Determination of topological charges of polychromatic optical vortices

We introduce a simple, single beam method for determination
\nof the topological charge of polychromatic optical vortices. It is based on
\nastigmatic transformation of singular optical beams, where the intensity
\npattern of a vortex beam acquires a form of dark stripes in the focal plane
\nof a cylindrical lens. The number of the dark stripes is equal to the modulus
\nof the vortex topological charge, while the stripe tilt indicates the charge
\nsign. We demonstrate experimentally the effectiveness of this technique by
\nrevealing complex topological structure of polychromatic singular beams.