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Photonics

Light in the right dose

New generation of optical fibers opens up room for increasing the capacity of the telecommunications networks and brings novelties to medicine

MIGUEL BOYAYANPreform: the fine optical fibers are produced from this material, which is some centimeters in sizeMIGUEL BOYAYAN

The refinement of a new generation of optical fibers – called photonic crystal fibers – is opening up new prospects for increasing the capacity of the telecommunications networks. It is a new technology that makes possible the production of fibers designed for specific uses, in areas as diverse as astronomy, industrial equipment, precision clocks, components for computers faster than the current ones, and diagnosis imaging – a prototype of an endoscope with a single fiber, dozens of times smaller than the conventional ones, has now been produced in Australia.

In telecommunications, the most  the researches aim to develop photonic fibers for specific uses in areas that are showing bottlenecks, such as the need for increasing the speed in transmission and reception equipment (amplifiers, converters etc). They are also candidates for replacing the old copper cables that connect the telecommunications network to the users. They are novelties that are in the technological vanguard at academic centers in the United Kingdom and in Australia or, as a development of applications, in over 60 research groups at the National Center for Frequency Metrology, in France, the Max-Planck Institute, in Germany, and the National Research Laboratory of Metrology, in Japan, besides several research institutes in the United States, Italy and Israel.

In Brazil, the new fibers are the object of study by some research groups from the Gleb Wataghin Physics Institute of the State University of Campinas (Unicamp), which is part of the Optics and Photonics Research Center (CePOF).  There are also groups studying photonic fibers at the São Paulo State University, in Araraquara, and at the Advanced Studies Institute of the Aerospace Technical Center, in São José dos Campos, where there is a team involved in theoretical works about the possibilities of photonic structures.

With over 30 years of researches in the area of optical fibers, Unicamp, besides having trained dozens of professionals, has now passed on technology and has teamed for developing innovations with several companies. Work that has made photonic fibers one of the pillars cutting edge technology of the area.  The studies have been intensified in the interchange with two of the world’s pioneer centers in the development of these fibers: the Photonics and Photonic Materials Group, from the University of Bath, in England, and the Optical Fiber Technology Center, of the University of Sydney, in Australia.

In the 1990’s, the researchers from Bath developed the concept of photonic fibers. In November 1995, they made the first photonic fiber in the world. Until 2001, this kind of fiber was manufactured in glass (mainly silica), when the group from Sydney prepared the same structure in polymers. At the beginning of November this year, one of the main researchers from Bath, Professor Jonathan Knight, who made the first photonic fiber, was in Campinas with sponsorship from FAPESP. At the same time, the group from Unicamp received a visit from Maryanne Large, an Australian researcher who developed the plastic photonic fibers. The two researchers gave talks and accompanied the work carried out at Unicamp.

To understand the novelties in the workings of the new fibers created by these researchers, you have to understand first how the traditional optical fibers work. They are made of a core and an external cladding, both almost always of silica. Their capacity for confining light and making it travel inside them with the information that one wants to transmit is based on the high transparency of the glass and on the fact that the core always has a higher refractive index than that of the cladding.

This difference of index makes it possible to trap the light, because the interface between materials with high and low refraction indices works like a mirror that facilitates the path of the light wave in the inside of these devices. To have a refraction index higher than that of the cladding, the silica of the core is enriched (doped) with atoms of another material, like germanium and boron. The process requires an excellent control of the chemistry of glass, because it is at this stage that a good part of the characteristics of the fiber is defined and, as a consequence, of the signal that will be transmitted.

One of the differences between the traditional fibers and the new ones is that the photonic ones are not based on chemical dopants to get variations in the levels of refraction. They have a core (which can be of silica, polymer or even air) surrounded by a regular set of air holes in the form of tunnels, which run parallel along the whole fiber. In the case of the fibers with a solid core (silica or plastic), it is considered that the guidance is due to the fact that the outer part of the fiber has been “doped” with air, a material with a lower refraction index. Surprisingly, though, light can also be guided in fibers with a hollow core, traveling in the air.

The whole of traditional physics shows that light prefers to travel in materials with materials with high refraction indices, and air has the lowest of them. A strange behavior that is possible to be applied to this kind of photonic fiber based on different physical principles from those that govern the traditional fibers. In the 1980’s, physicists discovered that materials structured on the scale of the wavelength of light – a fraction of a micrometer – could have their optical properties radically altered.

These are photonic crystals, so called because their internal structure, as regular as that of a crystal, makes it possible to control the guiding of the light. In the case of photonic fibers with a hollow core, the spaces between the air holes in the part that surrounds the fiber should have dimensions of the same order as the wavelength of the light that it is intended to guide in its core. The microstructured region thus creates, around the core, a zone prohibited for certain wavelengths, a band gap, obliging the light to remain confined in the core of the fiber. This is how the same silica as in the traditional fiber, now with a regular structure of tiny tunnels, starts to work like a new material, with unprecedented optical properties.

The advantages of the photonic fibers in relation to the conventional ones is the possibility of designing their microstructure in such a way that the fiber shows properties chosen according to the need in each case. Accordingly, it is possible to design and to manufacture fibers for a broad spectrum of applications, increasing the concentration of light or altering its very frequency, to mention just a few examples. “It is a new technology that makes it possible to have different kinds of fiber designed with specific properties”, stressed British physicist Jonathan Knight during his visit to Unicamp.

One of the good prospects for photonic fibers lies in telecommunications, an area in which optical fibers have been bringing about a veritable revolution for three decades, with speed gains, in relation to the copper wires. “The current limitations of the traditional optical fibers are due to the fact that the light travels in glass. Once set free from this tie, the potential is immense”, says Knight. He refers to the fact that the interaction between glass and light causes a loss of power and a dispersion of the light signal, a problem in the case of long distances. Amongst other characteristics, dispersion causes the broadening of the wavelength of the light signal, to the point of making it unrecognizable. And the loss of power comes to as much as 96% in 100 kilometers. Today, these problems are got round with signal amplifiers and other devices, but they limit the potential of the network, because they do not recover the sign in full.

“With photonic fibers, it is possible to control dispersion much better and, in theory, to reduce the loss to almost nil”, Knight guarantees. It was confidence in this potential, by the way, that led the physicist, together with his colleagues, in March 2001, to found a company, Blaze Photonics, to develop photonic fibers capable of replacing the present-day transatlantic cables, which today depend on extremely expensive amplifiers and on maintenance done by marines. The company ended up being sold for £ 3 million (almost R$ 15 million) to Crystal Fibre, a Danish company. The sale was before Blaze reached a commercial prototype, but Knight regards the achieved as promising. “We did not go so far as to get a loss lower than the loss with conventional cables, but these took three decades being perfected and are at the limit of their technological possibilities, while we have advanced very quickly in a short time.”

Inside the amplifiers
If for transmissions over long distances the photonic fibers have not yet shown advantages, their superior performance in many areas have already converted them into an option for developing new devices used in telecommunications, such as signal amplifiers, dispersion controllers and wavelength converters. To increase the quantity of information transmitted on this equipment today, for example, a wider range of different wavelengths has to be used for transmitting simultaneously a lot of data in the same fiber. The conventional amplifiers, though, only amplify a small band of wavelength. The solution is the parametric amplifiers, which operate in a much wider band (see Pesquisa FAPESP No. 81).

At Unicamp’s CePOF, Hugo Fragnito has been working on the development of these amplifiers since 2000, and this year he started a scientific collaboration with the group from Bath. The idea is to develop photonic fibers especially designed to increase even more the band of the parametric amplifiers. To put this collaboration into effect, Paulo Dainese, a pupil of Fragnito, worked three months with Knight’s group in Bath. At Unicamp, the group led by Luiz Carlos Barbosa, with his students Enver Chillcce and Sérgio Ozório, has also been studying the production of their own photonic fibers since 2002, a project that is now getting an impulse with the return Cristiano Cordeiro, who did postdoctoral studies in this area in Knight’s team, at the University of Bath.

Recently arrived in Brazil, and with a postdoctoral scholarship from FAPESP, Cordeiro will now continue his researches at the university, aimed at developing and characterizing photonic fibers with non-linear optical properties. They are fibers with a capacity for altering the wavelength of the light that passes through them. Cordeiro’s researches also intend to explore another possibility of photonic fibers, which is the generation of a supercontinuum. This is a very strong light with an extensive wavelength to be used, for example, in experiments in spectroscopy (characterization of materials), metrology and a special kind of tomography that provides, in a noninvasive manner, three-dimensional images of biological tissues (optical coherence tomography).

The new technological possibilities opened up with the supercontinuum were unthinkable with the traditional fibers. “I want to work on and to manufacture photonic fibers for these applications that are also the object of study for several other Brazilian groups”, Cordeiro says. One example is the team coordinated by Nilson Dias Vieira, from the Institute of Nuclear Energy and Research (Ipen). He plans to use photonic fibers for generating a supercontinuum to be used in optical coherence tomography experiments. The recent visit by Knight put the seal on the supply of photonic fibers produced in Bath for the Brazilian laboratory. This is going to happen for Cordeiro’s experiments as well, so long as the Brazilian photonic fibers are not being produced.

Another project of Cordeiro’s is to try, in collaboration with Maryanne, to produce in Brazil photonic fibers of plastic (polymer). These fibers have a potential market niche in the telecommunications networks: replacing the connections of the end users, an area still dominated by the technology of copper cables, which becomes an obstacle to increasing the speed of transmission. With plastic, the advantages in relation to glass are evident: it is cheaper, less fragile, and the method for manufacturing it makes it possible to develop a much wider range of photonic structures, besides it being possible to dope the material with a much greater diversity of substances and with larger quantities than those tolerated by glass.

Accordingly, plastic overcomes the disadvantage of it being less transparent, irrelevant in the case of small distances. In the stage of manufacture, polymer fibers have different characteristics from the traditional glass ones, which are produced from a preform of a few centimeters, made up of small glass tubes piled up in such a way as to make the desired structure. Heated up, this preform is pulled until it is converted into a fiber of the thickness of a strand of hair (125 microns), which maintains, on a microscopic scale, the same structure as the original preform. The system only permits the production of structures that can be made using this piling up. But plastic fibers make it possible to make any kind of structure, all that is needed being the use of a special drill, controlled by computer to produce in the preform the desired sequence of holes.

Connection of chips
The development of these fibers by Maryanne’s team not only in the network connections with the end users, but also in amplifiers and lasers and in the internal connections in computers and other equipment. “The electronic devices that connect the chips of a computer cannot operate above certain speeds, because they are transformed into antennas that emit and capture signals to the air. Our fibers will be able to act to make these connection with the use of light, making possible transmissions at speeds thousand times higher than the current ones”, Maryanne explains.

Some applications for photonic fibers are now ceasing to be mere speculations. The team from Sydney has, for example, produced a prototype endoscope a dozen times smaller than the conventional ones, because it transmits the image through a single optical fiber. Besides being more comfortable for patients who undergo several kinds of endoscopic examinations, the new technology allows a better visualization for the doctor. This happens because the microstructured plastic fiber has dozens of microscopic nucleuses through which the light is transported. The apparatus should first be used to guide the implantation of prostheses in the inner ear.

The Project
Unicamp’s Optics and Photonics Research Center (CePOF) (nº 05/51689-2); Modality Research, Innovation and Diffusion Centers (Cepid); Coordinator Hugo Fragnito – Unicamp’s Physics Institute; Investment R$ 1,000,000 a year (FAPESP)

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