Private communication using chaotic light

A major advantage of broadband carriers is that they enhance the robustness of communications channels to interference from narrowband disturbance. This technology is the basis, for example, of spread-spectrum communication methods such as the code division multiple access protocol, which combines highbandwidth digital information with a pseudorandom digital code. The result is a hard-to-interfere-with signal that is useful in military applications such as antijamming and secure messaging. So-called chaos-based communications1–3 is also a broadband technique, but it is based on a different concept. Here, the carrier is a chaotic (seemingly erratic but fully described by deterministic equations) analog optical waveform, generated at the physical layer. The message is encoded in such a way that the information is hard for an eavesdropper to extract. Decoding by the appropriate receiver requires synchronizing the carrier. The receiver architecture performs a nonlinear filtering process4 that is then used to subtract the encoded, transmitted information (see Figure 1). Chaos-based communications was proposed and demonstrated in the early 1990s in electronic circuits, and soon extended to optical systems.2–7 Photonics provides simple ways of generating high-dimensional chaotic carriers that offer both a substantial level of security and the possibility of excellent transmission rates. The simplest way to generate a chaotic optical carrier is to employ a semiconductor laser and feed back part of the emitted light into the device after a certain time delay (the all-optical approach). Alternatively, the light can be transformed into electrical current and, after a certain time delay, be used to feed, either the laser bias current or an external modulator (the electro-optical approach). Early laboratory experiments demonstrated successful back-to-back communications in both all-optical8 and electrooptical systems,9 where high bit rates were achieved. In all-optical chaos-based communication systems, the emitter architecture is usually composed of a semiconductor laser subject to optical feedback from an external mirror. The receiver Figure 1. Schematic representation of the encoding/decoding process.