Detection of multiple coaxial optical vortices with a Shack-Hartmann sensor in atmospheric turbulence

The Shack-Hartmann sensor has proved to be a valuable wavefront detector in the context of adaptive optics. In this work we explore its capacity of characterizing helical wavefronts and thus identifying orbital-angularmomentum (OAM) states under certain conditions in the context of OAM-based free-space optical communication links. We propose the estimation of \local OAM", which avoids the reconstruction of the phase field in the ShackHartmann array. The proposed method does not require prior knowledge of the OAM state, waist size or center location. We show the results of outdoor experiments for single-mode and two-mode OAM beams in turbulent conditions at which reliable detection is still feasible. We comment on the quality of detection as a function of turbulence strength, propagation distance, Shack-Hartmann resolution and number of acquisitions.