Optimization design of infrared dual-band common aperture optical system based on freeform surface

To meet the design requirements for miniaturization, integration, and common aperture Mid-Wave/Long-Wave Infrared (MWIR/LWIR) optical systems in current optoelectronic payloads, we established a compact re-imaging structural design model based on Gaussian optics and aberration theory. Simultaneously, to address the complex aberration problems introduced by beamsplitters in traditional dual-band infrared imaging systems, a novel beamsplitter design method incorporating freeform surfaces was proposed. This approach not only simplifies the system structure but also enhances image quality. Furthermore, considering the parameters of cooled infrared detectors, the system was designed as a cooled structure to eliminate background noise interference and ensure 100% cold stop matching efficiency. Finally, the proposed compact design concept was validated through a practical design case, resulting in a common aperture dual-band infrared compact optical system operating in the MWIR (3.7-4.8μm) and LWIR(7.5-9.5μm) bands. The total system length is ≤220mm, featuring a compact form factor. A thermal performance is achieved for both bands over a wide temperature range, utilizing a coaxial dual-mirror front group and a refractive rear lens group, minimizing the number of elements and simplifying material selection. Key parameters include: entrance pupil diameter 120mm, focal length 240mm, F-number 2, effective field of view 2.0°×1.6°, and operating temperature range -40°C to +60°C. Analysis confirms that over the entire temperature range, the Modulation Transfer Function (MTF) for the MWIR band exceeds 0.38 and for the LWIR band exceeds 0.3 at their respective Nyquist frequencies.