We propose a design for a large-field two-mirror afocal optical antenna based on an off-axis free-form surface to improve the working range of space laser communication systems and simplify the structure of optical systems. The optical antenna adopts an afocal structure without using collimating lens elements, which can greatly simplify the system structure, overcome the problems of traditional focusing optical antennae such as them being too large in volume or having a power density that is too high at the focus when us-ing high power light source. First, based on third-order aberration theory, the aberration-free formula of this class of two-mirror afocal optical antenna is derived, and relative results are analyzed. Then, an afocal optical antenna is designed according to the analyzed results and practical requirements. The effective aperture of the system is 100 mm, the magnification is 5, the range of the wavelength is 500 similar to 1 100 nm, the full field of view is 0.6 degrees, the primary mirror is part of the concave paraboloid and the secondary mirror is a freeform surface characterized by XY polynomials. MATLAB software is used to simulate the freeform surface of the secondary mirror. The design results show that the total field of view wavefront error of the optical system is better than lambda/14 (lambda=500 nm), the Strehl ratio is greater than 0.8, the system has a higher energy concentration, and the image quality is close to the diffraction limit. The field of view of the freeform surface optical system increased by 26.7% compared with that using traditional conic surface system. Therefore, this antenna structure is highly applicable and shows strong prospects for development in the field of laser communication.
