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Infrared polarization imaging can achieve faster perception speed and higher identification accuracy, which has been widely applied in diverse areas such as space remote sensing, biomedicine, and object detection. Limited by the aperture of the lens, the single-aperture imaging system can hardly meet the needs of high-resolution and multi-polarization imaging simultaneously. Aiming at achieving the properties of high-resolution, high-integration, and multi-channel performance, a sub-aperture infrared polarization imaging system based on freeform surfaces is proposed and demonstrated. The general scheme of the polarization imaging system mainly consists of a common aperture structure, a sub-aperture imaging group, and a relay imaging group. To compress the beam aperture, a Kepler telescope configuration is employed to build up the imaging objective. The field diaphragm is set at the primary image plane to effectively eliminate stray light. Polarizers with different orientations are added to the split aperture imaging group to form four polarization state channels. To reduce the assembly errors of the sub-aperture system, a freeform surface lens is utilized to replace the lens group in the multi-channel. The freeform surface profile uses the "XY polynomial" with eight coefficients. A diaphragm array is arranged on the front surface of the sub-aperture system, which is used to avoid the intersection between the image planes of different channels. To match with the cooled detector, the relay imaging group is designed as a finite conjugate structure with a magnification of -0.44×. The structure allows for simultaneous imaging of four infrared polarization states with the same system, and the MTF of each channel at 33lp/mm is higher than 0.45. Our research satisfies both miniaturization and engineering application requirements.
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