Signal-Twinning Trick Breaks Fiber Distance Record

Distortion-defeating system sends a 400-gigabit-per-second signal four times as far

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Signal-Twinning Trick Breaks Fiber Distance Record
Image: Courtney Keating/iStockphoto

05NWTwinPhasemasterImage: Courtney Keating/iStockphoto

A simple trick of physics effectively quadruples the distance a signal could travel over optical fiber, according to researchers from Bell Labs, who demonstrated the scheme experimentally. But some experts are skeptical that such a system will ever be commercially viable.

The Bell Labs team sent the same signal using two complementary light beams, and when the light was recombined at the end, the signal-smearing distortion that ordinarily limits both distance and data rate was canceled out. That allowed them to transmit data much faster and farther than today’s optical systems can, says Xiang Liu, a distinguished member of technical staff at the Alcatel-Lucent–owned Bell Labs. A paper on their work was published online by the journal Nature Photonics on Sunday.

Fiber optics is the backbone of modern telecommunications, with optical fibers each carrying multiple beams of laser light at slightly different wavelengths. Voice or data traffic is encoded onto each beam, often as a shift in the phase of the light. As the beams travel, though, various physical effects from the interaction between the fiber and the light start to distort the signals, until finally the distortions add so much noise that the signal becomes indecipherable. Today’s top commercial fibers carry 100 gigabits per second over a maximum of about 4000 kilometers—the distance from New York City to Los Angeles.

The Bell Labs team, by contrast, was able to send a 400 Gb/s signal through a single fiber over a distance of 12 800 km, more than a quarter of the way around the globe. To achieve that, they had to overcome a source of distortion called the Kerr nonlinearity limit. When engineers want to transmit more bits per second, they need a higher signal-to-noise ratio, so they boost the power of the light beams, essentially turning up the volume. But due to the Kerr effect, the higher power changes the index of refraction of the fiber, causing the signals to become distorted, limiting their reach.

The researchers overcame the effect by putting the same signal on two beams of light that were identical in all ways but two: Their phases were 180 degrees from each other and they were polarized in opposite directions. (For instance, one beam might be polarized vertically, so the light waves oscillated up and down, whereas its twin was polarized horizontally, with the waves oscillating right and left.)

Each of the twin beams still gets distorted as it travels down the fiber, but the distortions are the opposite of each other, so that when the signals are recombined at the far side, they’ll cancel each other out. “When we add the two twins at the receiver, we can get rid of all the distortion,” says Liu , whose Bell Labs collaborators included S. Chandrasekhar, Andrew Chraplylvy, Robert Tkach, and Peter Winzer.

Of course, there’s a trade-off: Using twin beams means half of the channels in the fiber are now given over to carrying duplicate signals. “We get the reach, but we sacrifice the capacity,” Liu admits.

Still, he says, the technique provides an overall improvement in the signal. Reducing the Kerr effect alone doubles the distance a signal can travel, but because the system uses two beams for one signal, the power of the signal, and therefore the data rate, can also be doubled, for an overall fourfold improvement.

Govind Agrawal, a professor of optics at the University of Rochester, in New York state, calls the paper “an excellent piece of work.”

“Having said that,” he adds, “I doubt this technique will ever become commercial. The systems engineers are always reluctant to employ nonlinear techniques in their design.” Since nonlinear effects are by definition out of proportion to their causes, they can be tricky to work with.

Jürg Leuthold, a professor of photonics and communications at ETH Zurich, agrees that the technique might not appeal to systems engineers, who would need to use two transmitters to send the same information on two beams. On the other hand, there might be some long routes, perhaps across oceans, where the price would be worthwhile.

Liu is optimistic, however. Changes to the transmitters would be easy to implement, he says. And unlike some other methods of dealing with distortions, the technique itself is not hard to pull off. “This scheme basically has physics to do the trick for us,” he says.

About the Author

Neil Savage, based in Lowell, Mass., writes about strange semiconductors, unusual optoelectronics, and other things. In the June 2013 issue of IEEE Spectrum, he reported on advances in making silicon emit light.

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