Recent studies have reported a new method for the determination of the azimuthal index (topological charge) ℓ of an optical vortex beam (eg. Optics Express 19, 5760-5771 (2011)). Here we consider the performance of such a system for determining both the azimuthal as well as radial indices of a Laguerre-Gaussian beam by measuring the far-field diffraction pattern from a tailored annular triangular aperture. Both a multi-annular triangle aperture and an annular triangle aperture with radial spokes are used.
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