Infrared remote sensing is widely used for Earth and Ocean observations. With the development of optical technology, infrared detection systems are developing toward higher resolution, hyperspectrum, and higher sensitivity. With the increasing scale of the detector array, the working wavelength also covers long waves. To ensure working performance, an infrared Dewar module must be more strictly designed to suppress stray radiation. On the one hand, to suppress stray radiation, the Dewar window adopts low-temperature optical technology. On the other hand, the Dewar window weight increases with array size, suggesting higher requirements for the supporting strength and heat insulation of the shell. In addition, the Dewar module goes through three working states, from assembly to application, and the window deformations in these three working states are different, affecting the design of the infrared optical system.
The temperature field of the large-aperture infrared and long-wave Dewar window was analyzed by finite element analysis and verified by experiments. The influence of the temperature of the Dewar window on the stray light and heat radiation of the cold screen was clarified. To reduce heat leakage between the low-temperature window and the flange surface, the strength and heat insulation performance of the three types of shell support materials were compared. The effects of force, heat, and force-heat coupling on the design parameters of the Dewar window, such as the thickness and aperture margin, were analyzed. The deformation of the Dewar window was fitted using a Zernike polynomial, and the modulation transfer function (MTF) and wave aberration were used as evaluation indices to control window deformation of the large-aperture long-wave infrared Dewar module. Subsequently, the influence of Dewar window deformation on the imaging quality of the infrared camera system under three conditions was analyzed.
When the window temperature decreases to 200 K, the window stray light and cold screen radiation are well suppressed (Fig. 3). When the target temperature is 220 K, the window radiation spurious ratio is less than 10%, satisfying the design requirements (Fig. 2). To reduce the heat leakage caused by the low-temperature window and ensure reliable use, a titanium alloy shell with higher strength and lower heat conductivity is adopted. Under the same temperature difference (300-250 K), the heat conduction of the titanium alloy shell is 278 mW; this is 49% and 43% lower than those of Kovar alloy (4J29) and stainless steel (304L), respectively (Fig. 4, Table 2). In mechanical-heat coupling, the window deformation caused by the heat load is dominant (Figs. 5 and 7), and increasing the window aperture is helpful to improve the window surface shape under mechanical-heat coupling (Fig. 8). When the thickness and aperture of the window are 4 mm and 8 mm, respectively, the influence of window deformation on the imaging quality can be neglected under the three conditions (Tables 4 and 5).
The application of a large-aperture long-wave Dewar window is studied, and the operating temperature of the window is determined according to the design requirements for stray light in engineering projects. Combined with the working temperature of the window, the deformation under the coupling of force and heat is analyzed, and the window design is optimized. Combined with the optical imaging design, the MTF and wave aberration are analyzed, and the influence of Dewar window deformation on the detector imaging quality under the three conditions is confirmed. This study provides a reference for optical system optimization and heat control-related designs.
