Research On Focusing System Of Rectangular Facula For High Power Diode Laser

High power diode laser is increasingly used in the surface modification, welding and cutting, it is the development tendency of laser processing. It has been analyzed for the beam shaping structure of high power diode laser in the paper, and a design scheme of the focusing system by using the Fresnel lens has been put forward, meanwhile, the factors that influence the uniformity of the output facula have been studied, eventually we gain the uniform rectangular output facula. The research contents are as follows:(1) As the direct source of surface modification, high power diode laser has the shape and the uniformity of the output facula that influence the quality of surface modification directly. Since the nonuniformity of the fast and slow axis, the solution of the Fresnel focusing system has been proposed, which optimized the facula uniformity by the beam split and rearrangement.(2) The Fresnel focusing system which consists of orthogonal Fresnel lens and cylindrical lens has been designed. The fast and slow axes were focused respectively to improve the facula uniformity. Two lenses were made of LAFN21, the dimensions were 40×40mm2, 20 mm apart and the focal distance of the focusing system was 380 mm, thus the output facula size was 2×10mm2.(3) The influence of the pitch to the uniformity of focal facula has been studied. The uniformity of the focal facula is best at about 94.90% when the pitch of Fresnel lens is less than 1mm and the uniformity becomes worse and worse with the increase of the pitch of Fresnel lens. As the pitch increases to 2.5mm, it has less influence on the uniformity of the focal facula, and the uniformity basically stables at around 93.85%.(4) It has been analyzed for the influence of divergence angle on the uniformity of focal facula. The uniformity will be improved with the increase of divergence angle, but large divergence angle will increase the difficulty of focusing and make the uniformity worse and worse. When the pitch of Fresnel lens is 1mm and the divergent angle is from 12.5mrad to 20 mrad, the focal facula uniformity is best at 95.22%.(5) The experiments have verified the feasibility of the Fresnel focusing system and its design scheme. The facula size was experimentally measured as 2.04×10.39mm2 since facula broadening caused by the incident divergence. The measured value was consistent with the designed on the fast axis for its small divergence angle, while there was a 0.39 mm broadening on slow axis caused by its larger divergence. Therefore the facula broadening should be taken into account while designing.This research work provides a new way of focusing system design for the high power diode laser, and it has certain reference value for the high power diode laser applied in the laser surface modification.