On the generation of clustered speckles due to ring-slit illumination

When a rough surface is illuminated by coherent light passing through a ring-slit aperture, a speckle pattern formed in its Fraunhofer diffraction region presents a clustering or snake-like appearance consisting of many winding chains of speckle grains. Some properties and origin of such clustered speckles are discussed. From experiments employing a ring-slit aperture as well as some segmented ring-slit apertures, it is shown that clustered speckles have different characteristics depending on the type of the apertures, and that the isotropically winding, snake-like feature of the speckles due to the ring-slit aperture is attributed to the circular symmetry of the aperture. An origin of the snake-like feature is discussed qualitatively on the basis of the random interference of many diffraction patterns. Speckle clustering in partially developed speckles is also examined. It is shown that a weak diffuser masked by the ring-slit aperture yields radial speckle chains, which turn into isotropic winding chains as the diffuser becomes stronger.