Study On The Diffraction Characteristics Of A Novel Spiral Zone Plates With Long Focal Depth

Vortex beam is a kind of beam with a spiral wavefront structure.During the propagation process,the light intensity exhibits a ring structure,and the central region is a dark core with a small size.This property makes it no heat damage effect when manipulating particles.In addition,the vortex beam carries orbital angular momentum during forward propagation.These unique properties make the vortex beam have important research value in many fields such as particle manipulation,optical communication.The methods for generating vortex beams developed in recent years mainly include geometric mode conversion method,liquid crystal spatial light modulation method,computational holography method and spiral phase plates method.With the development of research,more and more vortex beams with peculiar properties appear one after another.The main content of this paper is how to generate vortex beam with large focal depth and its application fields are studied in detail.The main contents of the paper are as follows:1.Firstly,the research background and significance of vortex beam are introduced,then how to generate vortex beam and its possible application fields are introduced.Then the basic theory of vortex beam and the scalar diffraction theory used in this research are described,which will serve as the theoretical support for the simulation part of this paper.Next,the design principle and diffraction characteristics of Fresnel zone plates and spiral zone plates are introduced.2.Spiral area-varying zone plates,firstly,the design method of the area-varying zone plates and its diffraction mode are introduced.Then,it is combined with the spiral phase to generate vortex beam with long focal depth.The diffraction characteristics are simulated by MATLAB,and the experimental verification is carried out with the spatial light modulator(SLM).The experimental results are consistent with the theoretical simulation results.3.Axial line-focused spiral zone plates,we introduce phase correction on the basis of the spiral zone plates,and give each zone plate different focal lengths in turn.According to the different focal lengths of different wave zones and the focal depth of the same focal length,the focused light spots at different focal lengths are combined to achieve a consistent axial light intensity distribution in a wide range near the focal plane,so as to realize the design of long focal depth.After theoretical simulation and experiment with the corresponding physical zone plates,the feasibility of this design to obtain vortex beam with long focal depth is verified.