Characterization of optical fields with quantized orbital angular momentum by invariants of higher order moments of radial coordinates

We show that the skewness and kurtosis parameters of optical fields with quantized orbital angular momenta (OAM) and integer topological charge, which depend on the propagation distance only through normalized transverse coordinates, remain invariant at propagation through axially symmetric first-order optical systems, if defined in terms of higher-order moments of the radial coordinate. The values of these parameters, which characterize the shape of optical fields, depend on the type of OAM beams (Gaussian, Laguerre-Gauss or spiral phase plates in far-field) and the topological charge. As a result, the skewness and kurtosis can be used to identify the type of OAM beam and the absolute value of the topological charge for Gaussian and Laguerre-Gauss vortices encountered in most applications.