We studied ring width of the Laguerre–Gaussian vortex beams, which is the outer radius minus the inner radius. Combining theoretical analysis and numerical simulation, the approximate expression of the ring width is obtained. From the expression, we find that the ring width is independent of the topological charge, and increases with the increase of the waist radius at z=0 of the host Gaussian beam and the propagation distance. This research has good application prospects in optical tweezers technology and OAM optical communication.
We presented a new method to measure the topological charge (TC) of the vortex beam (VB) by use a Shack-Hartmann wavefront sensor with sector microlens array (SMLA). The SMLA is circularly symmetric, which perfectly corresponds to helical phase and ring intensity of the VB. On the focal plane, there are spot array with circular distribution due to the special arrangement of the SMLA. In this letter, we also analyzed the error, and concluded that the error of the outer ring is smaller. As a result, we only employed the data of the outermost ring. This method easily can obtain the sign and magnitude of the TC by calculating offset direction and offset of the spots respectively. The simulation results show that our method can determine large TCs and has high accuracy. While this method has a simple structure and can be used in optical communications in the future.
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