With advancements in technology, the miniaturization and lightweight design of imaging systems have become inevitable trends. Traditional infrared systems often rely on refractive, reflective, or catadioptric structures, which involve numerous optical components, resulting in larger volumes and more complex configurations. While current infrared cooled optical systems utilize mirrors and other optical elements to fold the optical path, thereby improving performance without increasing system length, miniaturized designs still require multiple optical elements and complex surface profiles. Single-element imaging technologies, such as metasurfaces or diffractive lenses, are not yet applicable to cooled infrared systems. Additionally, the placement of the cold stop in traditional systems complicates the optical structure and introduces narcissus effects that degrade imaging quality.

To address these challenges, this paper proposes a cooled monolithic annular aperture folded imaging (AAFI) optical system. This design leverages the unique structure of annular aperture systems, which integrate multiple concentric reflective rings on a single substrate. Light enters through the outermost annular aperture and is focused onto the central image plane via concentric reflective rings. The system adopts a four-reflection configuration to optimize performance while minimizing complexity.

The annular aperture system is characterized by its integrated design, where multiple concentric reflective rings are mounted on a single substrate. Light enters through the outermost annular aperture and is focused onto the central image plane via a series of reflective rings. In cooled AAFI systems, the cold stop inherently blocks marginal rays, necessitating coordinated optimization of key parameters such as back focal length (L_cold), cold stop diameter (d_cold), system focal length (f), and entrance pupil diameter (D). These parameters—back focal length, obscuration ratio, and entrance pupil diameter—are critical to designing efficient cooled AAFI systems. This paper derives quantitative relationships among these parameters (Fig. 2) and proposes a design method for cooled infrared AAFI systems.

Stray light, a significant threat to imaging fidelity in compact optical systems, is addressed through a specialized baffle structure (Fig. 3). Designed based on the geometry of the annular aperture lens and the distribution of stray light rays, this structure effectively mitigates stray light interference.

The finalized imaging system features an outer diameter of 51 mm, an effective clear aperture of 38 mm, a focal length of 102 mm, and a total optical element length of 11.2 mm, resulting in a ratio of total optical length to system focal length of 0.11. Detailed parameters are provided in Tables 2 and 3, with the optical layout illustrated in Fig. 5. Analysis confirms that the absolute YNI values for all surfaces exceed 1, effectively suppressing narcissus energy (Table 4). Optical distortion remains below 1.5%, and the modulation transfer function (MTF) curves exceed 0.27 at 21 lp/mm across all fields of view (Fig. 7). The longitudinal chromatic focal shift curve (Fig. 9) shows a focal plane shift of 16.4 μm across the design wavelength range, which is negligible in practical imaging, confirming minimal chromatic aberration.

Given the sensitivity of annular aperture systems to ambient temperature variations, thermal performance analysis is critical to ensure imaging stability. The single-element design introduces defocus under temperature changes, which can be compensated mechanically. The linear relationship between detector compensation distance and temperature variation is shown in Fig. 10.

This paper proposes a cooled infrared monolithic AAFI optical system based on the unique structure of annular aperture imaging systems. Theoretical derivations establish relationships between key parameters such as entrance pupil diameter, back focal length, field of view, and baffle ratio. A design method for cooled infrared AAFI systems is presented, and an example system is designed and validated. Results demonstrate excellent imaging performance, with MTF curves exceeding 0.27 at 21 lp/mm across all fields of view within the 3.7?4.8 μm wavelength range. A baffle tailored to the structural characteristics of the annular aperture system is designed, and iterative optimization reduces the baffle length by 34%. Compared to the initial system without a baffle, the PST with a single-layer baffle decreases by 2?3 orders of magnitude in the 4°?24° range and by 1 order of magnitude in the 24°?30° range. The compact optical structure of the proposed system offers a novel approach for the integration and miniaturization of cooled infrared optical systems.