Research On The Focusing Characteristics Of Spiral Zone Plates Based On Fractal Structure

Vortex beam refers to a type of beam that carries orbital angular momentum and has a spiral phase wavefront structure.The center of the vortex beam is a dark core,and the phase here is variable and uncertain.Therefore,these unique properties make the vortex beam widely used in many fields,such as particle manipulation,micro-nano processing,optical communication and other fields.In recent years,researchers have discovered a variety of methods for generating vortex beams,including: computer generated hologram method,geometric mode conversion method,spiral phase plate method,spatial light modulator method.As scientific researchers continue to study vortex beams,some new vortex beams with strange qualities have been proposed one after another.In this paper,the diffraction characteristics of the spiral zone plates with fractal structure are studied in detail.The main contents of the full text are summarized as the following three points:1.Frist,the research background and significance of vortex beams,high-order diffraction phenomena,and large depth of focus diffractive optical elements are introduced.Several methods for generating vortex beams that are commonly used at present are briefly introduced.Then the basic principle of vortex beam and the theory of scalar diffraction and the focusing characteristics of Fresnel zone plate are introduced in detail.Subsequently,we introduced the design principles and diffraction characteristics of spiral zone plate,single focus spiral zone plate and fractal spiral zone plate.2.Cantor sequence spiral zone plate,based on the fractal spiral zone plate,modifies the sinusoidal apodization window by using two modulation parameters along the azimuth direction to eliminate higher harmonics.This article introduces the Cantor sequence spiral zone plate in detail.From the principle design,diffraction properties to the experimental verification process.The focusing characteristics of the Cantor sequence spiral zone plate are analyzed to prove that the experimental results are basically consistent with the theoretical simulation results.This type of zone plate provides potential for the realization of three-dimensional optical manipulation,optical communication,and lithography.3.The m-bonacci spiral zone plate,combined with the m-bonacci fractal structure on the basis of the spiral zone plate is used to generate two focused optical vortices of equal intensity along the optical axis.First,the theoretical design of the m-bonacci spiral zone plate is introduced,and analysis is performed in simulation and experimental verification.Simulation results and experimental results show that the m-bonacci spiral zone plate can produce two focal points of equal topological charge and equal intensity vortex.