Time Reversal in Diffusive Media

Abstract

Erreur ! Utilisez l'onglet Accueil pour appliquer Heading 1 au texte que vous souhaitez faire apparaître ici. 2 Yet, diffusion is not only important in understanding propagation at optical frequencies. Diffusion actually happens quite often with microwaves, too. As a matter of fact, non-line-of-sight propagation in rooms and urban canyons share some of the characteristics of wave diffusion, which is better known as the class of Rayleigh channels in telecommunication theory [3]. Perhaps more important is the fact that wave diffusion can be useful. Reverberation chambers are the champions of wave diffusion, as their peculiar characteristics heavily depend on the ability to turn coherent excitations into a chaos of waves propagating along (ideally) every possible directions at the same time. When used as the basis for time reversal applications, diffusive media can be shown to take a whole new dimension that makes them suddenly appear as very appealing media for wave propagation, rather than the messy and uncontrollable medium we are used to expect with harmonic excitations. This chapter presents a summary of the work done in applying time reversal to diffusive media. It is organized into three parts. First, the main features of diffusive media are presented, starting from their physical origin leading to the derivation of black-box statistical models; these properties will be fundamental in the derivation of the results presented later. Time-reversed excitations are then considered, first for the generation of fields and the transmission of signals. The powerful self-averaging enabled by diffusive media is demystified, proving why it can only happen with such media. Self-averaging powers all of the properties of time reversal in diffusive media, as its ability to generate well-polarized fields in a medium considered as incapable of doing it. These results are then pushed a step further, by studying the generation of coherent wavefronts. Diffusive media are shown to be a potentially more effective solution for this task than free-space-based wave generators: a single antenna is shown to be capable of generating a large number of focusing wavefronts just by playing on the signals applied to it. Applications presented throughout the chapter are based on the use of reverberation chambers. Erreur ! Utilisez l'onglet Accueil pour appliquer Heading 1 au texte que vous souhaitez faire apparaître ici. 6