Slower light for faster telecom networks

Promising research could yield better optical data storage

14 December 2007—A new method of temporarily storing light inside optical fibers may help speed up the optical telecommunications network of the future. Researchers at Duke University, in Durham, N.C., and the University of Rochester, in Rochester, N.Y., report in today’s issue of the journal Science that they have managed to store light pulses for several nanoseconds by converting them into sound waves. The ability to store data carried by light without first converting it into electricity could allow telecom networks to process that data more efficiently.

The scientists used a technique called stimulated Brillouin scattering to stop the light in its tracks. First, they sent light pulses encoded with data into one end of a 5-meter-long loop of optical fiber, the same sort of glass fiber that carries Internet and telephone communications around the world. Then they sent another set of pulses, called write pulses, with a slightly lower frequency than the data pulses, into the fiber in the opposite direction. ”When the pulses physically overlap in the fiber, they interfere with each other,” says Daniel Gauthier, a physics professor at Duke and one of the paper’s authors.

The interference pattern creates areas with high electromagnetic fields and areas with low electromagnetic fields. A process called electrostriction causes materials that do not conduct electricity—such as the silica glass in optical fibers—to slightly change their shape in the presence of an electrical field. So the optical interference pattern creates a pattern of higher- and lower-density areas in the glass—essentially a pressure wave. ”A sound wave is just a pressure wave,” Gauthier says.

Most of the energy of the data pulse gets transferred to the write pulse, which leaves the fiber in the same direction the data pulse entered from. The rest is converted into an acoustic wave in the fabric of the fiber itself. None of the data move forward in their original direction.

The acoustic wave doesn’t last forever; in this experiment, it lasted about 12 nanoseconds. During its lifetime, though, Gauthier, his postdoc Zhaoming Zhu, and Robert Boyd, a professor at the University of Rochester’s Institute of Optics, could send a third ”read” pulse through the fiber in the same direction as the original data pulse. Because changes in the density of the glass lead to changes in its index of refraction, the acoustic wave scatters the read pulse in a way that re-creates the original data pulse.

The method is analogous to how a hologram works. Two laser beams create an interference pattern that is inscribed into photosensitive material, then a third beam shone on the hologram re-creates the original pattern. ”I would call this real-time holography,” Gauthier says.

The new data pulses read out of the fiber are much weaker than the ones that entered. After storing the wave for 4 ns, the new pulse had about 29 percent as much energy as the original pulse. After 12 ns, the new pulse was down to 2 percent. For some applications, that might be enough power, Gauthier says.

Mehmet Yanik, as assistant professor of electrical engineering and computer science at MIT, calls the optical-fiber storage experiment ”a very exciting study and a promising method for slowing light in fibers.” One advantage the technique has over previous attempts is that it works with any standard telecommunications wavelength, Yanik says.

If the researchers can extend the lifetime of the acoustic waves, storing pulses for 100 ns or more, it might provide a buffer to hold on to a data stream coming into a router. Routers, which direct traffic from one fiber to another, have one physical switch but multiple input and output lines. With a buffer, the input from one line could be held briefly while simultaneous input from another line was processed. In fact, that’s the goal of the Defense Advanced Research Projects Agency’s Slow-Light Program, which funded the research.

Right now, routing is done by first converting light into electricity, processing it, and then converting it back into light. All that conversion generates ”waste heat,” and as the telecommunications industry moves from today’s data rates of 10 gigabits per second to 40 or 160 Gb/s, the heat problem will increase. ”Once we get to these very high-speed networks, it would be much better if we could keep everything entirely within the optical domain,” Gauthier says.

Gauthier would like to extend the amount of time he is able to store pulses. That can be accomplished, he believes, by using fibers made of materials with longer acoustic lifetimes and stronger electrostriction effects. One possibility may be chalcogenide glass, which is doped with a material like sulfur or tellurium. Some researchers have talked about using hollow fibers based on photonic band-gap crystals for higher-speed communications. Gauthier says filling the holes in such fibers with xenon gas or a liquid like carbon disulfide might lead to an excellent light-storage medium. Better materials might also allow him to get the power of the light pulses down. The pulses in this experiment were 100 watts. To be compatible with telecom uses, they should be less than 1 W. Gauthier says that if the research continues at its current pace, it might be possible to build a commercial light-storage device in four to five years.

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