Miguel BoyayanA group of Brazilian researchers set a new fiber-optic distance and data transmission rate record. Using 10 channels on the same fiber, each with a 400 gigabits per second (Gbps) traffic capacity, the team coordinated by electrical engineer Jacklyn Dias Reis of the Center for Research and Development in Telecommunications (CPqD) in Campinas, was able to send an enormous amount of data along 370 kilometers (km) of optical fiber without errors. It is the first time that light-encoded information has been able to travel so far, using this optical fiber configuration, without using repeaters, equipment installed along the path to amplify the signal. The data transfer rate used in the test would be sufficient to download 170 hours of films or TV series recorded in high definition (HD) in just one second.
In general, signal amplifiers are needed in order to send data over optical fibers for distances surpassing 80 km. This is because light’s intensity diminishes as it travels along the fiber. Installed at specific distances, the amplifiers provide energy for the signal, ensuring that the information arrives at its destination intact. The need to use amplifiers for transmission over distances greater than 80 km makes it hard to provide high-quality, high-speed Internet services to remote communities, such as on islands or in the Amazon forest. There are no energy sources along the path to supply the amplifiers and periodic maintenance is practically impossible. The cost of infrastructure to provide high-quality Internet in these regions could make this type of service impossible according to Reis, who is coordinator of Optical Technologies at the CPqD. The study also involved researchers from the School of Electrical and Computer Engineering (FEEC) of the University of Campinas (Unicamp).
In the experiment, the researchers used a configuration with three laser sources: one to send data and the other two, at each end of the fiber, acting like pumps, providing energy for two optical amplifiers located along the connection pathway. The optical amplifiers do not run on electricity like electronic amplifiers, but rather receive energy from the optical fiber that runs along the transmission fiber, in parallel. This amplifies the laser signal, which provides the same performance as before. The optical amplifiers are known in technical jargon as Erbium Doped Fiber Amplifiers (EDFA). They consist of a 10 meter fiber containing the chemical element erbium, which facilitates the transfer of energy from the extra laser sources at the ends of the other two fibers to the signal, increasing its intensity. This strategy used a known technique called remote pumping, which allows the elimination of electronic amplifiers. “The signal sent arrived exactly the same at the receiver, without errors,” says João Carlos Soriano Sampaio, an electrical engineer at the CPqD and one of the authors of the experiment.
The optical fibers used are thicker than those used by telecommunications companies in their networks, and have lower signal attenuation. About 40% more expensive that standard optical fibers, these devices are the same as those used in submarine connections between continents and reduce signal energy loss along the route.
Marcelo Martins Werneck, the electrical engineer who coordinates the Photonics Instrumentation Laboratory (LIF) at the Federal University of Rio de Janeiro (UFRJ), believes that the transmission carried out by the CPqD group is innovative because of the high transmission speed and the distance. Werneck states that several research groups are working on achieving results like this because the market needs transmission technology without electronic amplification. For a submarine connection between continents, for example, an amplifier has to be installed at the bottom of the ocean, along with a power cable to power it, explains the engineer. Maintenance, when there is a defect, is performed by a robot that finds the cable on the ocean floor and raises it to the surface so that technicians can repair it on board a ship. The service costs about $100,000 a day. “When transmission occurs without electronic amplifiers, the risk of defects is lower. There would be fewer pieces of equipment to cause problems distributed along the pathway.”
A potential market for these ultra high-speed data transmission devices over long distances, according to Sampaio, is high-sea oil exploration. The companies need to connect their platforms to land bases and the strategy used by the CPqD would allow them to eliminate the amplifiers — some Petrobras platforms, for example, are more than 200 km from the coast. Padtec, a company in Campinas that develops, manufactures and sells optical communication systems and is associated with the CPqD, has already tested the market with this electronic-amplifier-free transmission technology, although their solution works over shorter distances and at lower speeds.
In addition to improving the entire system, the researchers also used mathematical models to analyze the factors that influence the transmission of information. Consisting of nine researchers from different specialties, the group chose the best configuration of transmission, digital signal processing and error correction techniques to reproduce the experiment in the laboratory. The transmission and reception systems, plus the 370 km of optical fibers rolled in 50 km spools, were installed in one of the CPqD’s laboratories in Campinas. The result of this experiment was published online on July 18, 2016 in the journal IEEE Photonics Technology Letters. And the researchers do not intend to stop there. “We want to send signals at greater transmission rates over even longer distances,” says Sampaio.
JANUÁRIO, J. C. S. S. et al. Unrepeatered transmission of 10×400G over 370 km via amplification map optimization. IEEE Photonics Technology Letters. On-line. July 18, 2016.