| A new type of refractive-diffractive hybrid optical system consisting of diffractive optical elements and refractive elements can solve some challenging problems in imaging optical systems.The multilayer diffractive optical element(MLDOE)is a new type of diffractive optical element.The design of the conventional MLDOE is based on the assumption that the ambient temperature is constant at 20 °C and that light is incident on the surface of the diffractive optical element vertically.When the ambient temperature deviates from normal temperature or the incident angle increases,the diffraction efficiency of the MLDOE decreases.This dissertation studies the changes in the diffraction efficiency of MLDOE under wide ambient temperature and oblique incidence of light.It is found that the diffraction efficiencies of the MLDOEs that composed of different substrate materials are greatly affected by ambient temperature or oblique incidence angle.For a substrate material combination of MLDOE that is greatly affected by ambient temperature or oblique incident angle,when the ambient temperature changes or the oblique incident angle becomes large,the diffraction efficiency decreases rapidly or even becomes unusable.Therefore,it is very important to choose a proper substrate material combination for MLDOE.This dissertation proposes a temperature-substrate material optimization selection method and an incident angle-substrate material optimization selection method.By these methods,the optimal substrate material combinations of MLDOEs suitable for a wide temperature range or a large incidence angle range can be obtained.For a specific substrate material combination,the temperature-microstructure height optimization method is given,which effectively improves the diffraction efficiency of MLDOEs in a wide temperature range.The research methods and conclusions in this dissertation have important guiding effects on the design and image quality evaluation of the refractive-diffractive hybrid optical system in a wide temperature range or a large incident angle.Based on the scalar diffraction theory,this dissertation first studies the diffraction efficiency and the polychromatic integral diffraction efficiency(PIDE)of various DOEs.Based on the phase delay expression,the diffraction efficiency of traditional multi-stage phase grating is analyzed,and the diffraction efficiency and PIDE of continuous-surface single-layer DOE and MLDOE are also discussed.In addition,in order to correct the chromatic aberration of the ultra-thin imaging system with annular aperture,the formation of diffractive microstructures on the reflective surface is an effective solution.This dissertation presents the concept and design method of reflective imaging diffractive optical elements(RDOE).Based on the scalar diffraction theoretical model,the microstructure height expression,diffraction efficiency expression and PIDE of RDOE are given.The application of RDOE in ultra-thin imaging system with annular aperture is described in detail.The influence of ambient temperature change on the diffraction efficiency of MLDOE cannot be ignored.Therefore,a temperature-substrate material selection method is proposed in this dissertation,and the expression of the microstructure heights of MLDOE with temperature characteristics is derived.Through comparison and analysis of the difference in PIDE for different combinations of substrate materials,the substrate material combination that are less affected by ambient temperature is found to improve the diffraction efficiency of the MLDOE over a wide temperature range.This method effectively broadens the temperature application range of MLDOE,avoids the problem that the diffraction efficiency of MLDOE rapidly decreases with ambient temperature due to improper selection of the substrate materials.It provides theoretical guidance for the application of MLDOEs in athermalization system.For the selected substrate material combination of MLDOE,the temperaturemicrostructure height optimization method is proposed.By studying the effect of ambient temperature changes on the diffraction efficiency of MLDOE and formulating reasonable boundary conditions,a design wavelength combination that can increase the PIDE over the entire temperature range is selected,and further,the optimal design of the microstructure heights can be obtained.This method can realize the high diffraction efficiency design in a wide temperature range of the MLDOE.Because the large incidence angle will cause the decrease of the diffraction efficiency,this paper proposes an incidence angle-substrate material optimization selection method suitable for single-band and dual-band MLDOE.The expression of the microstructure height of the MLDOE with incident angle is derived in this paper.By comparing and analyzing the difference of the PIDE of different substrate material combinations,the optimal one suitable for a large incidence angle range is found.The incident angle-substrate material optimization selection method is proposed,which broadens the applicable range of MLDOE in the case of oblique incidence.This provides theoretical guidance for the design of refractive-diffractive hybrid optical systems at large incident angles.Finally,based on the proposed temperature-substrate material selection method and temperature-microstructure height optimization method in this dissertation,two refractive-diffractive hybrid athermalization optical systems in a wide temperature range are designed,and detailed optimization design processes are given.The design results considering the influence of diffraction efficiency are analyzed,and the effectiveness of these methods are verified.Based on the proposed incidence angle-substrate material optimization selection method,a refractive-diffractive hybrid zoom optical system with the midwave-longwave infrared dual-band is designed.The design process of the system is also given in this paper. |
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