Abstract
We analyze the time dependent response of strongly scattering media (SSM) to ultra-short pulses of light. A random walk technique is used to model the optical scattering of ultra-short pulses of light propagating through media with random shapes and various packing densities. The pulse spreading was found to be strongly dependent on the average particle size, particle size distribution, and the packing fraction. We also show that the intensity as a function of time-delay can be used to analyze the particle size distribution and packing fraction of an optically thick sample independently of the presence of absorption features. Finally, we propose an all new way to measure the shape of ultra-short pulses that have propagated through a SSM.
Highlights
The pharmaceutical and food industries commonly processes particulates in solid-liquid and solid-gas mixtures
The medium is considered to consist of scatters, the size of which are uniformly distributed over a specified size range, as one might expect from sieving
We have shown that the backscattering of near-infrared ultra-short pulses by random media looks rather promising for rapid monitoring, e.g., of aggregation during a granulation process
Summary
The pharmaceutical and food industries commonly processes particulates in solid-liquid and solid-gas mixtures. Analytical solutions can only be obtained in very specific circumstances, such as when the diffusion approximation can be applied, while numerical solutions still depend on knowledge of the absolute shape of the scatterer These limitations make the application of the radiative transfer equations problematic, in industrial processes, where the particle shape and the experimental geometry are not precisely known and difficult to control. A fast and effective analysis technique that does not depend on knowing the shape of the scatterers is desirable This tool could be used to separate geometrical parameters such as particle size distribution and packing fraction from other parameters, such as crystalline phase transitions, and chemical changes. The inclusion of light propagation within the particles allows us to examine both the influence of a wavelength dependent optical path length, and absorption within the particles on the temporal shape of the output pulse (aside from the obvious loss of photons)
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