A composite beam of radiation with orbital angular momentum allows effective local, single–shot measurement of topological charge

We introduce and report experimental realization of a method to measure the topological charge of radiation carrying orbital angular momentum by accessing a small portion of the wavefront. The method exploits the superposition of matched-curvature beams in separated frequency bands to achieve a differential measure realized by a pair of phase-sensitive detectors. The method is suitable for long-distance communications.