  The Fraunhofer diffraction intensity distribution of Laguerre-Gaussian beam
is studied in an angular-double-slit interferometer. We demonstrate that the
spiral phase structure of vortex light can be clearly revealed in this
interference geometry, and it gives us an efficient way to distinguish
different order of Laguerre-Gaussian beams. This angular-double-slit
interference gives us a better understanding to the nature of orbital angular
momentum and the interpretation of vortex beams interference phenomenon.
