Analysis of polarization aberrations of the off-axis pupil freeform optical system

Freeform surfaces are widely used in off-axis pupil optical systems with large apertures, wide fields of view, and long focal lengths. Polarization effects in such systems significantly influence imaging quality. We proposed a polarization aberration analysis method for off-axis pupil freeform optical systems. An analytical model of phase aberration, diattenuation, and retardation was established. Low-order Zernike coefficients were analyzed to evaluate the impact on polarization aberration. The analysis results show that off-axis pupil freeform surfaces break the rotational symmetry of polarization aberration. By optimizing the freeform surface in the designed off-axis pupil triple-inverse spatial optical system, diattenuation and retardation decrease by 8.70% and 6.94%, while PSF ellipticity under non-polarized, horizontal polarized light, and vertical polarized light decreases by 0.12%, 7.31% and 10.69%, respectively. This study provides a theoretical basis for the design and polarization aberration calibration of off-axis pupil freeform optical systems.