Chaos enables secure messages over all-optical network

By ROBERT V. PEASE

Researchers at the Georgia Institute of Technology (Atlanta, GA) may have found a productive use for an optical phenomenon that most engineers attempt to eliminate or control. The phenomenon is called "chaos," and Dr. Rajarshi Roy, chairman of Georgia Tech`s School of Physics, has produced some interesting results toward the use of chaos to transmit private or secure messages. With the help of fellow researcher Gregory D. VanWiggeren, Roy may well find a future for chaos in optical communications.

It is difficult to give a one-sentence definition of chaos, says Roy. Chaotic systems are characterized by their sensitive dependence on initial conditions. "Trajectories of a chaotic system [in phase space], starting from infinitesimally close initial conditions, diverge away from each other exponentially quickly," he says. "Chaotic systems also often display well-recognized routes to chaos, as a parameter is increased or decreased."

More simply, chaos is irregular fluctuations in light intensity produced by certain laser systems. The research at Georgia Tech to control chaos began in the early 1990s. In 1994, Roy and K. Scott Thornburg showed for the first time that two chaotic lasers could be synchronized. Following their initial research, Roy and Thornburg suggested potential communications-related applications for the work.

After studying erbium-doped fiber lasers and amplifiers with a colleague, Quinton Williams, Roy decided to apply them to chaotic communications. VanWiggeren and Roy explored this possibility in a series of experiments last year. Their success was published last February in the journal Science. Roy and VanWiggeren described how to use chaotic fluctuations to encode information being transmitted from one laser to another through fiber-optic cable. The work opens up the possibility of using chaotic carrier signals to hide private messages transmitted across existing optical fiber networks. By showing that information can be recovered from noisy and irregular signals, the work also challenges assumptions that underlie many forms of modern communications.

"We have developed a system that allows us to encode information onto chaos, transmit it, and then decode the information away from the chaos," says Roy. "In an ordinary digital signal, the message can easily be seen. But in our system, digital information is encoded in the chaos, so the message would not be obvious to a person who may intercept it."

Digital or analog information can be encoded onto chaotic carrier waves using amplitude or frequency modulation. The information then can be recovered with a synchronized chaotic system in a properly designed receiver. Roy and his collaborators have demonstrated this recovery in an all-optical system at up to 126 Mbits/sec.

Georgia Tech`s research into useful applications for chaos is still in its early stages, and the experiments of Roy and others there are only the first steps in developing systems that can be used practically. Thus far, the research is not ready for practical use.

A French group in Besancon has been working on chaotic communications with electro-optic systems. Other researchers have used chaos to mask information in electronic systems. However, Roy and his counterparts claim the first use of chaos to carry messages in an all-optical system. The advantage of the optical system is speed, as much as 100-fold over the electronic systems, making it attractive for modern communications applications.

The initial paper reported a transmission rate of 10 Mbits/sec. But Roy and VanWiggeren have since communicated random bits of information at speeds up to 150 Mbits/sec. Roy sees no theoretical limitation on how fast data could be sent, though the capabilities of detector equipment impose a practical limit.

The latest experiment conducted at Georgia Tech encouraged the optimism for chaos research. Using a scheme with an intracavity lithium niobate modulator, Roy and his team transmitted and received information at 126 Mbits/sec with less than one error in 100,000 bits. "Our system had two time delays and multiple parameters that need to be matched for accurate message recovery," says Roy. "A 1.6-km fiber communication channel did not noticeably degrade the signal."

Roy believes future chaotic carrier-transmission development will see gigabit and higher speeds for transmitting and receiving. Transmission over long fibers with very low bit-error rates also is in the forecast. Georgia Tech is continuing research of chaotic transmission and receiver configurations that will enhance the privacy of the communications process. q