Propagation Characteristics Of Orbital Angular Momentum Beam Based On Vortex Phase Modulation

Twisted beam,which is also called Orbital Angular Momentum(OAM)beam,has important applications in both classical and quantum fields because of their special phase structure.OAM as one degree of freedom of photon,can be constituted of infinite Hilbert space because of its special mechanical effect,therefore it has great potential in high dimension coherent optical communication,quantum information and precision measurement.OAM beam with phase structure and intensity distribution has made significant progress and application in the interaction with matter,such as imaging optical tweezers,phase control.However,the characteristics of OAM beam propagation make the nonlinear frequency conversion process low efficiency,especially the large topological charges.It is also a research hotspot in recent years to improve the nonlinear conversion efficiency of OAM beam.Therefore,based on the nature of OAM beam,this paper studies the propagation characteristics of OAM beam,and proposes to use 4f imaging system to improve its doubling frequency efficiency.Firstly,starting from the definition and concept of OAM beam,we analyze the generation method of OAM beam,and study the propagation speed of OAM beam respectively based on group velocity theory and the wave vector theory of structured light field.It is verified by the autocorrelation system experiment.Secondly,the generation and propagation of OAM beams are analyzed based on the integral diffraction theory.We have the evolution process that a Gaussian beam is converted into an OAM beam and point out the limits of theoretical calculation of OAM beams based on Laguerre Gaussian(LG)model.The interference characteristics of the OAM beam in the transformation process are also explored.We found that the starting points of evolution have Gaussian profiles for different charges.The dark spot size of high-order vortex is less than hundreds of micrometer in the range of propagation distance of 0.01 m.Therefore,using the starting point of evolution as nonlinear interaction area can solve the problem of low efficiency of nonlinear interaction between OAM beams with large topological charges.This provides important basis for achieving high-efficiency nonlinear manipulation and control of high-dimensional OAM light field.Finally,by taking the frequency doubling as an example,we explore the influence of nonlinear propagation in the process of OAM beam propagation,including theoretical and experimental analysis of the evolution characteristics of the frequency doubling beam and the influence if topological charge on the frequency doubling efficiency.The theoretical analysis of the characteristics of the 4f imaging system and the nonlinear OAM beam process can reduce the inefficiency caused by its evolution.