Controlling Chaotic Lasers
Over the past decade it has become clear that many different types of lasers exhibit chaotic fluctuations of intensity under a variety of operating conditions. These include exotic systems, such as the far infrared ammonia laser, as well as commonly used semiconductor lasers. Signatures of chaotic fluctuations in laser light and the comparison of experimental observations with predictions of numerical models will be reviewed. An important issue here is the development of measures that can separate noisy and deterministic components of dynamical behavior and provide information relevant for application of dynamical control techniques. While earlier work consisted of efforts to identify and classify chaotic behavior, the emphasis today is on the control of chaotic lasers, and possible applications.
Recent efforts to control chaotic systems have been remarkably successful in many different areas of science and technology. Dynamical techniques for the control of chaos allow a wide variety of waveforms to be stabilized and provide a completely new approach to the control of chaotic intensity fluctuations. Recent experiments require only small perturbations of parameters (about the ambient values). It has also been demonstrated that the stability regime of a laser system can be significantly extended through control and tracking of the unstable steady state. These results indicate the possibility of practical applications of nonlinear dynamics and show that it is possible to orchestrate the emission of light by large ensembles of atoms into complex or simple temporal patterns with rather small but judiciously chosen perturbations.
Several open questions have emerged from these studies. Can we control systems that have more than one positive Lyapunov exponent, i.e., more than one direction of instability in phase space? Second, how does intrinsic noise influence the dynamical behavior and controllability of chaotic systems? These questions will directly impact the control of spatio-temporal systems such as arrays of coupled lasers, or fiber laser systems. Can chaotic systems can be used to our advantage? The possibility of encoding information in a chaotic background and then decoding it in real time with a synchronized chaotic system will be reviewed. Experiments on synchronization of chaotic lasers will be described. The direct relevance of fundamental science to technological applications is clearly illustrated in this field.