Cooled infrared detectors can identify targets in complex environments through the temperature difference between the target and the background environment. However, the actual performance of infrared detectors is easily affected by the environment and their stray radiation. Stray light can affect the signal-to-noise ratio (SNR) of the detectors and imaging quality. For a large-format infrared detector, the large size of the detector chip corresponds to a larger cold shield installation interface and a higher height of the cold shield, and then there will be some problems, such as an increase in the mass and volume of the detector and a larger temperature gradient of the cold shield, which pose challenges for large-format infrared detectors with high sensitivity. At present, relevant studies are all focused on the design of the baffles of cold shields, the judgment of the advantages and disadvantages of cold shields, and structure optimization methods for cold shields. There are few discussions on the design of the cold shield structure and the vent hole under different F numbers. This paper discusses the selection of cold shield height and structure for different F numbers and the optimization of the vent hole structure and proposes an optimal design idea for the cold shield structure of cooled infrared detectors.
The cold shield model of cooled infrared detectors is built by optical simulation software, and the stray light simulation analysis is carried out by using the forward ray tracing method. Combining the non-uniformity of focal plane responsivity and the maximum temperature gradient at different cold shield heights, the selection of the cold shield height at different F numbers is analyzed. Four cold shield structures with different appearance are proposed. The point source transmittance (PST) at different off-axis angles of light sources is calculated, and the selection of cold shield structures under different F numbers is analyzed. Three kinds of vent hole structures are proposed, and the effects of different vent hole structures on the heat radiation suppression inside cooled infrared detectors are analyzed.
As the cold shield height increases, the maximum response gradient of the focal plane under cold shields with different F-numbers does not decrease uniformly, and the cold shield with small F numbers has a larger clear aperture, thereby introducing more stray radiation to the focal plane. The overall response uniformity of the focal plane is reduced. For small-clear-aperture cold shields with large F numbers, stray radiation has less effect on focal plane uniformity. Considering that the maximum response gradient of the focal plane should not exceed 25%, and the maximum temperature gradient of cold shields needs to be lower than 5 K, 25 mm is selected as the design cold shield height [Fig. 3(a)]. The PST of the cold shields of the four structures is calculated. It is found that when the F number is small, the clear aperture of the cold shields is large. At this time, the stray light suppression ability of a cold shield is determined by its structure (such as side wall inclination and maximum outer diameter). When the F number is large, the clear aperture of the cold shield is relatively small, and the width of the baffles increases. In such a case, the effect of cold shield structure on the stray light suppression ability is weakened (Fig. 5). Combined with the PST curve, the selection of cold shield structures under different F numbers is obtained (Table 2). Three vent hole structures are designed, and the vent hole structure at the top of the cold shield increases the transmission paths of internal stray radiation. The countersunk-head vent hole and the stepped vent hole structures make the incident stray radiation absorbed by the baffles and at the same time increase the number of reflections for the stray radiation entering the vent hole to reach the front of the image. The improved three vent hole structures reduce the internal stray radiation of the detector which is received by the image plane by about 87% compared with those before improvement (Fig. 7).
The optimal design analysis of the cold shield structure of cooled infrared detectors is carried out. Considering the non-uniformity of the focal plane responsivity and the maximum temperature difference of cold shields, this paper proposes design ideas of cold shields for large-format infrared detectors on the basis of the investigation of the stray light suppression effect under different F numbers and cold shield structures and the optimization direction of the vent hole position. The focal plane of 1280×1024 (pixel spacing is 10 μm) is taken as an example. At different stages of increasing the cold shield height, the focal plane uniformity exhibits different states (increased, decreased, and unchanged) for the cold shields with different F numbers due to the different influences of stray radiation caused by the clear aperture. The cold shield height is taken as 25 mm under the condition that the maximum response gradient of the focal plane does not exceed 25% and the maximum temperature difference requirement of cold shields is considered. Cold shields of four structures are designed, and their PST curves at F1.2, F2, F3, F4, and F5.5 are calculated. In addition, the stray light suppression effect and scheme of each cold shield structure at different F numbers are analyzed. Three optimization schemes are proposed for the vent hole position, which can completely suppress the stray radiation of the window frame from passing through the vent hole to the image plane. With the optimized vent hole structure, the stray radiation inside the detector which is received by the image plane is reduced by about 87% compared to that before optimization. The design idea proposed in this paper has guiding significance for the optimal design of the cold shield structure of cooled infrared detectors.
