We propose a method for identifying orbital angular momentum (OAM) states within a vortex superposition using a Shack-Hartmann (SH) sensor as a spatial-diversity detector. We define a local OAM at every pixel of the SH image, from which we construct an OAM spectrum. The topological charges are determined fromtheOAMspectrum using a low-complexity algorithm, resulting in estimates that are robust to beam wandering. Data from a 200 m experimental transmission are successfully tested using the proposed technique.
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