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Researchers Find Privacy in Chaos

Photonics Spectra
May 1998
Aaron J. Hand

Communication can be chaotic enough when the communicators stand face to face. Introducing technology into the equation can sometimes aggravate potential misunderstandings. Who, then, would want to throw even more chaos into the communication process?

Gregory VanWiggeren tests chaotic communications over optical fiber. Courtesy of the Georgia Institute of Technology.

Since the early part of this decade, researchers have been demonstrating ways to harness chaos for various applications, the idea being that it is possible to synchronize chaotic signals. Two researchers at the Georgia Institute of Technology have shown that chaotic lasers may be ideally suited for private and fast communications.

Rajarshi Roy, professor of physics, and graduate student Gregory VanWiggeren use noiselike fluctuations to encode information, transmit the signal from one laser to another through fiber optic cable, then decode the information out of the chaos. The experiment uses a stable laser diode to produce a message signal. The message is transmitted through a ring laser with a commercially available erbium-doped fiber amplifier; then it is received by a nearly identical laser. The chaotic part of the signal is subtracted and low-pass filtered, leaving the original message.

Similar research has exploited chaos using electronic and optoelectronic systems, but this is the first time an all-optical system has been used. One advantage of an optical system is increased bandwidth, which improves transmission speed by as much as 100 times over electronic systems. Georgia Tech's researchers have sent signals as quickly as 150 Mb/s but see no theoretical barrier to faster speeds. Concerned about potential problems with distortion over long distances, they ran the latest tests over a 1.5-km fiber. The researchers still encountered no problems.

For the encoding/decoding to work, sending and receiving lasers must be similar, though not necessarily identical. The timing of the signal and other factors such as the lasers' state and phase must be carefully set in both systems, prohibiting an interceptor with a similar laser from decoding the message without knowing the parameters.

Georgia Tech's researchers have been working on increasing the number of parameters that need to be set to improve privacy. Three or four parameters would make it very difficult to intercept, according to Roy. "Not only would somebody, first of all, have to have the right hardware," he said, "but, secondly, they'd have to have all the right parameters to receive the message."

Making interception even more difficult is the fact that the system uses the information signal to drive the chaos, making it a high-dimensional chaotic system, Roy said. This means that somebody intercepting the signal could not simply run through mathematical computations to decipher the message.

Chaotic communications could be used by banks or other companies transmitting sensitive information as an additional layer of privacy over encryption methods.

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