Statistical properties of a laser pointer speckle field at different propagating distances

Abstract

In this paper, we analyze the intensity distribution of the speckle pattern, its average size, and the optical memory effect at different propagating distances, z, when we scatter a light source with low spatial coherence. We produce the speckle field by illuminating the light source onto a ground glass and capture the speckle images at different propagating distances, z, and with different tilting angles to measure the optical memory effect. The probability density function tells us that the speckle field approaches being fully developed as the propagation distance increases. By tilting the ground glass from 0° to 1° at a 0.01° increment, we note a small increase in the full width at half maximum (FWHM) of the correlation as we increase the z, indicating that the optical memory effect of the speckle is slightly larger at greater distances from the scattering medium compared to regions closer to it. Lastly, we plot the autocorrelation function at different distances, z, and observe an increase in speckle size with increasing propagation distance as expected. Our results reveal a fundamental property of speckle, which will be significantly important for numerous applications, such as image reconstruction and the measurement of cerebral blood flow and brain function.