The spatial correlation singularity of a partially coherent vortex beam was demonstrated recently [Phys. Rev. Lett. 92, 143905 (2004)], and it was shown that the cross-correlation singularity disappears when the spatial coherence is high. In this paper, we demonstrate that the spatial autocorrelation function of a fully coherent beam in the far-field is equivalent to the Fourier transform of its intensity in the source plane. Our theoretical and experimental results show that, depending on both the radial and azimuthal mode indices (p, λ) of the incident light beam, the distribution of the far-field autocorrelation function displays a series of concentric, alternate bright and dark rings. This phenomenon may be used to determine the topological charge (the azimuthal index) of light beam with a nonzero radial index.
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