Research On The Measurement Method Of Orbital Angular Momentum Of Vortex Beams

Because of its novel properties,orbital angular momentum(OAM)has been investigated in several realms,such as optical manipulation,quantum information process and optical remote detection.Recently,OAM has been used as an additional degree of freedom to improve the performance of optical communication systems.For most of these applications,the determination of OAM in advance is very important.In this paper,the basic principles of OAM generation and measurement are studied.Two novel OAM measuring methods are proposed,which are the gradually changing duty-cycle grating method and the variable cylindrical lens pair method.The main work is as follows:First,two types of beams carrying OAM are introduced,namely the Laguerre-Gaussian(LG)beam and the astigmatic elliptical Gaussian beam.The typical methods of vortex beam generation and OAM measurement are summarized.Secondly,two special gratings are proposed to measure OAM,which are horizontal and vertical gradually changing duty-cycle gratings.They are realized by the superposition of binary amplitude gratings to produce moiré pattern.The property of the light field is studied by Fresnel diffraction.The OAM measurement capability of the two gratings is evaluated by numerical simulation.Experiments verify that the the horizontally gradually changing dutycycle grating can measure low-order OAM.At last,we proposed a novel method to measure the high-order OAM with a variable cylindrical lens pair.The optical system consists of two cylindrical lenses with opposite focal length.It produces a rotating position—spatial frequency transformation of by changing the angle between the two cylindrical lenses.With this feature,the method can flexibly measure the OAM states of the vortex beams with different beam waists.The OAM state is determined by the number and direction of dark stripes of the diffraction pattern.The measurements of the topological charge up to 200 and the radial index of LG mode are demonstrated.Simulation results show that the optical system is tolerant of misalignment.As a compact high-order OAM measuring device,the variable cylindrical lens pair may find potential applications in OAM optical communication or fiber optics.