A Hybrid Refractive-diffractive Optical System Design for Mid-wave Infrared Interference Spectral Imaging

Mid-wave infrared interference spectral imagers possess unique advantages in gas detection and identification due to their wide characteristic spectral coverage， high-throughput detection capability， and multi-channel sampling. Particularly in daily inspection and early warning of chemical gases and rapid emergency response to leakage scenarios， they can provide early warnings of disasters at the initial stage of gas leakage， preventing the situation from escalating. After a gas leak occurs， without requiring personnel to enter the site， they can quickly identify the type of leaking gas and the distribution of its diffusion concentration， providing first-hand on-site data for the evaluation of emergency response plans. However， traditional point-scan and area-scan interference spectral imaging configurations face significant limitations in enhancing lightweight， compact design， and stability. It is of great significance to research and design interference spectral imaging instruments that meet the high demands for mobility， adaptability to complex environments， field of view coverage， and lightweight compactness in specific application scenarios. Additionally， as interference spectral imagers require high precision in detector pixel intensity detection， it is necessary to study the analysis and suppression methods of infrared stray radiation in complex scenarios.This paper proposes a spectral imaging system based on static interference modulation using a stepped micro-mirror. The optical configuration of the system is shown in Figure 1. The scanning telescope imaging lens assembly achieves wide-field one-time convergence imaging onto the stepped micro-mirror for interference modulation， and the image is then relayed onto the detection focal plane through the relay imaging lens group. A cylindrical design is introduced to compensate for the high-order astigmatism introduced by the interference beam-splitting system. Achromatic broadband design and athermalization across a wide temperature range are achieved through hybrid refractive-diffractive and aspheric designs， resulting in a static Modulation Transfer Function （MTF） better than 0.7 at the cutoff frequency. To analyze infrared stray radiation， a stray radiation model for the interference spectral imaging system was constructed. The materials and surface radiation scattering characteristics of the optomechanical structure were defined， and ray tracing was performed. Based on the tracing results， a stray radiation suppression structure was designed， achieving infrared radiation suppression at the level of 10⁻⁵.The telescope imaging lens group was optimized with an image-space telecentric design， while the relay imaging lens group adopted an object-space telecentric design， achieving a perfect match in a dual-high telecentric optical system. By employing hybrid refractive-diffractive and aspheric designs， the system demonstrated excellent performance in the mid-wave infrared band of 3 μm-5 μm across a temperature range of -40 ℃ to 60 ℃. The static MTF exceeded 0.7 at 17 lp/mm， distortion was controlled within 0.2%， and the RMS radius of the imaging spot was less than 5.9 μm， meeting all imaging quality requirements. A stray radiation simulation model for the interference spectral imaging optomechanical system， as illustrated in Figure 7， was constructed. Stray radiation tracing was performed for typical field light paths， and based on the analysis results， efficient suppression was achieved through blackbody extinction treatment and dedicated stray radiation suppression structures. This reduced the infrared stray radiation energy ratio from an initial level of 10⁻² to 10⁻⁵. Imaging detection experiments on long-distance target scenes using an engineering prototype demonstrated excellent image quality. The MTF test of the prototype of high-precision alignment principle shows that the actual MTF of the designed imaging system is better than 0.32 at the cut-off frequency.The infrared spectral imaging system based on static interference modulation with a stepped micro-mirror successfully achieves high-quality remote sensing imaging， identification and analysis of gases. The adoption of achromatic methods using cylindrical lenses and hybrid refractive-diffractive lenses， together with the stray radiation suppression method proposed in this paper， has all yielded the expected effects in optimizing imaging quality， which is proven to be effective and can serve as a reference for research in the same field. In addition， the instrument's volume is reduced by a quarter and its weight by approximately 60%， which also provides a valuable contribution to the miniaturization process of such imaging systems.