  It is well known that when a laser is reflected from a rough surface or
transmitted through a diffusive medium, a speckle pattern will be formed at a
given observation plane. Speckle is commonly produced by laser beams with a
homogeneous intensity, for which, well-known relations have been derived,
relating the speckle size to the area of illumination. Here we investigate the
speckle generated by higher-order Laguerre-Gaussian (LG) modes, characterized
by a non-uniform intensity distribution of concentric rings.We show that the
ring-structure of the LG modes does not play any role in the speckle size,
which happens to be the same as that obtained for a homogeneous intensity
distribution. This allow us to provide with a simple expression that relates
the speckle size to the spot size of the LG modes. Our findings will be of
great relevance in many speckle-based applications.
