  Based on the Kirchhoff-Fresnel approximation, we consider behavior of the
optical vortices (OV) upon propagation of the diffracted Laguerre-Gaussian (LG)
beams with topological charge |m| = 1, 2. Under conditions of weak diffraction
perturbation (i.e. the diffraction obstacle covers only the far transverse
periphery of the incident LG beam), the OVs describe almost perfect 3D spirals
within the diffracted beam body, which is an impressive demonstration of the
helical nature of an OV beam. The far-field OV positions within the diffracted
beam cross section depend on the wavefront curvature of the incident OV beam so
that the input wavefront curvature is transformed into the output azimuthal OV
rotation. The results can be useful in the OV metrology and for the OV beam's
diagnostics.

Optics