Flexible Source of Optical Chaos for Use in Communications
Jonathan N. Blakely, Daniel J. Gauthier (Duke Univ., Durham, NC 27708-0305)
(Presented at the OPTO-Southeast Meeting in Charlotte, North Carolina, Sept. 19, 2000.)
A recent exciting development in nonlinear dynamics is the proposal that chaotic fluctuations in optical systems may be exploited in a novel ‘chaotic’ communication system. The transmitter contains a laser that produces a chaotic waveform in which a message is encoded. The transmitted waveform resembles random noise, possibly fooling would-be eavesdroppers. The receiver contains a second chaotic system that produces an identical chaotic signal but without the encoded message, enabling the transmitted message to be extracted from the chaos. This scheme for communications relies on two surprising properties of chaotic systems. First, message recovery relies on the ability of coupled chaotic systems to ‘synchronize’ or produce identical chaotic behavior. Second, message encoding utilizes ‘chaos control’ or applying small perturbations to stabilize one of the many unstable periodic behaviors typically displayed by chaotic systems. A chaotic communication system may possess advantages of privacy and efficiency over conventional systems. Successful communication with chaos has been demonstrated experimentally using chaotic erbium doped fiber ring lasers.
The potential for practical application drives the current widespread interest in understanding fundamentally chaos control and synchronization. To that end, we have designed and built a new chaotic optical source. Our system produces chaotic fluctuations on a time scale that is directly controllable over a range from milliseconds to nanoseconds. For communication at a high data rate, gigahertz fluctuations are desirable. However, such fast oscillations are difficult to observe experimentally due to the practical bandwidth limitations of laboratory instruments. For experiments, a speed of a few hundred megahertz is more desirable since we can obtain accurate time series from the system and hence accurately test our understanding of this new device. Our optical source could be useful both for experimental investigation of synchronization and control and for use in a practical high-speed communication system.
Our chaotic optical source is an electro-optic hybrid device consisting of a diode laser with time-delayed electrical feedback. Starting at the output of the laser, the feedback loop consists of a Mach-Zender interferometer with unequal path lengths, a photodetector, an amplifier, and a delay line. The loop is closed by a bias-T that combines the signal from the delay line with the dc injection current of the laser. The interferometer with unequal path lengths produces intensity fluctuations that are nonlinearly related to input wavelength fluctuations. The photodetector converts the intensity fluctuations into an electrical signal that eventually modulates the laser’s injection current producing fluctuations of the laser’s wavelength and, to a lesser extent, its intensity. With enough loop gain, the combination of non-linearity and time delay produces chaos. We can vary the time scale of the oscillations by simply changing the length of the delay line. Oscillations from several kilohertz to hundreds of megahertz have been observed. With modifications to the experimental setup, we believe that chaotic oscillations at several gigahertz are possible.