved with semiconductors such as silicon. That control depends on a phenomenon called the "band gap", a range of energies in which electrons are blocked from traveling through the semiconductor.
2) Researchers have produced materials with a "photonic band gap" -- a range of wavelengths of light blocked by the material -- by structuring the materials in carefully designed patterns (e.g. arrays of holes) at the nanoscopic size scale. These photonic crystals function as "semiconductors for light" and promise innumerable technological applications.
3) Many researchers greeted the idea of a photonic band gap with skepticism and disinterest when it was first proposed in the late 1980s, but today photonic crystals are rapidly becoming big business, with applications such as high-capacity optical filters, color filters photonic integrated circuits that would manipulate light in addition to electric currents, nanoscopic lasers, light-emitting diodes, and radio-frequency antennas and reflectors. Integrated circuits that combine conventional electronics and photonic crystals would represent the ultimate limit of optoelectronic miniaturization.
Scientific American http://www.sciam.com
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