Technique boosts data rate in light pipes

Like runners racing to a finish line, a burst of photons fired into some glass fibers arrive at the end as a drawn-out blur. In today’s computer networks, those optical fibers in which such smearing, or dispersion, is great are used to carry only a limited volume of data per fiber and only for relatively short hauls. Nonetheless, these fibers align easily with lasers, so systems using them are comparatively inexpensive and popular.

Now, Howard R. Stuart of Lucent Technologies’ Bell Labs in Holmdel, N.J., has found a way to transform the Achilles heel of these so-called multimode fibers into a source of new strength. Using several lasers to send data and as many detectors to receive it, Stuart expects to multiply bit flow by the number of lasers, he says. No longer a curse, dispersion becomes the key to unraveling the data threads at their destination, he notes.

The new scheme is “intellectually beautiful. It’s really, really nice stuff,” comments George Papen of the University of Illinois at Urbana-Champaign. Many network-equipment developers are frantically pursuing new ways to boost data flow across multimode connections. Such links are used in data networks that cover areas up to the size of a college campus or business park.

Stuart says his method, which he describes in the July 14 Science, has great potential but remains far from becoming a product. Because the latest multimode fibers eliminate dispersion and optical networking is in rapid flux, “it’s not clear [Stuart’s] technique will keep up with the rate of change,” Papen remarks.

Stuart’s laboratory prototype, with two lasers, two detectors, and a circuit that sorts out the received signals, has mixed and separated data streams. It has yet to double the transmission rate of a one-laser system, however.

An optical fiber consists of a glass strand coated with a thin layer of another glass with different optical properties. The mismatch causes light to remain confined primarily to the core.

Physicists attribute the spread of light traveling through fibers to different patterns, or modes, of oscillation that electromagnetic waves can experience in the glass. The modes travel at different velocities through the fiber.

To achieve high data flows over long distances, network designers depend on so-called single-mode fibers. These have cores 10 micrometers or less in diameter, and the light they carry doesn’t disperse. But aligning laser beams to the narrow fibers adds cost.

In contrast, fibers with cores roughly 50 to 60 micrometers across often host hundreds of modes—hence the name multimode. In theory, engineers can design optical equipment that taps each mode as a data channel, but “there is really no practical way of doing it,” Stuart explains.

Instead, he took cues from an innovation in wireless communications.

The radio signals used by wireless equipment bounce off buildings and other obstacles en route to a receiver. Multiple copies of the signal arrive at slightly different times due to the multiple paths the signal takes, usually creating a nuisance. In 1998, other researchers at Bell Labs demonstrated a way to exploit such scattering to cram extra radio channels into a single frequency band. In their setup, separate antennas broadcast different parts of each signal. At least as many antennas pick up the jumbled broadcasts, feeding them into an unscrambling circuit.

Stuart’s brainstorm was to note that the spread of pulses in a multimode fiber was analogous to the effect of multiple paths on radio waves. In principle, his optical scheme can boost flow through multimode fibers without limit as new lasers and detectors are added. In practice, however, losses of light intensity at the fiber entrance and a rapidly growing load on the signal-processing circuitry may limit the increase to about 4 to 8 times the single-laser rate, he predicts.

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