Scatter could boost fiber capacity
By Eric Smalley, Technology Research NewsThe tendency of light beams to scatter inside a type of fiber optic cable commonly used in computer networks limits its capacity, but a technique used to cram more data in radio transmissions could turn this limitation around.
Several years ago researchers found that, when a radio transmission was scattered by obstacles like buildings, increasing the number of transmitters and receivers while keeping the total transmission power constant increased the transmission's data capacity. The scattering expands the capacity by spreading the transmission in space. The capacity doubles with each doubling of transmitter-receiver pairs.
Howard R. Stuart, a researcher at Bell Labs, noticed that the way light beams in multimode optical fiber scatter resembled the scattered radio transmissions. "Multimode [optical fiber can] behave very similarly to a very strong multipath scattering environment in wireless [communications]," he said.
Multimode fiber cables, which transport several light beams at once, are wide enough -- usually 50 or 62.5 microns -- that the beams bounce their way down the sides rather than shooting straight through. In addition, some beams bounce more often than others, which means they travel farther and take longer to traverse the cable. This spreading out of the beams, usually a detriment to data communications, is called dispersion.
Stuart demonstrated the feasibility of his technique in an experiment reported in the July 14 issue of the journal Science. He put two data streams on two separate light waves of the same frequency over a single multimode optical cable. The two detectors at the opposite end of the cable received different mixes of the data streams. The key to the scheme, called dispersive multiplexing, was sorting out the data streams using a signal processing circuit, Stuart said.
It's interesting work, but it's not likely to have as much impact in fiber optics as it does in radio, said Joseph M. Kahn, a professor in the electrical engineering and computer science department at the University of California, Berkeley.
Sorting out the data streams is a complicated process, Kahn pointed out. "One of the issues is that the difficulty of the signal processing that he's proposing gets harder and harder the higher the speed of the data transmission and the more channels." At this point, “this is more of a scientific curiosity," said Kahn.
Solving the signal processing issue is one of the challenges to making dispersive multiplexing over multimode fiber a practical technology, Stuart said. Another challenge is solving the engineering problem of getting the light beams from multiple lasers into a single fiber with an acceptably low level of signal loss, he said.
It's too difficult to determine when or if the technique will be commercially available, Stuart said. The research was funded by Bell Labs' parent company, Lucent Technologies.
Timeline: Unknown
Funding: Corporate
TRN Categories: Networking
Story Type: News
Related Elements: Diagram; Technical paper "Dispersive Multiplexing and Multimode Optical Fiber" in July 14 Science
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July 19, 2000
Page One
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