Feb. 25, 1998 During the early 1990s, researchers at the Georgia Institute of Technology learned to control the chaotic fluctuations in light intensity produced by certain laser systems. In 1994, Rajarshi Roy and K. Scott Thornburg of Georgia Tech showed for the first time that two chaotic lasers could be sychronized. At that time, they suggested potential communications-related applications for the work.
Now, in a report published February 20 in the journal Science, Roy and collaborator Gregory D. VanWiggeren describe how to use those 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. And by showing that information can be recovered from "noisy" and irregular signals, the work also challenges assumptions that underlie many forms of modern communication.
"We have developed a system that allows us to encode information onto chaos, transmit it, and then decode the information away from the chaos," said Roy, who is chair of Georgia Tech's School of Physics. "In an ordinary digital signal, the message can immediately 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."
The research was sponsored by the U.S. National Science Foundation and the U.S. Office of Naval Research.
In the experimental system, a square wave "message" signal is produced by a stable semiconductor diode laser. That signal is then amplified by an erbium-doped fiber amplifier (EDFA) and introduced into a chaotic signal produced by an erbium-doped fiber ring laser of the type commonly used in the communications industry today.
The resulting combined signal, containing a mixture of the message and chaotic carrier, is transmitted through an optical fiber to a second EDFA nearly identical to the first. Upon receiving the combined signal, the receiving EDFA begins generating chaotic fluctuations synchronized with those produced by the transmitting laser. The chaotic portion of the signal, measured by a digital oscilloscope, is then subtracted from the combined signal and low pass filtered, recovering the original message to be read by the recipient.
The semiconductor diode laser and erbium-doped fiber ring lasers used in the experiment operate at approximately 1.53 micrometers, a wavelength ideal for fiber transmission.
Roy believes that the sending and receiving EDFA systems must be similar, though not necessarily identical, in order for the chaotic encoding-decoding scheme to work. The timing of the signal and other factors such as the lasers' state and phase must be carefully set in both systems. Thus, a person intercepting the message with a similar laser could not decode it without knowing these parameters.
Other researchers have used chaos to mask information in electronic and hybrid opto-electronic systems, but the work reported in Science is the first use of chaos to carry messages in an all-optical system. The optical system provides speed improvements of as much as 100-fold over the electronic systems, making it attractive for modern communication systems.
The Science paper describes sending signals at a rate of 10 megabits per second, but Roy and VanWiggeren have since communicated random bits of information at speeds of up to 150 megabits per second. They see no theoretical limitation on how fast data could be sent, though the capabilities of detector equipment impose a practical limit.
Before the chaotic system can be put into practical use, researchers must further develop the techniques demonstrated -- and verify that they can be used with longer lengths of fiber. Optical transmission networks can introduce distortions that may affect the chaotic fluctuations and hamper recovery of the message signal.
Theoretical analysis and numerical computations developed by researchers at the University of California at San Diego and Cornell University are related to these experiments. In a paper to be published in Physical Review Letters, collaborators Henry Abarbanel and Matthew Kennel from the University of California at San Diego suggest that the encoding-decoding technique should be robust even if the lasers aren't identical -- and there is noise in the optical fiber.
Beyond the potential communications applications, the work also shows that communication does not have to rely on regular waveforms.
"In science, when we look for information, we usually look for some kind of analog or digital form of information that is readily recoverable," Roy noted. "But we are seeing here that information can be hidden in signals that look noisy and chaotic. Information can be transmitted and received with a chaotic waveform instead of the standard sinusoidal waves that have always been used to carry information in radio or television."
This knowledge may encourage scientists to take a new look at how they transmit information and could one day lead to a better understanding of how biological systems such as neurons communicate.
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