For some infrared (IR) optical systems, due to the wide field of view of the instrument, some cryogenic optical lenses must be packaged near the detector, otherwise the entire optical system will be very complicated. Moreover, for weak signal and multi-spectral detection, it is necessary to reduce the background. Except IR detectors, if several cold filters and lenses are housed in Dewar, then it is conducive to eliminating the infrared radiation background, improving system sensitivity and integration. This paper presents the package of mid-wave infrared (MWIR) and long-wave infrared (LWIR) detectors Dewar with integrated cryogenic optics. The micron-scale alignment requirement of a 32×4 array detector with a pitch of 120 μm×120 μm and a dual-lens module at each band is comprehensively described. The key parameters such as detectors temperature uniformity, differential temperature packaging and low thermal mass are analyzed. We hope that Dewar package structure integrating 4 lenses, 2 optical filters and 2 detectors will be successfully developed.
First, several lenses and detectors are packaged, so the size of the Dewar cold platform is large, and the mass of the infrared detector Dewar cold finger and its top load reaches 364.7 g. If the acceleration in space application is 500 m/s2, the maximum stress at the root of the cold finger can be calculated to be 352.9 MPa. In order to ensure the reliability under environmental vibration, it is necessary to use a new titanium alloy TC4 as the cold finger material, which can not only ensure sufficient mechanical strength, but also effectively reduce Dewar thermal loss. Secondly, in order to solve the problems of large longitudinal and axial thermal resistance between the detector, the cold lenses and the cold filter, as well as the low temperature uniformity of the detector array, both low thermal resistance heat transfer and the structure to realize differential temperature of the detectors are required. The cold platform is designed as the shared base of alignment for MWIR and LWIR detectors and cryogenic optics lenses. The structures of the sapphire cold link of the LWIR and the titanium alloy heat insulation ring of the MWIR are shown in Fig. 4. Through the combination of cold platform, sapphire cold link and titanium alloy heat insulation ring, the cooling capacity from the tip of cooler is non-uniformly introduced to the detector and the cryogenic optics lenses and filters, and the single-point cooling capacity is effectively transferred to different temperature zones. Thirdly, considering the material matching and assembly thermal stress of the detector at low temperature, the material whose linear expansion coefficient at low temperature matches the detector, lens, and filter is selected as the supporting structure material for assembling the cryogenic optics modules. The thermal stress simulation analysis of the infrared detector and optics is carried out, and the analysis results are shown in Fig. 6. The maximum cold shrinkage stress of the detector LWIR HgCdTe material is 15.8 MPa when it works at a low temperature of 65 K. Such a stress is relatively low.
The selection of titanium alloy cold fingers can not only ensure the mechanical reliability, but also reduce the heat conduction between the top of the cold fingers and the environment. Measured under liquid nitrogen environment, the thermal loss of titanium alloy cold fingers is about 160 mW smaller than that of stainless steel cold fingers. The brazing of the titanium alloy cold finger and the Kovar cold platform is realized by Ag-based solder. The weld structure after multiple temperature cycles from 300 K to 77 K is normal. The photo of the brazing sample is shown in Fig. 7. The average thermal loss value of the four Dewar assemblies tested at 77 K is 818.8 mW. In the orbit application, when the LWIR detector works at 65 K and the temperature window of Dewar is 195 K, the thermal loss is about 620 mW. In experiment, the LWIR detector works at 65 K, and its temperature uniformity is 0.08 K. Meanwhile, the temperature of the MWIR detector is about 73 K, and its temperature uniformity is 0.36 K. The temperatures of the long-wave lens 2 and lens 3 are stabilized at about 68 K and 70.5 K, and the temperatures of the mid-wave lens 2 and lens 3 are stabilized at about 80.5 K and 81 K, as shown in Fig. 10. According to the experimental data in Fig. 10, the longitudinal thermal resistance of the MWIR and LWIR detector-optics is relatively large. There is a bit difference about the temperature between the actual MWIR detector and the designed one, which should be related to too many heat transfer interfaces, the contact thermal resistance controlled by the screw torque during installation, and the shape of special heat-insulating titanium alloy TC4 rings. The adjustment of axial distance and pitch is adopted by partially adding different polyimide shims with the thickness of 5?20 μm between the detector substrate and the cold shield, combined with assembly and testing by several instruments. Finally, the maximum measured value of alignment between the detector and the cryogenic optics lenses is 7.8 μm, and the axial center deviation value between the detectors is 14.3 μm.
This paper presents the MWIR and LWIR detectors Dewar assembly with integrated cryogenic optics. The new structures in Dewar such as monolithic cold platform and lens support (cold shield) with low thermal mass are designed to realize the alignment of single detector and its related cryogenic optics lenses, to solve the high-precision alignment between two detector-lens modules, and to solve new brazing processes of single high-strength cold finger and so on. This paper also proposes the key parameters of the Dewar such as detectors temperature uniformity, differential temperature packaging and low thermal mass. The thermal mass of Dewar at liquid nitrogen temperature is less than 0.85 W. The LWIR detector allows for focal plane array (FPA) operation at the temperature of 65 K, with a temperature uniformity of 0.08 K. Meanwhile, the MWIR detector is balanced at the temperature of 73 K, with a temperature uniformity of 0.36 K. The misalignment between the detector and the lenses is less than ±10 μm, and the misalignment between two detector-lens modules is less than ±15 μm. The Dewar integrating cryogenic optics has been testified by relevant environment reliability, and has been successfully applied to Geostationary Interferometric Infrared Sounder of the Fengyun-4 meteorological satellite.
