This paper discusses the optical design of an uncooled dual-band MWIR/LWIR optical system using a circular
unobscured three-mirror system which is particularly suitable for wide spectral range , large aperture and small volume
imaging systems. The system is designed at focal length 310mm, F-number 1.55 with field of view 1.77°×1.33°. A
coaxial three-mirror system is calculated by the paraxial matrix as a starting point. With the condition that the focal point
of each conic mirror is placed to coincide successively, elements in the system are tilted and decentered properly to make
the system unobscured and the mirrors are arranged to form a round configuration for compactness. The optical path is
folded inside the region surrounded by the mirrors. Zernike polynomial surfaces which are limited to be symmetric about
tangential plane are used to correct aberrations and to improve the image quality. The modulation transfer function of
this system is above 0.65 in MWIR band and above 0.5 in LWIR band all over the field of view at the Nyquist frequency
of 20 line pairs per millimeter. The result shows that the space can be utilized efficiently, the system is compact and
image quality is favorable.
In this paper, the effect of environmental temperature change on multilayer diffractive optical elements (MLDOEs) is evaluated from the viewpoint of the diffraction efficiency and the polychromatic integral diffraction efficiency (PIDE). As environmental temperature changes, the microstructure heights of MLDOEs expand or contract, and refractive indices of substrate materials also change. Based on the changes in microstructure height and substrate material index with environmental temperature, the theoretical relation between diffraction efficiency of MLDOEs and environmental
temperature is deduced. A practical 3-5μm Mid-wave infrared (MWIR) optical system designed with a MLDOE, which made of ZNSE and GE, is discussed to illustrate the influence of environmental temperature change. The result shows that diffraction efficiency reduction is no more than 85% and PIDE reduction is less than 50% when environmental
temperature ranges from -20°C to 60°C. According to the calculated diffraction efficiency in different environmental temperatures, the MTF of hybrid optical system is modified and the modified MTF curve is compared with the original MTF curve. Although the hybrid optical system achieved passive athermalization in above environmental temperature range, the modified MTF curve also remarkably decline in environmental temperature extremes after the consideration of diffraction efficiency change of MLDOE. It is indicated that the image quality of hybrid optical system with ZNSE-GE MLDOE is significantly sensitive to environmental temperature change. The analysis result can be used for optical engineering design with MLDOEs in MWIR.