| 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. |
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