Design of annular beam shaping system based on freeform surface

Laser is widely used in manufacturing, information transmission, biotechnology and other fields, but these fields often need the input beam with specific intensity distribution, and the Gaussian beam needs beam shaping to achieve the required illuminance distribution. Annular flattened beam has great demand in satellite communication, laser high-speed welding and other fields. The existing beam shaping system needs to compensate off-axis aberration and chromatic aberration, and its design process and system structure are complex. In this paper, we propose a design method of coaxial reflection annular beam shaping system based on geometric optics principle. Through ray mapping method to establish a mapping relationship, through geometrical optics principle to derive the freeform surface equation. We constructed the beam shaping system of the double freeform mirror, which can realize the annular flattened beam with a uniformity of 96.35%. Compared with the existing beam shaping system, the coaxial double reflection system has fewer optical elements, more compact structure, less calculation of optical element structure, and avoids off-axis aberration and chromatic aberration under the condition of similar shaping uniformity.