Analysis of laser diode array pump coupling system based on microlens array

In order to improve the performance of laser diode (LD) array pumping field in high-power solid state laser, an LD array pumping coupling system based on microlens array is used to achieve a high-uniformity pumping source with a longer transmission distance. The homogenizer has two structures based on microlens array, which are called diffracting homogenizer and imaging homogenizer. In this paper, we mainly study imaging microlens array due to its advantages of simple structure, better output homogeneity, flexibility of changing pumping field size, and insensitive to change in the input beam. First, the mathematical expression of the intensity distribution of target surface is derived based on the theory of geometrical optical. According to the geometrical optical formula, we obtain the relationship between the intensity distribution of target surface and system parameters, i.e., maximum incident angle of LD array, the distance between two microlens arrays, and the aperture and focal length of microlens. The boundary condition of microlens Fresnel number is derived based on the LD array beam parameters. Second, the influence of the number of microlens array elements on the output field homogeneity is studied theoretically by the mathematical statistics method. As the input beam is considered to be divided randomly, the central limit theorem is employed to derive the mathematical expression of calculating the integrated output field non-homogeneity. The formula shows that the non-homogeneity is in inverse proportion to the root of the number of microlens array elements and the related maximum and minimum value of input field intensity distribution. And the spatial period of microlens array is designed to be unrelated to the spatial period of LD array to reduce the coherence of LD beam. According to the luminescence field parameters of an LD array consisting of 25 bars, an LD coupling imaging microlens array homogenizer test system is designed and constructed based on the theoretical analysis above. Another contrast system with a different microlens array which is not optimized is constructed at the same time. The coupling characteristics of two coupling systems with different microlens arrays are compared. The simulation and experimental test are carried out. The experimental result accords well with the simulated result, and thus proving the correctness of the theoretical studies. The coupling system with optimized microlens array shows better homogeneous effect with an output field non-homogeneity of 7.9%, and a coupling efficiency of 90.7%, proving the feasibility of the system for LD array pumping field homogenization.