  It is known that the cross-correlation function (CCF) of a partially coherent
vortex (PCV) beam shows a robust link with the radial and azimuthal mode
indices. However, the previous proposals are difficult to measure the CCF in
practical system, especially in the case of astronomical objects. In this
letter, we demonstrate experimentally that the Hanbury Brown and Twiss effect
can be used to measure the mode indices of the original vortex beam and
investigate the relationship between the spatial coherent width and the
characterization of CCF of a PCV beam. The technique we exploit is quite
efficient and robust, and it may be useful in the field of free space
communication and astronomy which are related to the photon's orbital angular
momentum.
