【作者】 刘涛；

【导师】 崔庆丰；

【作者基本信息】 长春理工大学 ， 光学工程， 2013， 博士

【摘要】 与传统的以光的折射和反射原理为基础的光学元件相比,衍射光学元件具有质轻体薄、设计灵活、并且对入射光波可以自由调制等优点。通过折衍混合的方式,可以在提高光学系统成像质量的同时,简化系统结构,减轻重量,缩小体积。对于致冷红外光学系统,衍射元件在简化系统的同时,从致冷红外探测器上发出的冷光线会被衍射表面反射而发生多级衍射。本文首次系统地研究了致冷红外光学系统中单层和多层衍射表面的冷反射特性,提出了红外系统中衍射光学元件冷反射分析的概念、理论和方法,给出了衍射冷反射的评价标准。本文的研究方法及结论对于衍射表面冷反射的分析具有重要的理论意义和实用价值。以光波的标量衍射理论为基础,本文系统地研究了单层衍射光学元件和多层衍射光学元件的衍射特性,分析了单层衍射光学元件的色散特性,部分色散特性与温度稳定特性,研究了单层衍射光学元件的消色差、复消色差和消热差理论。通过将单层衍射元件和多层衍射元件的衍射表面用二元光学元件的台阶表面进行模拟,计算出斜入射的光线在衍射表面产生的位相延迟,进而分析了单层和多层衍射元件的衍射效率与入射角度和入射波长变化的关系。多层衍射光学元件增加了衍射光学设计的自由变量,通过不同材料的配合,可以设计出不同衍射表面微结构的高度,以提高成像波段范围内入射光波的衍射效率,解决单层衍射元件衍射效率随着入射波长对中心波长的偏离而下降的问题。但是多层衍射光学元件的衍射效率仅在一定角度范围内对入射角的变化不敏感,当入射角度增加,衍射效率会迅速降低,增加了对入射角度的灵敏度。此外本文对多层衍射光学元件在不同温度变化下的成像特性进行研究,分析了衍射离焦量与衍射元件材料热膨胀系数的关系。对于致冷红外光学系统,可以把衍射光学元件的消色差和消热差特性等应用到光学系统优化设计中。但是致冷探测器会通过前方光学系统的光学表面反射而探测到自身的冷像。当单层衍射元件应用于致冷红外光学系统时,从致冷的红外探测器上发出的冷光线逆向追迹到单层衍射元件的衍射表面,被反射的光线会由于衍射表面引入的相位延迟而发生衍射。衍射的冷反射光线在整个设计波段内会发生多级衍射,从探测器发射出的冷光的能量也分配在不同的衍射级次中,且不同衍射级次的冷光线成像到致冷探测器上形成冷像的大小尺寸也各不相同。这样,在进行冷反射分析时,首先要进行衍射效率分析,然后针对各衍射级次进行光线追迹,进而分析衍射表面的冷反射特性。与单层衍射元件相比,多层衍射元件需要分析多个衍射表面的衍射冷反射。对双层衍射元件进行冷反射特性研究的结果表明,双层衍射表面的冷反射与单层衍射表面冷反射的最大不同在于,被双层衍射表面第一个谐衍射表面反射的冷光线实际上经历了三次相位延迟,一次反射,两次透射。此外,相对单层衍射元件,多层衍射元件表面的微结构高度更高,造成衍射的冷反射光线在整个设计波段内会发生多级衍射。本文对衍射光线采用近轴光线追迹的方法,首先分析了从致冷探测器发出的冷光被衍射表面反射后衍射级的衍射效率,在此基础上定量分析了单层衍射表面以及多层衍射表面的冷反射,给出了修正后的衍射表面的近轴冷反射评价标准,为致冷红外光学系统中衍射表面的冷反射提供了一种全新的分析与评价方法。文章最后给出了三个红外光学系统设计实例,分别为含有单层衍射表面的中波折衍射混合红外光学系统,含有单层衍射表面的长波折衍射混合红外光学系统和含有多层衍射表面的长波折衍射混合红外光学系统。通过实例分析了多个含有衍射表面的红外系统中的冷反射,并给出了衍射表面的冷反射分析结果。更多还原

【Abstract】 Compared with the conventional optical element based on light refraction and reflection principle, the diffractive optical element has the characteristics of light, thin and flexible. And it can be used to modulat the incident light waves freely. With the combination of diffractive and refractive elements, the imaging quality of the optical system can be greatly improved. At the same time, the diffractive surfaces can be used to simplify the system structure, reduce weight and volume of the optical system. Except from simplification of the optical system, diffractive elements could reflect the cold rays emitted from the cooled infrared (IR) imaging sensor. There will be multi-order diffraction for the reflected light which is backward traced to the diffractive surface. In this dissertation, the narcissus phenomenon for single layer and multilayer diffractive elements in cooled IR imaging system were thorouly discussed. Based on the anaylisis, the concept, theory and discussion method of diffractive elements used in IR system are given, and the modified evaluation criteria for the narcissus effect of diffractive elements are presented. The analyze method and conclusion have both theoretical significance and practical utility.In this dissertation, based on light wave scalar diffraction theory, the diffraction characteristics for single-layer diffractive optical element and multilayer diffractive optical element have been systematically studied. The dispersion characteristics, partial dispersion characteristics and thermal characteristics of single-layer diffractive optical element have been analyzed. The achromatism, apochromatism and athermalization for single-layer optical elements have been discussed. Using multi-level binary optical surface model to simulate the diffractive surface of single-layer diffractive element and multilayer diffractive elements, the phase delay for oblique incident light were deduced. Besides, the relationship between diffraction efficiency and both the changes of incident angle and the incident wavelength were analyzed for single-layer and multilayer diffractive element. Compared with the single-layer diffractive optical element, the multilayer diffractive optical element has more variables for the optical design of diffractive element. Through the mating of different materials, and optimum design of the height of the diffractive structure for the diffractive optical element, the diffraction efficiency of the incident light in the imaging wavelength range can be improved. But analysis showed that the diffraction efficiency of the two-layer diffractive optical element is relatively high only in a certain incident angular range. When the incident angle continues to increase, the diffraction efficiency decreased rapidly with the increasing angle of incidence. In addition, through the research of the imaging properties of the multilayer diffractive optical element in different temperatures, it can be found that the changes of the height of the diffractive structure and the refractive index for the diffractive optical element are relatively small, which means that the thermal influence for diffraction efficiency of multilayer diffractive element is insensitive. Besides the relationship between the defocus amount of the diffraction element and the thermal expansion coefficients of the lens materials is analysed in this dissertation.As for the cooled IR imaging system, diffractive optical elements, with the characteristics of achromatism, apochromatism and athermalization, could be used in the optimization process of optical system. The narcissus phenomenon is a well-known problem for cooled IR imaging systems, which is generated by the cooled detector detecting its own image through the reflection of the optical system surfaces. When there are diffractive surfaces, the narcissus effect is different from the transmitted and reflected radiation. And due to the phase delay introduced by the diffractive surface, there will be diffraction for the reflected light which is backward traced to the optical surface. The energy of the cold rays through the design wave band would be distributed in different diffraction orders. And the cold image of different diffract order would be different too. Thereby the narcissus calculation must be considered with specific orders of diffraction based on their diffraction efficiencies.Compared with single-layer diffractive element, as for the multilayer diffractive element, it is necessary to analyze the cold reflection caused by all the diffraction surfaces. The narcissus influence analysis showed that the cold reflection of the multilayer diffractive element is more complicated. Taking the two layer diffractive element as an example, the cold rays reflected by the front diffractive surfaces actually experienced three times of phase delay, one reflection and two times of transmission. For the multilayer diffractive element, the height of the diffractive structure is relatively big, which means that there will be more diffractive orders throughout the design waveband for the cold rays that reflected by the diffractive surfaces.Paraxial ray tracing analysis of the diffracted light emitted from the cold detector is used in this dissertation. Considering the diffraction efficiency, the narcissus influence of single layer diffractive surface and multilayer diffractive surfaces are discussed. The modified evaluation criteria for the narcissus effect of diffractive surfaces are given, which provides a new evaluation method for narcissus in cooled IR optical system.At the last chapter of this paper, three practical cooled LWIR(Longwave Infrared) imaging optical system containing diffractive elements are given, one MWIR(Midwave Infrared) optical system containing single layer diffractive elements, one IR optical system containing single layer diffractive elements, and one IR optical system containing one two-layer diffractive element. These examples showed the analysis results of the narcissus phenomenon for diffractive elements in cooled IR optical system.更多还原