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The strength of fiber

Researchers from Unicamp are developing latest generation optic amplifiers that are going to improve telephony and the Internet

MIGUEL BOYAYANTransmission by light: optic fibers have room to be put to good useMIGUEL BOYAYAN

When we key in a number on the telephone or send an e-mail, we never imagine the route that our call covers to a neighboring district or a distant city. The transmission from one point to another always has been and continues to be carried out by equipment, wires and, more recently, by optical fibers, all of them equally hidden from our eyes. That is why following up on the technological transformation in the telecommunciations networks over recent years is no easy task for the uninitiated. The most important revolution began in the 70’s, when optical fibers appeared. Since then, everything that has to do with this means of transmission continues to instigate researchers in the area.

By permitting the installation of a filament a little larger in diameter than a filament of hair in the place of hundreds of copper wires, and using light for transmission, the fibers have created other technological needs. At the moment, one of the great battles of the professionals from this sector all over the world is to get amplifiers capable of meeting the demand for the growth in the number of fixed telephones, and of the appearance of fax, mobile phones, data networks and the Internet. Installed mainly between cities and in the international connections, amplifiers receive, from a stretch of between 20 and 100 kilometers, the laser signals that run through the inside of the fibers.

They recover the light wave, which loses strength in the course of transmission. The development of new amplifiers is indispensable for increasing the capacity and the speed of telecommunications systems and for diminishing the costs of implanting new networks. With these goals, four laboratories on the whole of the planet are running a technology race to perfect a new generation of optic amplifiers called Fiber Optic Parametric Amplifier (Fopa). One of these laboratories is at the Institute of Physics (IF) of the Campinas State University (Unicamp), which is part of the Optics and Photonics Research Center (CePOF) financed by FAPESP. The others are the laboratories of Bell, a Lucent company, of Stanford University, in the United States, and Chalmers University of Technology, in Sweden.

A Fopa will be able to increase dozens of times the capacity for transmission of an optical fiber. Today, in long distance connections, transmission takes place in a wavelength band of between 1,53 and 1,56 micrometers (µm), in the infrared region, where fibers have minimal losses. But the capacity of this fiber goes from 1,10 to 1,75 µm. The fine structure of fiber also has the capacity of transmitting more than 100 terabits per second (Tb/s; one terabit is equal to a thousand gigabits). Transmission nowadays takes place at 4 Tb/s at the most.

As a comparison, this transmission speed (technically called transmission rate, because the speed is always the speed of light) is sufficient for 62 million phone calls (of 64 kilobits per second – kb/s) at the same time, or, looking to the future, about a thousand simultaneous digital television channels or the complete text of all the books from the largest library in the world as well, in just one second. Between 100 Tb/s and 4 Tb/s, then, there is plenty of space to be made use of in an optical fiber.

Increased bandwidth
In the laboratory, the Fopa now manages to transmit at 10 Tb/s. “Our intention is to arrive at 40, 50 Tb/s”, explains Professor Hugo Fragnito, coordinator of the CePOF segment at Unicamp (the others are at the Institute of Physics of the University of São Paulo, in São Carlos, and at the Nuclear and Energy Research Institute – Ipen). With Fopa, it will be possible to work with other bands within the spectrum between 1,1 and 1,75 µm. “It amplifies successfully at bandwidths (frequencies) greater than the erbium doped amplifier used today. Erbium is a chemical element classified as rare earth. The parametric version is equal to or better than the erbium one in various regions of the transmission spectrum”.

The Erbium Doped Fiber Amplifier (EDFA) had already set off technological revolution back in 1990. Announced in 1987, in the form of a prototype prepared in conjunction by British Telecom, the DuPont company, and University of Southampton, in Great Britain, erbium showed many advantages over the electronic repeater used since optical fibers started to be used, the 3R (regenerator/reshaper/retimer). “The speed of transmission went up from 1 Gb/s to 4 Tb/s with erbium”, says Fragnito.

Another important factor was the decrease in the cost of installing the networks. The price of an amplifier has fallen from US$ 500,000 to US$ 30,000. The “miracle” of this chemical element called erbium is that its ions absorb light from a laser called a pump laser of a wavelength of 0.98 µm or of 1.48 µm and (amplify) the light from the signal laser (who makes the transmission) in the 1.53 to 1.56 µm band. The revolution set off by this kind of amplifier was also strengthened with the adoption of the Wavelength Division Multiplexing (WDM) system. It was erbium that made WDM feasible, and instead of using one fiber for each laser signal, as in the old system, it made it possible for several lasers to be transmitted by the same optical fiber.

Accordingly, multiplexation allows various transmissions bands (each one with dozens of millions of calls at the same time) to be sent by a single optical fiber.The Fopa does not need erbium and can operate at any wavelength. Parametric amplification takes advantage of the effects that any material shows when illuminated at a high intensity, using the transfer of energy from one laser to another. “The effects of high intensity light in the inside of the fibers usually distort the transmission signals”, says Fragnito.

Fopa’s secret is to put these effects to good use, and not to bad. To do so, it transfers the energy from a pump laser, placed inside the fiber, to the transmission laser, amplifying the signal. “We can put one or two pump lasers in each fiber, to have more amplification and a greater bandwidth”, explains José Manuel Chávez, who is doing post-doctoral studies at the optical fiber laboratory of Unicamp’s IF. “With the Fopa, we will succeed in making better use of the immense capacity that the fibers have for transmission”, says Fragnito. The Fopa’s benefits do not yet have a date for going onto the market. Many tests and adjustments are still needed.

Another innovation that arose at Unicamp’s School of Electrical Engineering and Computing, which is also part of the CePOF, is related to another kind of amplifier. This is the Semicondutor Optical Amplifier (SOA), now made by the Alcatel company, only for use in laboratories. This equipment carries out the amplification of the laser through a chip, and it is to be used in the telecommunications networks in metropolitan areas.

“For this amplifier, we have developed a time reducer of the electro-optical switching that makes the system switch on and off to the SOA”, says Professor Evandro Conforti. This switching was made ten times quicker, cut from 2 billionths of a second or 2 nanoseconds (the time currently taken by other systems used on Internet networks) to 0.2 nanoseconds. This kind of reduction in time makes things easier and offer the system greater capacity for receiving and dispatching laser light in the transmissions.

Everything optical
The new technique had the co-authorship of Cristiano Gallep, who is studying for a doctorate and won an award in the United States with this work, and is in the process of patenting it through Fapesp’s Nucleus for Patents and Licensing of Technology (Nuplitec). “There are no interested parties yet, but we believe in this system’s potential for improving future optical amplifiers”, explains Conforti. “In particular, if a few nears from now totally optical systems are adopted, when, unlike today, there will be no conversion of an electrical signal (from the copper wires) to laser signals”.

If this happens, the transmission of an e-mail, for example, would leave the computer already in the form of light, and would arrive at the server like that, and from there go on to another computer, always running through optic fibers. A future in which the e-mail is going to become o-mail, o for optic, and is going to need a lot efficient optic amplifiers, of a broad band and very quick. A future that is already under production.

Students under international spotlights

Two students from the Optics and Photonics Research Center (CePOF) at Unicamp have won awards in international competitions connected with associations of optics in the world-wide ambit. The first was Paulo Dainese, who studied for a master’s degree at the Institute of Physics (IF). In October, he won a prize from the American Optic Society (AOS) for the best Chapter, the designation given to groups of students who are carrying out scientific work and disseminating optics.

Under the guidance of Professor Hugo Fragnito, Dainese leads the Brazilian group made up of 25 students that won the contest this year between another 26 teams from the United States, South Africa, Canada, Belgium and England. The AOS is a scientific society that gathers together 13,000 members, including researchers, engineers and technicians in 70 countries.

During the annual meeting of the AOS in Orlando, in the United States, the students from Unicamp showed several events that they had organized, like Optic Workshops for secondary school teachers and an Applied Optics School for postgraduates and students who are finishing their undergraduate studies. They also reported on a Program of Visits to the industries in the region, as a way of showing Unicamp’s research activities and opening the doors for future working relations. “The student goes to the industrial concern and establishes a contact, which may result in him being hired or in intern period”, Dainese explains. The students also gave courses for the technicians who work at Unicamp. “They learn to work at the benches and the meaning of our experiments.” The prize for the group was US$ 1,000.

In November, it will be the turn of Cristiano Gallep, who is studying for a doctorate at the School of Electrical Engineering and Computing, and will be going on to Glasgow, in Scotland. He will be receiving a scholarship grant from the Laser and Electro-Optics Society (Leos), in the amount of US$ 5,000. With the guidance of Professor Evandro Conforti, he has developed a new kind of switching for optic amplifiers.

The Project
Unicamp’s Optics and Photonics Research Center (CePOF); Modality Centers for Research, Innovation and Dissemination (Cepids); Coordinator Hugo Fragnito – Unicamp’s Institute of Physics; Investment
R$ 1 million per year