Solitons under the sea.

Solitons under the sea

Over long distances, information traveling along an optical fiber as a sequence of light pulses can become scrambled. To rectify the problem, communications engineers install "repeaters" along the optical-fiber cable to clean up and amplify the degraded signal. But each repeater must convert the cable's optical signal into an electronic form, then reconvert it to light pulses, slowing the entire system down.

Researchers are now testing an alternative scheme that overcomes these problems and promises extremely rapid communication over very long distances. Their approach uses specially shaped light pulses, described as solitons, along with optical amplifiers to boost the signal when necessary. Created by a small semiconductor laser, optical solitons can travel long distances along optical fibers without spreading out and losing their identity, and can thus counter the normal tendency of light pulses to disperse.

Because solitons furnish an extremely clean signal that remains virtually unchanged over tens of kilometers, optical-fiber cables carrying them convey a much larger volume of data and require fewer repeaters than systems now in use. Furthermore, the replacement of repeaters by optical amplifiers -- which in effect turn pieces of the cable into lasers -- significantly increases transmission rates by eliminating the need to convert light pulses into an electronic form.

"This began just a few years ago as the purest of physics, and now it's entering engineering development," says Linn F. Mollenauer of AT&T Bell Laboratories in Holmdel, N.J. An undersea cable based on this technology, stretching 9,000 kilometers from Seattle to Tokyo, may enter operation by 1996.