In recent years, with increasing demands for imaging optical systems and continuous advancements in manufacturing technologies, dual-band infrared zoom optical systems have been widely applied in various fields such as military reconnaissance, precision guidance, airborne electro-optical pods, aerospace, security, and night vision surveillance. Continuous zoom infrared lenses enable target searching over a wide field of view and rapid switching to a narrow field of view for precise tracking and observation. This capability is particularly advantageous for tracking high-speed moving targets, addressing the issue of losing targets during field-of-view switching in stepped zoom lenses. However, dual-band infrared zoom systems face challenges such as complex structures, difficult aberration correction, suboptimal image quality, and limited material selection. Therefore, developing a high-performance dual-band infrared zoom optical system is crucial for advancing imaging optical systems.

First, in the dual-band infrared range of 3.7?4.8 μm and 8?10 μm, a double-layer diffractive optical element (DOE) is designed using a method that maximizes polychromatic integrated diffraction efficiency (PIDE) considering angular-wavelength characteristics. The wavelengths corresponding to the maximum PIDE values are selected as the design wavelengths, and the corresponding microstructure height parameters are calculated. The designed double-layer DOE achieves a comprehensive PIDE of 98.29%, demonstrating high diffraction efficiency across the dual-band infrared range. Second, a dual-group linked zoom model is derived. Using this model, the component distances at different focal lengths are calculated. These parameters are then input into optical design software to develop a dual-band infrared zoom optical system with a continuous zoom range of 30?300 mm. However, the initial system exhibits suboptimal image quality. Finally, the high-efficiency double-layer DOE is integrated into the zoom system, significantly improving image quality. This method offers a feasible solution for enhancing the image performance of zoom optical systems.

The double-layer DOE consists of a silicon first layer and a germanium second layer, separated by a 0.05 mm air gap. The microstructure heights in each layer are 270.00 μm and -205.61 μm, respectively. Under the dual-band infrared (3.7?4.8 μm and 8?10 μm), the optimal design wavelengths for maximizing PIDE are 4.18 μm and 8.67 μm (Fig. 3). Based on the dual-group linked zoom model, the component distances at different focal lengths are calculated (Table 3). Using this data, a cooled 10× zoom optical system is designed (Fig. 5). Throughout the zooming process, the modulation transfer function (MTF) values at a spatial frequency of 17 lp/mm remains above 0.5, and distortion stays consistently below 3% (Figs. 6 and 7). Finally, a comparison of spot sizes before and after the introduction of the double-layer DOE (Fig. 8) shows a significant reduction in spot size and a notable improvement in image quality.

The introduction of the double-layer DOE effectively addresses the challenges of structural complexity, difficult aberration correction, and suboptimal imaging quality in dual-band infrared zoom optical systems. By utilizing only two common infrared materials (silicon and germanium), an eight-lens, 10× zoom system with excellent imaging quality has been developed. This system adopts a dual-group linked zoom mechanism, enabling wide-field target searching and narrow-field target tracking. Moreover, the tolerance allocation is well-optimized, and fabrication errors in the double-layer DOE have minimal influences on diffraction efficiency, ensuring consistently high imaging quality. With its simple structure and ease of fabrication, this system has broad application potential in both military and civilian fields.