Characterization of Coherence

This thesis discusses the development of a heterodyne based system designed to detect the coherent component of light transmitted through a turbid medium. The system can detect coherent light intensity as low as 100 dB of an initial sub-milliwatt level input beam. It employs a very narrow bandwidth approach for generating the required heterodyne signal so that the bandwidth of the detection is about 10 Hz, limited predominantly by vibrations of the optical components. With this system, we studied the extent of the preservation of coherence in light that is transmitted through a turbid medium. By tilting the transmission beam by about 20 mrad, we were able to detect coherent photons which were scattered only a few times. The coherent intensity was experimentally found to be always larger than the ballistic intensity but smaller than the transmitted intensity. In addition, this coherent intensity fluctuated slowly over time. A model that assumed scattering is coherent and that the scattered light wavefront is non-uniform and fluctuating due to the Brownian motion of the microspheres, was applied and was found to be sufficient to explain the main features of the experiment.