Underwater Optical Beam Tracking

Abstract

The use of blue-green laser frequencies for establishing an air ocean underwater communication channel has been well accepted. However any attempt to initialize or maintain such a link will invariably require some method of accurately spatially pointing and tracking the penetrating beam. In this paper we present results of a study concerned with determining the ability to spatially track an optical after undergoing underwater propagation. By invoking the concept of modulation transfer theory and substituting established propagation models for underwater coherence functions, the focal plane intensity patterns generated in wide angle optical lensing systems can be determined, as a function of the link characteristics (e.g. sea state, depth into the ocean, turbulence, etc.).With the intensity pattern modeled, the behavior of various forms of optical trackers can be analyzed by the application of standard tracking loop theory. Of particular interest here is the application of well known mathematical tools, such as Kolmogorov theory, which allows generalized statistical analysis to be performed on both linear and nonlinear dynamical systems. The result of such an approach is the development of a differential equation whose solution yields the statistics of the tracking error. Theoretical studies of this type have been examined previously for generalized scattered optical fields [1]. With these basic approaches as a guide, mean squared tracking errors can be derived, which assesses the perfomance of the beam tracker in relation to the channel characteristics.