Speckle clustering in diffraction patterns of random objects under ring-slit illumination

Abstract

Some optical conditions are discussed for producing speckle patterns having long intensity correlations in the diffraction geometry. A random rough surfaces under illumination of coherent light with a circular ring slit produces the speckles with a zero-order Bessel function for the intensity correlation when observed in the Fraunhofer diffraction region. This function has relatively long oscillating tails and, consequently, string or network structures appear in speckle patterns, which we may call speckle clustering. This clustering phenomenon can explain the similar appearance observed in diffraction patterns produced by random Koch fractals.