• Acta Optica Sinica
  • Vol. 45, Issue 12, 1222002 (2025)
Tingcheng Zhang*, Xu Yan, Xiaolin Liu, and Zheng Wang
Author Affiliations
  • Beijing Institute of Space Mechanics and Electricity, Beijing 100190, China
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    DOI: 10.3788/AOS250722 Cite this Article Set citation alerts
    Tingcheng Zhang, Xu Yan, Xiaolin Liu, Zheng Wang. Design of Cooled Long-Wave Infrared Continuous Zoom Optical System Followed by Thermal Stray Light Analysis[J]. Acta Optica Sinica, 2025, 45(12): 1222002 Copy Citation Text show less

    Abstract

    Objective

    Infrared optical systems possess excellent penetrability and offer advantages in applications such as target tracking. Particularly, cooled long-wave infrared optical systems exhibit superior transmittance and detection performance, enabling all-weather operations. Combined with continuous zoom functionality, these systems can seamlessly transition from wide-field-of-view search to narrow-field-of-view detailed inspection while maintaining image stability and clarity. As a result, cooled long-wave infrared optical systems are widely used in fields such as coastal defense and ground-based air defense. To adapt to detectors with smaller pixels and avoid the use of binary surfaces, while also achieving a larger zoom ratio, the general rules of power distribution and the conditions for cold aperture matching based on the theory of mechanical compensation zoom are discussed. A method is proposed for rapidly obtaining initial structures by studying the Gaussian layout of cooled long-wave infrared optical systems, which can significantly improve optical design efficiency. Additionally, the irradiance values of thermal stray light introduced onto the image plane by the system itself are calculated. This effectively shifts the subsequent stray light analysis to the optical system design stage, allowing for the anticipation and avoidance of potential risks.

    Methods

    By integrating the rear fixed group into the secondary imaging system, the structure of the cooled long-wave infrared optical system is simplified while still achieving 100% cold shield efficiency and avoiding system vignetting. Based on the theory of mechanical compensation zoom systems, we propose a design method for rapid zooming to meet the requirement of a large zoom ratio. The core of this method is the allocation of optical power among different lens groups and the smooth zoom transition, which can be achieved by solving a quadratic equation. The quantitative relationships are provided between the first-order parameters and the general principles for determining these parameters. By tracing ideal rays, we study Gaussian structure layouts, which can quickly and in real-time verify the rationality of the solved first-order parameters, greatly improving design efficiency. Considering the inherent thermal stray light problem in long-wave infrared systems, we establish a linear mapping relationship between the surface temperature of the optical system and the irradiance on the image plane from the same surface. When the optical surface temperature changes, new irradiance values can be obtained without performing a new round of ray tracing. Moreover, this method of thermal stray light analysis can also be extended to the thermal stray light analysis of general optomechanical systems.

    Results and Discussions

    With the reasonable allocation of optical power and research on Gaussian structure layouts, the initial structure can be obtained immediately. Further optimization by implementing the CODE V software results in an optical system that meets all the technical requirements. The materials chosen for the lenses are Germ, ZnS, Germ, Germ, ZnS, and GaAs (Table 2), and the introduction of sulfur glass helps control chromatic aberrations. In this optical system, four even-ordered aspherical surfaces are employed (Table 3), with the remaining surfaces being standard spherical surfaces. The modulation transfer function (MTF) at all focal lengths is close to the diffraction limit (Fig. 8). The root mean square (RMS) diameter is within the pixel size at all focal lengths for all fields of view (Fig. 9). Meanwhile, the distortion is also well corrected, with a maximum value of 1.66% (Fig. 10). Considering the needs of manufacturing and alignment, the zoom cam has been optimized to ensure that the rise angles of the two zoom paths can be controlled between 5.19° and 30.47° (Fig. 7). The design example is a cooled long-wave infrared continuous zoom optical system with a magnification ratio of 20, an image plane size of 9.60 mm×7.68 mm, a maximum focal length of 400 mm, a constant F# of 3, and distortion and chromatic aberrations corrected, which provides new ideas and insights for the design of such optical systems. The results of the thermal stray light analysis indicate that zooming does not significantly affect the irradiance on the image plane.

    Conclusions

    For the cooled long-wave 640 pixel×512 pixel infrared detector with pixel dimensions of 15 μm×15 μm, a cooled long-wave infrared zoom optical system with a smooth zoom transition has been designed to meet the need for reducing axial dimensions. The system achieves MTF values close to the diffraction limit at all focal lengths, which indicates excellent image sharpness. It features a compact structure, minimal aberrations, and high overall imaging quality. The system achieves a zoom ratio of 20, a large relative aperture (with a constant F# of 3), a short and smooth zoom curve, and excellent correction of various aberrations. Additionally, the irradiance values formed on the image plane by the thermal stray light of the optical system itself have been calculated, which can serve as supplementary data for subsequent calculations of dynamic range, contrast, and other parameters. The optical system designed in this paper can be applied in fields such as surveillance, reconnaissance, and air defense.

    Tingcheng Zhang, Xu Yan, Xiaolin Liu, Zheng Wang. Design of Cooled Long-Wave Infrared Continuous Zoom Optical System Followed by Thermal Stray Light Analysis[J]. Acta Optica Sinica, 2025, 45(12): 1222002
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