Modeling Low-coherence Enhanced Backscattering (LEBS) Using Photon Random Walk Model of Light Scattering

Interference effects caused due to the coherent waves traveling in time reversed paths produces an enhanced backscattering (EBS) cone, which is known to be inversely proportional to the transport mean free path length (ls) of a scattering media. In biological media, ls (0.5-2mm) >> wavelength λ, results in an extremely small (~0.001) angular width of the EBS cone making the experimental observation of such narrow peaks to be difficult. Hence, we developed a low coherence enhanced backscattering (LEBS) technique by combining the EBS measurements with low spatial coherence illumination and low temporal coherence detection. Low spatial coherence behaves as a spatial filter preventing longer path lengths and collects photons undergoing low orders of scattering. The experimental angular width of these LEBS peaks (~0.3) are more than 100 times the width of the peak predicted by conventional diffusion theory. Here we present a photon random walk model of LEBS cones obtained using Monte Carlo simulation to further our understanding on the unprecedented broadening of the LEBS peaks. In general, the exit angles of the scattered photons are not considered while modeling the time reversed interference phenomenon in diffusion regime. We show that these photon exit angles are sensitive to the low orders of scattering, which plays a significant role in modeling LEBS peaks when the spatial coherence length of the light source is much smaller than ls. Our results show that the model is in good agreement with experimental data obtained at different low spatial coherence illumination.