Design of MWIR and LWIR infrared common-path optical system based on freeform surface

This paper analyzes the advantages and disadvantages of imaging in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) spectral bands, and elaborates on the necessity of fusing MWIR and LWIR imaging. Based on the selection analysis of MWIR and LWIR common-aperture imaging system, it is determined to adopt an off-axis reflective optical system to achieve MWIR and LWIR common-path imaging. On the basis of off-axis reflective optical system imaging, by introducing a freeform surface primary mirror optical element, the compactness and simplicity of the dual-spectral infrared optical system are achieved. The final MWIR and LWIR common-path optical system has a focal length of 2000mm, an F-number of 2.86, a field of view of 2.1°×0.6°, a spectral range of 3-5μm/8-10μm, and a cold stop efficiency of 100%. The mean MTF of the MWIR system across the full field of view and full spectral band at the Nyquist frequency (25lp/mm) is 0.60, and the mean MTF of the LWIR system across the full field of view and full spectral band at the Nyquist frequency (25lp/mm) is 0.22. Through analysis and calculation, it is found that the introduction of freeform surface optical elements reduces the maximum material removal of the primary mirror during the manufacturing process by approximately 37.8%, increases the mean MTF of the MWIR imaging spectral band by approximately 22.4%, increases the mean MTF of the LWIR imaging spectral band by approximately 5.5%, and reduces the total length of the optical system by 18.4%. This MWIR and LWIR common-path optical system can meet the requirements of short-cycle engineering projects that demand rapid processing and implementation.