Diffraction diagnostics of phase singularities at optical beams

A diffraction technique for diagnostics of phase singularities occurring at optical beams is presented. It is shown that both the phase twirling (clockwise or counterclockwise) of a vortex optical beam and the modulus of an azimuthal mode index of the beam are directly and unambiguously determined on the bending of interference fringes in a strip Young's interference experiment. Applicability of the proposed technique into diagnostics of phase singularities kind of non-localized dark interference fringes and into revealing of optical vortices at developed speckle fields at far zone is demonstrated. Advantages of the diffraction diagnostics of phase singularities at optical fields against the classical interference testing are argued.