TRANSMISSION AND INTENSITY CORRELATIONS IN WAVE-PROPAGATION THROUGH RANDOM-MEDIA

We report on an experimental study of wavelength-dependent intensity fluctuations in coherent light that passed through a random dielectric medium. For the intensity that is scattered into a narrow solid angle and for the total transmission, we find exponential- and power-law decay in the intensity-intensity and transmission-transmission correlation, respectively. We derive an expression for the intensity-intensity correlation in the medium under the prevailing experimental conditions: Gaussian incident beam, slab geometry, and some absorption. From this expression we calculate the short-range and long-range correlation functions, using a Langevin approach to find the latter. Two earlier calculations of the long-range correlation function disagreed by a prefactor. The present theory quantitatively describes the long-range correlation, and therefore removes the uncertainty about this factor. With the Boltzmann diffusion constant for light in the random medium as the only fitting parameter, we obtain quantitative agreement between theory and experiment.