For a long time, the intensity and phase fluctuations determined by multiple light scattering were regarded as “optical” noise that degrades the radiation properties. Recently, remarkable advances in fundamental understanding and experimental methodologies proved that light propagation in random media is a source of unexplored physics with a wide range of potential applications. Among them, the medical applications occupy a special place, since it has been proven that scattering of optical radiation can be successfully used as a noninvasive investigation technique.
In many cases of practical interest, light propagating in dense scattering media can be described by the diffusion of scalar photons and therefore is fully characterized by their distribution of optical pathlengths. This is a comprehensive quantity that describes the statistics of photon random walk through many scattering events in a random medium. Experimental techniques such as diffusive wave spectroscopy and coherent backscattering rely on different models of the probability density of optical pathlengths in order to describe the measurements. The theoretical models typically use the time-resolved diffusion equation, which provides a satisfactory description of the photon transport phenomenon in scattering media whenever the absorption is not significant and the medium can be considered as infinite. Refined boundary conditions for the diffusion equation extend its applicability closer to the interfaces of finite-sized media. Nevertheless, there are many situations in which the scattering process cannot be treated as diffusive and, therefore, a direct way to measure the pathlength distribution is highly desirable when approximate theoretical values are not available anymore. For quite some time, direct time-of-flight measurements were the only experimental techniques able to provide direct information about the pathlength distribution of scattered light. However, low dynamic range and limited resolution impose severe limitations in using time-resolved measurements to characterize light propagation through highly scattering media.
Due to characteristics such as beam directionality and intensity, the use of highly coherent radiation produced by lasers has been the undisputed choice for many light scattering procedures. Recent developments in light sources and detection techniques offer new, more sophisticated experimental possibilities. By adjusting the coherence properties of light, one can use interferometric approaches to select specific orders of scattering and, therefore, directly infer the pathlengths distribution of photons scattered by a random medium. The background of this approach is introduced, different implementations are presented, and several applications are discussed in the next sections.
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