Nonlinear Frequency Conversion Of Beams Carrying Orbital Angular Momentum

In recent years,structured beams with special optical field distribution such as phase and polarization have become one of the research hotspots in the world.Especially since Allen et al.put forward in 1992 that the orbital angular momentum of the beam is independent of the spin angular momentum,and carries the orbital angular momentum beam,also known as the optical vortex beam.Because of its dynamic and quantum properties,it has the spiral phase wavefront which brings the central phase singularity and thus produces the hollow light intensity distribution.It has important potential value and application in many fields,such as particle manipulation,large-scale optical communication with high capacity and high speed,quantum information processing,super-resolution microscopic imaging and so on.It attracts more and more attention and research interest.The beam carrying orbital angular momentum can be controlled in a linear way and also in a nonlinear way.By means of nonlinear frequency conversion,on the one hand,the working wavelength of the beam carrying orbital angular momentum can be expanded;on the other hand,in the process of frequency conversion,the orbital angular momentum carried by fundamental wave light will transfer to harmonics.In particular,for the frequency up-conversion process,high-order orbital angular momentum can be obtained simultaneously with short-wavelength laser output.In this paper,we use optical superlattices as nonlinear crystals to achieve efficient frequency doubling output of high topological charge vortex beams.The characteristics of frequency doubling conversion are analyzed and studied,focusing on phase matching conditions and conversion efficiency.Specifically include:1.In the introduction,we briefly review the development of laser technology and nonlinear optics;introduce the background,application prospects,research significance,problems and research topics related to optical vortex beams,and the progress of literature on the nonlinear frequency conversion of orbital angular momentum beams.2.Chapter 2 mainly introduces the basic characteristics of orbital angular momentum,including the derivation of photon orbital angular momentum,the main typical method of loading orbital angular momentum with beams and the detection of orbital angular momentum,and the intensity distribution,group velocity and wave vector variation of LG beams,which are representative basic beams in optical vortex beams.It lays a foundation for the following research on the nonlinear frequency conversion of vortex beams with orbital angular momentum.3.In Chapter 3,the frequency doubling characteristics of LG beams with high topological charge are studied.The numerical solution of nonlinear optical coupled wave equation using program is used as the basic research tool.Based on 1342 nm acousto-optic Q-switched nanosecond laser,quasi-phase matching technology is used to realize the frequency doubling process with high efficiency and high topological charge through a single pass on periodically polarized lithium tantalate(PPLT)crystal.The optimum matching temperature drift of LG beam frequency doubling is found and explained by correlation analysis.Nonlinear process is a phase sensitive process.Through a simple frequency doubling process,the optimal matching temperature shift of LG beams can be observed.Whether the additional radial wave vector caused by the radial parameter waist radius wo and radial order p of LG beam or the additional angular wave vector caused by the angular vector subnumber topological charge l of LG beam can lead to the generation of the transverse wave vector component,thus changing the longitudinal wave vector,which will help us to correct and compensate the phase mismatch introduced in the design of superlattice artificial microstructures.It makes momentum conservation meet the phase matching condition again and improves the efficiency of optical nonlinearity conversion.4.In Chapter 4,based on the numerical solution of nonlinear optical coupled wave equation by program as a basic research tool,the theoretical study of perfect vortex frequency doubling characteristics is carried out.Considering that the frequency doubling efficiency of LG beams decreases with the increase of the topological charge number,and the radius w,of the maximum intensity of perfect vortex beams does not change with the topological charge value l,it is hoped that the frequency doubling efficiency of the perfect vortex beam will be higher than that of the LG beam when the topological charge is greater than a certain number under a certain condition by comparing the frequency doubling efficiency of the perfect vortex beam with that of the LG beam.