| Free space optical communication is an important means of high-speed information transmission in space information network,which is facing some problems,such as low spectrum utilization and insufficient channel capacity.In theory,the above problems can be solved by using the vortex beam as the carrier and the additional orbital angular momentum(OAM)degree of freedom to encode information.Uncertainty of beam angular position can also improve the security performance of communication system.In addition,the vortex beam can also be used in optical micro-manipulation and other fields.Relying on the light intensity distribution characteristics of its dark hollow structure,an optical potential well can be formed to capture and translate submicron particles without physical damage.At the same time,it can also be used as an optical wrench to transfer orbital angular momentum to rotate around the optical axis.However,these applications depend on the special helical phase structure and orbital angular momentum parameters of the vortex beam,and OAM can be expressed by the topological charge(TC).Therefore,it is of great significance to study the spatial distribution characteristics of vortex beams and to detecttopological charge information,which is the basic and key for the wide application of vortex beams in various fields.In this paper,taking the vortex beam of Laguerre-Gauss(LG)as an example,based on the principle of interference and diffraction of light waves,the methods for detecting the topological charges carried by single-OAM vortex beams and double-OAM composite vortex beams are studied.At the same times,studied the spatial distribution characteristics of the composite vortex beam with different composite parameters of the sub-beams,and the variation of the vortex distribution with the distance in the beam during free-space transmission.The main work of this paper is as follows:(1)In order to detect the topological charge carried by a single orbital angular momentum vortex beam,a new far-field diffraction method using an elliptical aperture with partially azimuthal occlusion was proposed.Based on Fraunhofer diffraction principle,the diffraction model of elliptical aperture with shielding azimuth angle was established.The diffraction patterns of the incident vortex beams is simulated numerically,and the corresponding relationship between the diffraction patterns and the topological charges carried by the vortex beams is deduced.(2)Two sub-LG beams are collinearily superposed to form a new composite vortex beam.The effects of the topological charges,waist-to-waist ratio and propagation distance of the two beams on the intensity and phase distribution of the composite vortex beams are analyzed.(3)In order to establish the variation of the number and distribution characteristics of the peripheral phase singularities(vortices)with the propagation distance in the composite vortex beams under different parameters.Based on the zero intensity characteristic of the vortex,the conditions for the position of the vortex in the composite beam are deduced.The effects of waist parameters,amplitude parameters and transmission distance on the number and distribution of the vortices in the beam are analyzed.The numerical simulation is used to verify the results.It is found that the results are in good agreement with the theoretical derivation.(3)Aiming at the problem of measuring the topological charge information of composite vortex beams,a method is proposed to detect the topological charge information of each sub-beam by using its intensity distribution characteristics and the diffraction patterns of an elliptical aperture occluded specific azimuth angle. |
