Coherent components of forward light propagation through scattering media

The transmitted light through random media is studied at lambda = 633 nm with a Mach-Zehnder interferometer. The optical heterodyne detection used allows a coherent detection with a sensitivity corresponding to an optical thickness (Lnl/l(0)) of 25. In addition to the standard exponential decrease for the transmitted light in accordance with the scattering cross section value computed from the Mie theory, we observed a distinct component for optical thicknesses above lei and 17 for polystyrene beads with diameters of 0.94 and 0.43 mu m respectively. This second component, involving light scattered onto the detector, was hardly detected with smaller scatterers but is clearly present in tissue samples. A measurement of the angular distribution of the scattered light confirmed the coherent detection of non-ballistic light for particle suspensions and blood samples.