Studies On Modified Scalar Diffraction Theory Of Multilayer Diffractive Optical Elements

There are huge potential application values to multi-layer diffractive optical elements(MLDOEs)in the field of hybrid refractive/diffractive optical systems.MLDOEs can reach high diffraction efficiency in the wide waveband,and prior to the single-layer diffractive optical elements.In addition,MLDOEs brings new design freedom to the optical systems,which can break through the limitations of traditional optical systems.It means a lot to optical performance improvement,structural simplification,and realize the modern optical system of lightweight,integration,miniaturization.The paper aimed to modify the analysis model of MLDOEs,and MLDOEs optimization design,model of MLDOEs with oblique incidence,fabrication error analysis and fabrication method are also included.Through the study of diffraction theory and relative work,it can be found that the scalar diffractive theory(SDT)is widely used in the research of MLDOEs.However,the validity of SDT to MLDOEs is unclear,it makes part of MLDOEs designed with SDT cannot reach the designed performance.It means that the MLDOEs analysis model of SDT is not perfect.In this thesis,inspired by the research of the existing SDT analysis method,combined with the verification of the vector diffraction theory,a more accurate analysis method of MLDOEs was established.This method will broaden the application range of the MLDOEs.The study theoretically improves the existing design method of MLDOEs and can be used to provide assistance to the design and analysis of MLDOEs.The research results can be commonly used in the design of hybrid refractive/diffractive optical systems with MLDOEs.The development history and research status of single-layer diffractive optical element,harmonic diffractive optical element and MLDOEs were reviewed in the paper,and the basic characteristics of diffraction optical element,processing and testing technology,as well as the basic theory of diffraction optics were introduced;Single-layer diffractive optical elements is analyzed with the SDT,extension scalar diffraction theory and vector diffraction theory,limitations of the three diffraction theory and their respective applicable scope were also studied.Based on SDT and vector diffraction theory,the diffractive characteristics of MLDOEs with different period width and microstructure height were studied.The limitation of SDT to MLDOEs was given through the two methods comparison.Then an analysis method based on the shielding effect of MLDOEs was established,which is called the effective area method(EAM).The method can calculate the effective period width and effective height of MLDOEs through comparing the actual wavefront and ideal wavefront under the conditions of oblique incident,wavelength change and fabrication error.Then a more accurate diffraction efficiency of MLDOEs can be obtained.According to the design principle of Polychromatic Integral Diffraction Efficiency(PIDE)maximization,the optimal design process and results of MLDOEs with different period width were obtained based on the mathematical model of the relationship between diffraction efficiency,microstructure height and period width.Besides,the valid of EAM were checked by the vector diffraction theory.According to the analysis method of EAM,the incident angle is introduced into the analysis model of MLDOEs.The effect of incident angle on diffraction efficiency was analyzed and the general expression of diffraction efficiency was obtained.The mathematical model among diffraction efficiency,incidence angle,period width and microstructure height was established for the first time.In addition,limitations of SDT were studied when the incidence angle is large.In addition,effect of fabrication errors on diffraction efficiency of MLDOEs was analyzed based on the EAM.The limitations of SDT were analyzed when MLDOEs owns fabrication errors by comparing those two methods.Those results have been proved by the vector diffraction theory.For the passive facet of MLDOEs,the relationship between diffraction efficiency and passive facet errors was given for the first time.Research of microstructure height errors,period width errors and passive facet errors provide theory foundations of MLDOEs' tolerance distribution,and means a lot to fabrication quality improvements and cost reduction.Based on single point diamond ultra-precision machining technology,effect of diamond tool shape,feed rate and tool radius on the shape and roughness of MLDOEs were analyzed through EAM.The relationship between fabrication parameters and optical performance was established.Those consequences can be used to guide the fabrication parameters rapid choice and achieve better processing quality.