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The measurement of the topological charge of a vortex beam is demonstrated using the diffraction pattern produced by hole wheel. The resulting mandala-like interference pattern depends on the number of holes relatively to the topological charge. The interference at the centre of the pattern —bright or dark—enables us to determine the topological charge in a procedure when hole wheels with different number of holes are applied. This method is direct and wavelength independent. It does not require any image analysis and could find applications in classical telecommunications or quantum optics using twisted light. [ABSTRACT FROM AUTHOR]

The self-interference pattern of a Laguerre Gaussian beam using a Fresnel bi-prism is shown to be very different from what could be expected from a usual laser beam. It resembles the interference pattern that could be obtained using a double slit experiment. The interferences are shifted and the topological charge and its sign can be readily determined considering the shift order of the pattern only. However, since there is no diffraction nor absorption losses unlike in a double slit interference, such a set up could be used even for low power twisted beams or beams with high topological charge. Even fractional topological charges could be determined with an absolute precision of 0.05. [ABSTRACT FROM AUTHOR]