Secret Rhapsody in Infrared

Computers may someday have a new way of transmitting secret information--by scrambling it with chaotic static. In an experimental technique, physicists sent a series of infrared laser pulses through a chaotic amplifier, then decoded the pulses by passing them through a closely matched amplifier. The method, reported in the current issue of Science, could cut the cost and tighten the security of coded messages.

The idea of communicating by modulated chaos was first proposed in 1991 by Lou Pecora and Tom Carroll, physicists at the Naval Research Laboratories in Washington, D.C. Since then, decodable chaotic transmission has been demonstrated in electronic circuits and, last year, in a hybrid electrooptical copper wire system (Science, 2 May 1997, p. 679). But copper wires cannot transmit data at the gigabits-per-second speed that will soon be standard for computer communications. For such speeds, optical fiber is required.

Now two physicists at the Georgia Institute of Technology in Atlanta, Rajarshi Roy and Gregory VanWiggeren, have demonstrated the first all-optical system to encode data chaotically. In their setup, the sender shoots a staccato burst of laser pulses (the message) into a ring of optical fiber that both amplifies and distorts the pulses. By "folding" the message into its own distorted feedback, over and over, the loop creates a completely garbled message that then shoots out through an outbound fiber. "It's not even clear [to an eavesdropper] when the message is sent," Roy says. At the receiving end, the transmission--which includes the embedded message--is decoded by a matched amplifier that filters out the noise.

Experts say that the work brings chaotic waves a step closer to marketable uses such as encoding the laser pulses that travel through transatlantic fiber-optic cables. "The wavelengths and optical devices they use are [the] industry standard," says Daniel Gauthier of Duke University. Chaotic encoding might in principle save money over current fiber-optic transmissions: Only low-quality amplifiers have the right kind of discord to generate chaos, Gauthier says.