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Doped fiber

A company dominates the process of manufacturing optical equipment that amplifies a light signal

MIGUEL BOYAYANThe optical fiber manufacturing process at Sun Quartz: domination of the technology is a credential for the market abroadMIGUEL BOYAYAN

A small company born at the State University of Campinas (Unicamp) is placing Brazil in the vanguard of communications by way of optical fiber networks, in which the transmissions are done by laser. Founded in 2003, the company named Sun Quartz is one of the few companies in the world that masters the technology for manufacturing optical fibers known as VAD (Vapor-phase Axial Deposition). This process presents various advantages over similar methods and permits the development of amplifying fibers that intensify the signal of a laser. The mastery of the manufacturing process of these components is important because the major part of the world structure of current telecommunications bases itself optical networks, capable of transmitting large volumes of data with much greater safety and speed between cities close to each other or from one point of the globe to another.

Sun Quartz, which is installed at Unicamp’s Technology Based Companies Incubator (Incamp) and can count upon funding through FAPESP’s Small Business Innovation Research (PIPE) program, has the expectation of placing its optical fibers on the market during the first semester of 2006. At the moment they are undergoing tests at the Padtec laboratories, one of the largest Brazilian manufacturers of optical communications systems. Currently the amplifier fibers used in Brazil are all imported.

Contrary to ordinary optical fibers, the special fibers are doped with erbium, a natural chemical element known as a rare earth metal. Doping is the introduction of a chemical element in order to change a material’s properties. In this case, the erbium serves to amplify the luminous signal that is propagated in the fiber cable in the form of a laser beam. They are fundamental because, in the manner in which the light traffics through an optical network, as it goes along it is absorbed and its signal is weakened. In order to recover the original signal’s amplitude, the solution is to install optical amplifiers along the network, whose main  component are fibers with erbium, as manufactured by the company from the city of Campinas.

“For a link between the cities of São Paulo and Campinas, a distance of around 90 kilometers, one needs to place in the middle of the path an optical amplifier. Of between 20 to 30 meters of erbium doped fiber are used for this system, known as Erbium Doped Fiber Amplifier (EDFA). In cities, these special fibers are used at the distribution points of cabling”, explains the physicist Carlos Kenichi Suzuki, a professor at Unicamp’s Mechanical Engineering College and a partner in Sun Quartz.

With a diameter of a little more than a hair fiber, optical fibers were invented during the  1970s and since then have been more and more in use in the transmission of data via the internet; cable TV; fixed and mobile phones and various other applications involving image and sound. This is happening because optical cables can transmit much more information than the conventional communications system, which makes use of copper wires and radio or microwave frequencies. Optical fibers are formed with a central nucleus, which is where the light is transmitted, and have a diameter of  3.5 micrometers (in the case of the erbium fibers) and 9 micrometers (in common optical fibers), where 1 micrometer is equal to 1 millimeter divided by 1,000. The external covering of the fiber that envelops the nucleus, known as the jacket, serves to provide optical isolation and has an average width of 125 micrometers or 0.125 mm. The basic raw material for optical fiber manufacture is quartz, also used in the production of solar cells and microchips. And Brazil has the largest reserve of this mineral on the planet.

The mastery of the optical fiber manufacturing technology is important because of the high aggregated value of this product. Whilst 1 kilogram of quartz costs around US$ 0.10 and 1 kilogram of silicon does not go beyond US$ 1, an optical cable doped with erbium is valued a million times more and can reach a cost of US$ 100,000 per kilogram. These fibers are also much more expensive than the conventional ones. A kilogram of common optical cable sells between US$ 20 and US$ 30, whilst the same number of meters of the amplifier cable is valued between US$ 10,000 and US$ 15,000. “We’re dealing with a millionaire market. In 2001 it is estimated that the sector optical amplification apparel ran at about US$ 4 billion. Today this number must be much more”, advised the researcher. When they begin to produce commercially, the Sun Quartz company is going to stock the internal market, which today purchases the erbium doped fibers abroad, and will then become an exporter of the product. “Besides the fibers, we could also sell the VAD technology, because we have complete control over it and the instrumentation used in the manufacturing process was entirely developed by us.”

Carlos Suzuki explained that all of the national optical fiber manufacturers use the MCVD (Modified Chemical Vapor Deposition) technology, created by Bell Laboratories, in the United States, more than 30 years ago. “The main commercial advantage of the VAD technology, developed in Japan, is that, contrary to the MCVD process, it doesn’t need the importation of silica tubes. This makes our product much cheaper”, says the scientist. The tubes, used to make the nucleus and the fiber jacket, clarified researcher Suzuki, are not produced in the country. Furthermore, the technology of axial deposition in the vapor phase uses as a raw material a by-product of silicon which is ten times cheaper than that used in the MCVD methodology. Another important advantage when comparing the VAD technology is that the optical fibers of the new generation, capable of transmitting light for much longer distances, are only possible to be produced by this process, because it corrects an undesirable effect that weakens the luminous signal, known as the Rayleigh spread phenomenon, something that the MCVD technology does not do.

Elevated automation
The optical fiber manufacturing process by Sun Quartz can be divided into five stages and is completely automatic. It allows for the control of the final product’s priorities such as the distribution of nano-porosity and of the refractive index.  The first stage consists in the manufacture of a nano-structured porous pre-form of silicon, a sort of milky-white round coil molding of around 60 millimeters in diameter by 30 centimeters in length. Each pre-form, the fore-runner element of the optical fiber, can originate 4 kilometers of fiber. It is produced with the use of a special blowpipe installed within a deposition chamber and then possesses all of the characteristics of the future fiber. The next stage is to carry out the addition of the elements that will furnish the amplification properties. The doping of the silicon is done with the immersion of the pre-form in a solution containing erbium ions, which bring about the special effect of amplification. The doping of the pre-form guarantees that the erbium ions penetrate into its structure at the atomic level.

When this stage is completed, the pre-form undergoes a thermo chemical treatment for drying and purification. The objective is to remove from its structure undesirable constituents such as the hydroxyl ion (OH)- and transition metals (iron, chrome and nickel etc.) that can cause a loss in the signal’s power. Afterwards, the material undergoes a new thermal treatment in a high temperature consolidation oven and with a controlled atmosphere in order to transform it to be totally transparent and free of micro bubbles, an imperfection that prejudices the perfect functioning of optical fibers. Finally, the transparent glassy material is stretched out and suffers an external deposition of nanoparticles of silicon for the formation of the jacket, responsible for the mechanical protection of the nucleus along which the light is transmitted. From this point onwards the fiber of nano-structured silicon undergoes a series of elongations and new depositions until the nucleus and the jacket attain the desired diameters. “This is a highly complex process and one that depends on an enormous variety of factors in order to work out well”, emphasizes Suzuki. In order to certify that the fiber has the quality and possesses the desired properties, it passes through a wide ranging series of tests, in order to describe its structural, optical and amplification properties, using a sweep electronic microscope, as well as its spread and X-ray spectrometer.

The optical fiber amplifier is only one of the products developed by the Sun Quartz company, which operates within the Integrated Quartz Cycle Laboratory of Unicamp’s Mechanical Engineering Faculty and maintains a link with the Optics and Photoptics Research Center (CePOF), installed at the university, one of the Research, Innovation and Diffusion Centers (Cepids) funded by FAPESP. The company also masters the process of the manufacture of sensorial optical fibers and of highly homogenous optical lenses for use in the ultraviolet region of the electromagnetic spectrum. The sensorial fibers have a wide range of applications and are used as pressure and temperature sensors, bringing about measurements in real time in operations of the monitoring of crude oil, natural gas and water pipelines. They can also be used in phone booths for the control of passing vehicles and as detectors for the presence of applications linked to the security sector. “These fibers have a distinct structure of erbium doping, in such a way that the nucleus is thicker and can reach 0.8 millimeters, in diameter. Their cost starts at US$ 1 per meter, and as far as we know, national manufacturers of the product don’t exist”, related the Sun Quartz executive.

The high resolution lenses are products that are on the cutting edge of knowledge. They are important components in the manufacture of the new generation of microchips and are fundamental for increasing the spatial resolution of their components (transistors, capacitors, diodes etc.), this is in virtue of their ultra-high homogeneity to ultraviolet light of small wavelengths and, consequently,  to the velocity of the operator of the computer processor. These lenses are used in a piece of equipment called the Stepper that manufactures microchips by way of a technology known as optical lithography. “Few manufacturers exist for this equipment in the world, among them are Canon and Nikon. These industries need high precision homogenous lenses in the ultraviolet region, which can only be manufactured beginning with the VAD technology”, explains Suzuki.  It so happens that these companies that master this technology are only directed towards the manufacture of microchips. Consequently, Sun Quartz wants to occupy a niche of the market only lightly explored in this sector. The manufacturing process of the lenses is already dominated and now the company is making adjustments in order to finalize the product with the specifications requested by the market. In order to increase the company’s commercial perspective, professor Suzuki was, at the end of October, in Japan, where he visited companies and discussed partnerships.