Research Partnerships
Switching in a Flash
Transmission via fiber optics is the only way to handle the rapidly growing amount of data traffic. Researchers from Siemens and the Technical University of Eindhoven have developed a process that dramatically increases the capacity of fiber opticsthanks to an extremely fast light-controlled switch.
The amount of data speeding through our communication networks doubles every six to 12 months. Today, about 2 Gbit/s (gigabits per second) are transmitted between any two typical large cities. But in five or six years, it could be more than 1,000 Gbit/sfar too much for conventional copper-wiring technology. But even if fiber optics were used, it would still be necessary to transmit 100 different light wavelengthsat least given todays rate of 10 Gbit/s per wavelengthan expensive proposition by any stretch of the imagination.
In other words, its high time to develop a different, less costly technology. Researchers working in a EU-sponsored partnership between Siemens, the COBRA Institute of the Technical University of Eindhoven (the Netherlands), and BTexact Technologiesthe research branch of British Telecom (United Kingdom)have developed a fiber-optic system that can transmit 160&nbp;Gbit/s per light wavelength. The scientists working on the project have christened it FASHION (UltraFast Switching in High-Speed OTDM Networks). As a result of their efforts, only seven wavelengths instead of 100 would be needed to transmit the 1,000 Gbit/s mentioned above. Such a development would reduce costs because fewer components would be needed. In a process called Optical Time Division Multiplexing (OTDM), one laser sends several data streams via 16 different channels at staggered intervals, reaching a total of 160 Gbit/s. The key to achieving this is turbo-charged switching technology. "When a different channel comes through the distribution node every 6.25 ps, you have to switch very fast in order to read a certain data set and replace it with a new one," says project leader Dr. Gottfried Lehmann of Siemens Corporate Technology, who coordinates the FASHION project. "But electronic components simply arent fast enough to accomplish that," says Lehmann. "So we switched to light."
The researchers needed two components: one that could read data and another one that could filter out a data set and replace it with another set (add/drop multiplexer). In both components, a precisely timed laser pulse changes the light signals in such a way that they can be separated and read by electronic components (see box).
Siemens researcher Gottfried Lehmann tests a new data-transfer process that uses fiber optics
"Four years ago, a colleague from Siemens Netherlands realized that we were working on something at the COBRA Institute in Eindhoven that could be used as an add/drop multiplexer," recalls Professor Huug de Waardt who was involved in the project from the start. "So although the basic physics behind the development of the switch was developed at MIT in Boston and at the Heinrich Hertz Institute in Berlin, we were the ones who came up with the idea of using it as an add/drop multiplexer."
The FASHION project got under way in 2001. In the laboratory, the technology soon worked flawlessly. "But real life is a different matter," Lehmann says. "For instance, fiber lines expand when the outside temperature rises, which results in slower rates of data transmission." To study the technology under field conditions, British Telecom made available a 70-km, eight-strand fiber link between Newmarket and Ipswich, England in the fall of 2003. FASHION engineers sent signals back and forth on four connected fibers.
"The add/drop multiplexer worked flawlessly over 280 km. But over longer stretches, the intensity became too weak for this component. Nevertheless, we could still receive the signal up to 550 km away," Lehmann says. For Edmund Sikora of British Telecom, the results were also positive. "Although we had to make manual adjustments in order to react to changes in the fibers, the bottom line is that the technology worked."
The nodes, those points where data come together and are separated, are particularly critical. They must be designed to handle the maximum transmission rate. Otherwise, data traffic breaks down. If video-on-demandthe downloading of entire moviescatches on, the amount of transmitted data will climb even faster. Meeting this increase can be accomplished only with faster connections to the end user. "In the U.K., fiber to the home will happen in the next five to ten years," Sikora says. "Until then, we need a cost-effective technology for the current fiber network. The FASHION project indicates that were getting there."
But that goal has not yet been reached. "We still have to learn how to stabilize signals before they reach the add/drop multiplexer, and automatically adjust the channels," says Lehmann.
Bernhard Gerl
The Building Blocks of Tomorrows Fiber Optic Networks
In Optical Time Division Multiplexing (OTDM), several data channels are transmitted on the same wavelength using only one laser. This works because the light pulses that carry the data bits are extremely short (2 ps = 2 trillionths of a second) in comparison with the 100 ps separating two successive pulses in a signal transmitted at the conventional rate of 10 Gbit/s (gigabits per second). As a result, it is possible to insert pulses from other channels into the gaps between the signals. Using this method in the context of the FASHION project (see main article), scientists transmitted 16 channels in pulses staggered at intervals over a fiber optic line, resulting in a total data rate of 16 × 10 Gbit/s = 160 Gbit/s per light wavelength.
Basic concept of an add/drop multiplexer
In order to feed the data, the 2-ps laser pulses are distributed over 16 fiber of different lengths. On each path, electro-optical modulators add binary information by either letting the light pulse through (1) or not (0). The pulses belonging to the 16 channels are staggered due to the varying path lengths and are ultimately reunited on a single fiber and sent to the recipient. In the FASHION project, the delay of the individual pulses amounts to 6.25 ps. Data bits belonging to one channel continue to be transmitted at intervals of 100 ps (6.25 ps × 16 channels). In order to read them, individual pluses have to be detected every 100 ps.
The recipient can read the data set intended for him by using the so-called non-linear four-wave mix process. In this process, a control laser sends a short pulse at a slightly lower light wavelength to the receiver component in synch with the desired data signal. The superimposition of the two pulses creates a further signal whose wavelength is just under that of the control laser.
This new light wavelength is isolated by an optical filter that allows only its wavelength to pass. In this technology, only the light pulses that belong to the desired channel land on a photodiode behind the filter. The photodiode converts the light signals into electrical ones, enabling them to be processed by conventional electronic components. The information contained in the other channels is lost in the process.
This cannot be allowed to happen when an individual signal channel is not entirely read on the network nodes and is instead directed to a branch line. Add/drop multiplexers are used to perform this job (the graphic shows four channels). They leave the other channels unchanged and feed in another channel on the site of the decoupled channel when necessary. This ensures that the fibers data-transmission capacity is always optimally used. This is accomplished by a component in which bi-refringent fiber, control lasers and optical amplifiers work together to polarize the pulses that are to be decoupled with respect to each other. Using polarization filters, the channels can then be easily separated from one another.