This paper presents a theoretical study on propagation and scattering characteristics of a short optical pulse in a dense distribution of scatterers. Examples include pulse diffusion in whole blood and in a dense distribution of particulate matter in the atmosphere and the ocean. The parabolic equation technique is applicable to the forward-scatter region where the angular spread is confined within narrow forward angles. When the angular spread becomes comparable to the order of unit steradian, there is as much backscattering as forward scattering and diffusion phenomena take place. We start with the integral and differential equations for the two-frequency mutual coherence function under the first-order smoothing approximation, and a general diffusion equation and boundary conditions are obtained. As examples, we present solutions for diffusion of a pulse from a point source and a plane wave incident on a slab of scatterers.
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