Athermalization design of short-to-mid-wave infrared broadband interferometric hyperspectral imaging optical system

ObjectiveFourier transform infrared hyperspectral imaging technology has gained extensive application across various fields and scenarios, including environmental monitoring, military reconnaissance, industrial inspection, and scientific research, owing to its unique spectral resolution and throughput advantages. Hyperspectral imaging instruments with broad detection spectral bands will significantly enhance the breadth of target identification and classification, while a wide temperature adaptability design will improve stability and reliability in complex temperature environments. Currently, the designed spectral range of most infrared hyperspectral imaging systems falls within the mid-wave infrared (3-5 μm) and long-wave infrared (8-12 μm) regions. Except for reports from Canadian companies ABB Bomem and Telops on hyperspectral imaging instruments covering the 1.5-5 μm band, there have been no reports in China on the design and research of short-wave to mid-wave broadband infrared imaging systems. Particularly in the design of Fourier transform infrared hyperspectral imaging systems, a short-wave to mid-wave broadband range can expand the detection breadth of target characteristic spectral peaks, improve identification accuracy, and enhance analytical depth. Research on short-wave to mid-wave infrared imaging system design is of great significance. It is necessary to conduct research on the design of a highly environmentally adaptive, steady-state short-wave to mid-wave infrared hyperspectral imaging system. Therefore, this paper presents the design of a spatially interferometric modulated hyperspectral imaging system with wide temperature adaptability for the short-wave to mid-wave infrared band.MethodsThis paper presents the design of a hyperspectral imaging system based on stepped multi-order spatial interference modulation. The principle and layout of the paraxial thin-lens optical path are shown in Figure 1. This system is a secondary imaging configuration, comprising a telescope group and a relay lens group. To compensate for the high-order astigmatism introduced by the beam splitter and compensator plate, and to solve the issue of sagittal and tangential image plane defocus, Toroidal surfaces were incorporated into the telescope imaging group and the relay imaging group, respectively. To enhance the degrees of freedom for optical alignment, the Toroidal surfaces were designed to be located on the sixth optical surface of the telescope imaging system and the first optical surface of the relay imaging system. To improve lightweight performance and optical transmittance, and to achieve aberration correction with the minimum number of lenses, the design realized a large linear field of view, object-space telecentricity, and low distortion for the relay imaging system. High-order aspheric surfaces were introduced during the optimization process. Athermalization design across a wide temperature range was achieved through multi-configuration simultaneous least-squares optimization. The optical path layouts of the designed system are shown in Figures 2, 4, and 5.Results and DiscussionsThe telescope group was optimized to be image-space telecentric, while the relay lens group was designed to be object-space telecentric, achieving a perfect match of dual high telecentricity in the optical system. The relay lens group realized cold stop matching design. The overall system operates in the short-to-mid-wave infrared band of 1.5-5 μm, with a static modulation transfer function (MTF) exceeding 0.55@34 lp/mm at the cutoff frequency. The system distortion is less than 0.23%, and the RMS radius of the imaging spot is smaller than 6.5 μm, with a diameter less than the single pixel size of 15 μm. All imaging quality metrics meet the design requirements, as shown in Figure 6. When the lens mounts and barrel structures are made of aluminum with a linear expansion coefficient of 23.6×10−6/K, the MTF performance after athermalization within the temperature range of −20 ℃ to 60 ℃ is illustrated in Figure 7. The MTF@34 lp/mm at the cutoff frequency remains above 0.5. A focusing mechanism was designed for the front lens group to ensure that the MTF@34 lp/mm exceeds 0.55 across the depth of field from 20 meters to infinity. Additionally, stray radiation analysis of the infrared optical system showed that the total power of narcissus reflection on the detection focal plane is 5.07×10−6 W, with a peak irradiance of 2.34×10−4 W/cm2. Tolerance analysis indicated that the system possesses high manufacturability and assemblability. Imaging detection experiments on target scenes at a distance of 2 km using an engineering prototype demonstrated excellent image quality, with highly clear detailed information.ConclusionsA broadband athermalized Fourier transform infrared hyperspectral imaging system was designed, operating in the spectral range of 1.5 μm to 5 μm and within a temperature range of −20 ℃ to 60 ℃. This spatially modulated infrared hyperspectral imaging system exhibits high MTF, with characteristics such as broad bandwidth, large depth of field, excellent environmental adaptability, high stability, and clear imaging. Design and optimization using Zemax software demonstrated that the achromatic design across the broad spectrum and the athermalization over a wide temperature range achieve an MTF above 0.5 at the cutoff frequency under all imaging conditions. Narcissus analysis verified the rationality of the curvature design for each lens surface. Tolerance analysis determined the acceptable limits for manufacturing and alignment errors of the optical system. After assembling the prototype, imaging tests of distant scenes confirmed excellent image quality.