Spatial-diversity detection of optical vortices for OAM signal modulation

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 from the OAM spectrum 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.