Optical communications systems are well established in the infrastructure of the global communications net, providing enormous bandwidth for present and future applications. Various approaches have been tried over the last decade to assure the utmost privacy and security of these systems. Among the approaches, chaotic optical carriers are very promising for secure communications.
Figure 1 shows the operating principle of chaos-based optical communications. The transmitter consists of an optical oscillator forced by external feedback to operate in the chaotic regime, producing an optical carrier with a broad (GHz-wide) spectrum. Information is encoded on this chaotic carrier using different techniques. Assuming a high complexity in signal carrier and low message amplitude, it is practically impossible to extract this encoded information using techniques like linear filtering, frequency-domain analysis, or phase-space reconstruction. At the receiver side, a second chaotic oscillator is used, ‘similar’ to that of the transmitter. This similarity refers to structural, emission, and intrinsic parameters of the semiconductor laser, to the feedback loop characteristics, and to the operating parameters.
Figure 1. In chaos-based optical communication, a data message encoded on a deterministically chaotic carrier is recovered by using a receiver incorporating a similar deterministically chaotic oscillator.
Extraction of the signal is based on a process called synchronization. Synchronization means that the time evolution of the chaotic emitter can be perfectly reproduced by the receiver, provided both transmitter and receiver chaotic oscillators are similar. Even minor discrepancies between the two oscillators can result in poor synchronization, that is, poor reproduction of the emitter's chaotic carrier.
The receiver's operation is very important. Part of the incoming message with the encoded information is injected into the receiver. Assuming efficient synchronization, the receiver generates, at its output, a chaotic carrier almost identical to the injected carrier, but without the encoded information. Therefore, subtracting this chaotic carrier from the incoming chaotic signal, which includes the encoded information, reveals the transmitted information.
The feasibility of chaos-encoded communications systems has recently been proved in a transmission experiment carried out by our optical communications group at the University of Athens.1,2 An all-optical chaos-encrypted transmission system, operating on an installed optical network infrastructure of approximately 120km that covers the metropolitan area of Athens, was implemented with bit-error rate (BER) on the order of 10-7 for the extracted message.
The map in Figure 2 shows the topology of the link. It consists of three fiber rings, linked together at specific cross-connect points. A dispersion-compensation-fiber module at the beginning of the link cancels the chromatic dispersion induced by the single-mode fiber transmission. A number of erbium-doped fiber amplifiers and optical filters are used to compensate for optical losses and amplified-spontaneous-emission noise filtering, respectively.
Figure 2. A 120km total transmission link in the metropolitan area of Athens was used to validate the chaos-encoded communication scheme.
The system's encryption and decryption performance was studied by BER analysis. The message amplitude is tuned so that the filtered, encrypted message is not discernable by standard methods, with BER values no better than 6 × 10-2. Figure 3 shows eye diagrams of the 27 – 1 length, 1Gb/s message, both encrypted (left trace) and decrypted (after the transmission link: right trace). The good synchronization performance of the transmitter-receiver setup leads to efficient chaotic carrier cancellation and hence to a satisfactory decrypting process.
Figure 3. Eye diagrams show that a 1 Gb/s message encrypted using a carrier from a chaotic transmitter (left) was recovered by using a similarly chaotic receiver.
The performance of the chaotic transmission system has been studied for different message bit rates up to 2.4Gb/s. All BER values have been measured after filtering the subtraction signal, using filters with bandwidth adjusted each time to the message bit rate. For sub-gigahertz bit rates, the recovered message exhibits BER values lower than 10-7, while for higher bit rates a relatively high increase is observed.
The transmission effects induced by the optical medium in optical communication systems can play an important role in the final performance, because the receiver's output must be synchronized with the signal that reaches it, rather than the signal generated at the transmitter. Nevertheless, our results show that information can be transmitted at high bit rates using deterministic chaos in a manner that is robust to perturbations and channel disturbances that are unavoidable under real-world conditions, for distances on the order of 200km.