Large aperture flat-top beam shaping and long-distance transmission based on off-axis reflective freeform surface optical system

Flat-top beam is widely used in laser applications such as holography, material processing, and nuclear fusion. However, it is difficult to maintain the flat-top effect over long distances due to the limitations of wavefront modulation and natural diffraction effects. This study aims to shape a circular Gaussian beam into a canonical flat-top beam and preserve its flat-top characteristics during long-distance (meter-level) transmission. Based on the principle of energy conservation, an energy mapping relationship between the incident plane and the output plane is constructed, and a circular flat-top intensity distribution is obtained at the output plane. The wavefront quality of the outgoing beam is controlled by the principle of equivalent optical length between mapping point elements. An off-axis reflective free-form surface optical shaping system is designed. The incident Gaussian beam has a spectral range of 1060±15nm, a beam waist diameter of 40mm, an energy truncation diameter of 60mm, and a beam quality β of 3. After the shaping system, a flat-top beam is shaped at the position of 5m output plane behind the mirror. The energy uniformity is more than 95% and the energy utilization rate is more than 90% within the diameter of 60mm. The flat-top effect can be maintained within 10m. The results show that this system can effectively shape and transmit a flat-top beam over long distances. This study provides a novel and practical method for flat-top beam shaping and transmission, which has potential applications in various laser fields.