Topologically protected measurement of orbital angular momentum of light

We develop a weak measurement scheme for measuring orbital angular momentum (OAM) of light based on the global topology in wave function. We introduce the spin-orbit coupling to transform the measurement of OAM to the pre- and postselected measurement of polarization. The OAM number can be precisely and promptly recognized using single-shot detection without the need for spatial resolution. More significantly, the measurement results exhibit topological robustness under random phase perturbations. This scheme has the potential to be applied as a paradigm in the OAM-based optical computing, metrology and communication.