Characterization of thick scattering media via speckle measurements using a tunable coherence source

Abstract

Summary form only given. Non-intrusive characterization of thick optically scattering media and detection of embedded inhomogeneities is of interest for various industrial and biomedical applications. We recently demonstrated that laser speckle statistics, which previously had been used for characterization of surface roughness, could also be used for characterization of thick, optically scattering media. Our measurements are based on the dependence of the speckle contrast on the laser coherence time relative to the variance in photon travel times through the medium. Therefore, for an appropriate coherence time, material parameters can be inferred from the observed speckle contrast. We have previously determined material thickness and scattering parameters and detected inhomogeneities embedded within turbid media, using a fixed coherence length (HeNe) laser. In our current experiments, we perform the speckle contrast measurements as a function of coherence length, which we vary by frequency modulating a diode laser, and demonstrate good agreement with a theoretical model based on photon diffusion. By varying the laser bandwidth, we can extract material parameters without adjusting material thickness or we can optimize the coherence time over a wide range as needed to characterize materials with different scattering parameters. We note also that our method is conceptually similar to photon diffusion imaging, where one measures variation in an output RF modulation as a function of modulation frequency relative to variances in photon travel times, but the implementation is completely different.