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Harnessing chaos for optical communications.

Police radio messages are vulnerable to interception. A wireless communication system based on the erratic, unpredictable output of an electronic circuit or a laser, however, offers the possibility of enhanced security. An eavesdropper would detect only static, even though an intelligible message rides atop the chaotic carrier.

Researchers have now shown that chaotic signals transmitted from one laser directly to another can be used to carry information. Such an optical scheme has the advantage of reducing the amount of power required and potentially increasing the number of signals that can be delivered along an optical fiber, says Henry D.I. Abarbanel of the University of California, San Diego.

Abarbanel and Matthew B. Kennel describe the principles underlying such a technique in a report scheduled for publication in Physical Review Letters. Gregory D. VanWiggeren and Rajarshi Roy of the Georgia Institute of Technology in Atlanta report in the Feb. 20 Science the first experimental demonstration of chaotic communication using an optical system.

"What we've done is learned how to attach a message -- it could be one voice, it could be hundreds of voices -- to a carrier that is very irregular," Abarbanel says.

"In an ordinary digital signal, the message can immediately be seen," Roy adds. "In our system, digital information is encoded in the chaos, so the message would not be obvious to a person who may intercept it."

Using chaotic signals for communication requires synchronization of two highly unpredictable, virtually identical -- but separate -- systems. In 1990, Louis M. Pecora and Thomas L. Carroll of the Naval Research Laboratory in Washington, D.C., showed that it is possible under the right conditions to synchronize the wildly fluctuating voltages of a pair of properly matched chaotic electronic circuits (SN: 3/24/90, p. 191). Several years later. Kevin M. Cuomo and Alan V Oppenheim of the Massachusetts Institute of Technology demonstrated that such synchronized circuits could be used for communication.

Abarbanel and Kennel focused on whether erbium-doped lasers, widely used in optical-fiber communication devices, could serve as the basis for transmitting information when such lasers are allowed to generate chaotic signals. They showed theoretically that the distant lasers can be synchronized by injecting light from one directly into the other via an optical fiber. Moreover, the sending and receiving lasers don't have to be perfectly matched for the system to work.

Roy and VanWiggeren developed an erbium-doped fiber ring laser to produce chaotic electromagnetic radiation with a wavelength of 1.53 micrometers. They combined that erratic signal with a message signal, consisting of a uniform string of pulses at a frequency of 10 megahertz, and sent the mixture through an optical fiber to the receiving laser system. In response, part of the receiving system began generating just the chaotic fluctuations to which it is synchronized. By subtracting that chaotic portion from the combined signal, the researchers recovered the message.

Compared to an electronic circuit like the one his group used, Oppenheim notes, lasers are more convenient and provide higher frequencies, which are needed to send information faster.

Using their chaotic lasers, Roy and his team have recently communicated random bits over optical fibers at rates of up to 150 megabits per second. "We'd like to go faster," Roy says. "We'd also like to be able to send multiple signals simultaneously."

The Department of Defense's Army Research Office is now funding an ambitious project to demonstrate the usefulness of chaotic signals in optical and wireless communication.
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Title Annotation:using lasers in wireless communications
Author:Peterson, I.
Publication:Science News
Date:Mar 21, 1998
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