One of the solutions of the Helmholtz Equation is the vortex beam. In the recent decades, production and applications of this type of beams have found serious attentions. Determination of the vortex beam topological charge and its sign are very important issue. Odd number of reflections of the vortex beam changes its vorticity. In this paper, we used a q-plate (QP) to generate a vortex beam from a plane wave of a He-Ne laser beam. A two-channel moiré based wave-front sensor is used to measure wave-front gradient of the vortex beam. In two different arrangements the vortex beam posses odd and even number of reflections, respectively, and from the moiré patterns deformation, value and sign of the topological charge were determined.

Keywords: moiré deflectometry, topological charge, vortex beam, reflection effect on the topological charge

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