Diffraction Theory And Experimental Study Of Vortex Beam Incident 2D Amplitude Grating

A vortex beam is a beam with a helical phase wavefront and orbital angular momentum,characterized by a helical phase structure,and every photon in the vortex beam carries an orbital angular momentum.This beam has a phase singularity with a topological charge,a beam intensity distribution similar to a donut shape,dark empty,and zero central light intensity.The advent of vortex beams reveals the subtle correlation between macroscopic physical optics and microscopic quantum optics.As the vortex beam function is constantly developed,its application field is also more and more extensive.For example,optical tweezers,quantum communication,biomedicine,optical sensing and other fields.Similarly,the different interesting phenomena presented by different gratings of vortex beams have attracted great attention.The main content of microoptics is to study the optical phenomenon occurring on optical elements in the order of micron and the optical microprocessing of optical components,and diffraction microoptics is one of the important branch.Grating is an important object in exploring the diffraction phenomenon,which can be divided into amplitude diffraction diffraction grating and phase diffraction one.2D amplitude and secondary grating are used to study the diffraction of vortex beams.A 2D amplitude grating is a grating with a periodic structure and an array of aperture is arranged.The Talbot effect of 2D amplitude grating plays an important role in optical measurement,holographic storage,optical interconnection and other fields.The grating trajectory of the quadratic grating is composed of a fixed point in the circle and composed of many different radii,which enables the imaging of multiple planes in the same plane.Laguerre Gaussian beam is a common and typical vortex beam,which can maintain high stability in the transmission process.Moreover,there are many ways to make Laguerre Gaussian beam,so Laguerre Gaussian beam is used in both simulation and experiment in this paper.In this thesis,the diffraction phenomena of the 2D amplitude grating and secondary grating of the vortex beam are simulated and experimentally studied according to the Fresnel-Kirchhoff diffraction formula,and the influence of grating period and diffraction distance on the light field distribution is discussed.The main work of this article is as follows:(1)The diffraction of the incident 2D amplitude grating of the Laguerre Gaussian beam is simulated.In the simulation,square aperture array gratings with grating periods of 60 um and 150 um are designed respectively,and the diffraction pattern at the diffraction distance of 360 mm is obtained.According to the simulation results,it is found that the diffracted spot and the incident beam are consistent in the shape of a doughnut,and the radius of each spot is the same,and the arrangement is uniform,which is similar to the grating aperture arrangement.Take the central spot as the zero diffraction level,the zero diffraction level has the strongest light intensity,and the light intensity around the zero diffraction level is gradually weakened;When the grating period is changed,the diffraction level spacing changes,and when the grating period becomes larger,the diffraction level decreases.(2)The Laguerre Gaussian beam incidence 2D amplitude grating was experimentally studied.The red light emitted by the He-Ne laser is expanded by the laser collimator,and then irradiated to the spiral phase plate to generate a vortex beam,and the diffraction pattern at 360.85 mm after incident to the grating is collected,and the grating periods are selected to be 150 um and 200 um,respectively.The experimental results obtained are consistent with the simulation results: the spot shape and arrangement are the same as the simulation results,the grating period is increased,and the diffraction level spacing is reduced.According to the light field distribution,it is found that the quasi-Talbot effect occurs at the Talbot diffraction distance,that is,there is self-imaging of the incident vortex beam,so each diffracted spot has the same topological charge as the incident beam,and the orbital angular momentum is not dispersed.The experimental results confirm the correctness of the analysis of vortex beam to grating diffraction by Fresnel diffraction theory.(3)Experimental study of the incidence secondary grating of the Laguerre Gaussian beam.A secondary grating with a central grating period of 0.085,a grating defocus of 15λ and a grating aperture radius of 3 mm was selected for experimental exploration.The diffraction pattern shows that the third-order diffraction stage,+1,-1 and 0 levels,the shape of the diffracted spot is consistent with the incident beam,and the center of the three-diffraction grade spot is on the same line;Change the diffraction distance,the spot intensity and radius change accordingly;When the diffraction distance becomes larger,the strongest point of the light intensity of the three diffraction grades gradually changes from-1 to 0 to +1,the radius of the central spot remains unchanged,the-1 stage changes from small to large,and +1changes from large to small.By exploring the diffraction phenomenon of the incident two-dimensional amplitude grating and the secondary grating,it is found that the period,the topological charge number of the vortex beam and the diffraction distance will affect the light field distribution of the diffracted light,which provides a reference for controlling the application of beam splitting and generating the array vortex beam.