Measuring orbital angular momentum of helical beams by spatially variable retardation plates

The transformation of Laguerre-Gaussian (LG) beams by a spatially variable retardation plate (SVRP), composed of half wave plates, is investigated. When a LG beam with topological charge of m(0) passes through an mth order SVRP, it was transformed into two orthogonal circularly polarized beams with topological charge m(0) + m and m(0) - m. This property is used to measure topological charges of LG beams. We also propose an efficient method which can be used to check mixed orbital angular momentum states.