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We report what is to our knowledge the first all-optical detection of the frequency beats between Gaussian and Laguerre-Gaussian LG0(1) modes in their axial superposition, caused by the rotational Doppler effect. The relation between the observable off-axis optical vortex rotation and the rotational frequency shift of the Laguerre-Gaussian component is ascertained. The results can be used as a physical basis for recognition of Laguerre-Gaussian mode spectra along their orbital angular momenta.

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We report the result of investigation of the screw, edge and mixed edge-screw dislocations of wavefront in monothromatic light waves. Methods for the experimental determination of the topological charge value and helicity of screw dislocation are elaborated. By use of computer-synthesized binary gratings waves with edge, mixed screw-edge and anisotropic screw dislocation were obtained and Studied experimentally. 1JNTRODUCTIONIn last years optical wavefront dislocations (phase singularities, or optical vortices) attract a great attention15. First analysis and description of phase defects in wave train and monochromatic wave was perfonned in a paper of Nye and Berry'. The presence of screw dislocations in speckle field was demonstrated later. The optical waves possessing single screw dislocation or a system of several dislocations were experimentally obtained in laser cavities3'4 oscillation, diffraction by specially synthesized holograms57, transmission of light beams through phase mask8'9, observed in laser light scattered by random phase diffuser or transmitted through multimode fibers.Existence ofthe phase singularity in an optical wavefront requires that the amplitude Eofthe light wave vanishes at this point, so as real, as imaginary part ofcomplex amplitude are equal to zero and phase becomes indeterminate.Our aim is to give general rule for the determination of sign and topological charge of the screw dislocation and to report the analytical approach and experiments with holographically synthesized waves possessing edge, mixed screwedge and anisotropic screw dislocations.