Infrared (IR) imaging technology has become a research hotspot in different countries because of its advantages such as not being limited by day and night, being able to work all day, strong ability to penetrate smoke, and good detection concealment. In recent years, with the development of high-performance and high-resolution large-array IR detector technologies and the requirements of remote observation tasks such as border and coastal defense, various advanced IR imaging systems have emerged. The IR continuous zoom optical system is widely used in military and civilian fields. It can search for targets with a large field of view and observe distant targets with high resolution. In order to improve the IR system's ability to identify distant targets at long focal lengths while ensuring target search with a large field of view at short focal lengths, it is hoped that IR zoom optical system has a longer focal length and large zoom ratio. However, the longer focal length makes the diameter of the zoom optical system increase sharply. In addition to the inherent secondary spectrum, a large number of chromatic aberrations and advanced spherical aberrations in optical systems with long focal lengths will be introduced, which makes it difficult to design mid-wave infrared (MWIR) continuous zoom system with a large zoom ratio. Some scholars have also carried out relevant research and design work, but at present, the long focal length of the MWIR zoom system is less than 1000 mm; the detector resolution is mostly 640×512, and the optical path structure of the MWIR zoom optical system is complex and large. It is hard to meet the urgent demand of the new generation of photoelectric pods for high-definition MWIR zoom imaging systems with compact sizes.

In order to realize a compact design of IR zoom lens with a large zoom ratio, we propose a design idea and method which adopt secondary imaging, positive group mechanical compensation of zoom lens, and smooth root replacement and introduce a warm shield by switching the rear group of the zoom lens to change F-Number of the optical system at long focal length. The optical path of the MWIR zoom lens is ingeniously folded by two mirrors. First, the IR zoom optical system adopts a kind of optical path structure form with intermediate image planes and uses the zoom differential equation to solve the initial structure of the zoom lens to meet the required zoom ratio (Fig. 1); second, pupil aberration, especially pupil coma, is controlled in the optimization of the optical system to minimize the diameter of the front group; third, the optical system adopts positive group compensation zoom lens. It has a negative zoom group and a positive compensation group. The magnification of the zoom group and compensation group at a certain focal length position during optimization is controlled to keep zoom group magnification and compensation group magnification at -1, so as to reduce zoom travel length and overall length of the MWIR zoom optical system as much as possible. Finally, two mirrors are cleverly used to fold the optical path, and by switching the rear group of the zoom lens, a warm shield is introduced to change F-number at a long focal length, which further reduces the diameter of the front group and keep IR zoom lens more compact.

Based on the proposed design method of a compact MRIR zoom lens, this paper uses a high-resolution MWIR-cooled detector with a resolution of 1280×1024. The pixel size is 15 μm, and an MWIR continuous zoom optical system with a zoom ratio of 48 times and focal length from 25 mm to 1200 mm has been designed (Figs. 2 and 3). While ensuring 100% efficiency of cold shield, the compact IR zoom lens is realized. The optical system has good imaging quality within the operating temperature range of -40-60 ℃ (Fig. 6), and the maximum optical diameter of the front group is 230 mm. The total optical length after folding is only 350 mm. This compact MWIR zoom optical system has many advantages, such as a compact structure, large zoom ratio, long focal length, high resolution, and good imaging quality, which can meet the requirements of the new generation of IR imaging systems (Fig. 9).

In this paper, an MWIR continuous zoom optical system with a large zoom ratio and long focal length is designed. The secondary imaging, positive group mechanical compensation, and smooth root replacement are used, and a warm shield by switching the rear group of the zoom lens is introduced to change F-Number of the optical system at a long focal length. The optical path is ingeniously folded by two mirrors, which realizes the compact and miniaturization design of the MWIR continuous zoom system with a focal length from 25 mm to 1200 mm. The MWIR continuous zoom lens has excellent imaging quality within the operating temperature range of -40-60 ℃. The optical diameter of the front group is 230 mm, and the overall length after folding is only 350 mm. The overall size of the zoom thermal imager based on this optical system is less than 360 mm (L)×238 mm (W)×290 mm (H). This compact MWIR zoom optical system has many advantages, such as a compact structure, large zoom ratio, long focal length, high resolution, and good imaging quality, which can be used in the new generation of high-performance photoelectric pods.