High-power lasers are widely used in various fields for industrial, scientific, and military applications. Generally, the intensity distribution of a laser beam is not uniform and generally exhibits a Gaussian distribution, which may lead to material damage during laser processing owing to the uneven energy distribution. Different application fields have different demands for the spot shape and intensity distribution of laser beams. Recently, flat-topped laser beams with a uniform distribution of beam intensity have become commonly used, with a wide range of applications in material processing, semiconductor substrate annealing, optical holography, and laser lighting. A flat-topped beam can be obtained using beam shaping technology, and common beam homogenization technologies include the light field mapping method and beam integration method. Light field mapping is realized using an aspheric lens group, a birefringent lens group, and diffractive optical elements, which are suitable for single-mode laser light sources. Beam integration is mainly performed using mirror arrays, prism arrays, and microlens arrays, which are particularly suitable for excimer lasers, multi-mode lasers, or laser light sources with irregular light intensity distribution. The microlens array homogenization system is generally wavelength insensitive, and the output spot shape is modulated by the sub-lenses. It is widely used owing to its simple structure, high damage threshold, and low transmission loss.

Based on the superior properties of microlens arrays, a beam homogenization and shaping system based on cylindrical microlens arrays was designed. The microlens arrays were placed orthogonally to homogenize and shape the vertical and horizontal directions of a Gaussian circular beam, respectively, achieving a square beam output with a near-flat-top intensity distribution. Based on the theories of matrix optics and Fourier optics, the light transmission mode was analyzed, the structural parameters of the microlens array were optimized using Zemax software, and a simulation model was constructed to shape the homogenization effect of the system. The research system was established using an experimental platform. First, one pair or two pairs of orthogonal microlenses were employed to compare the Gaussian beam homogenization. With two pairs of cylindrical microlens arrays, the effect of the incident Gaussian beam diameter on the uniformity and size of the homogenized light spot was investigated experimentally and theoretically. When the input laser beam had a diameter of 8 mm, the relationship between the output spot shape and the interval distance of the microlens arrays was analyzed. Moreover, the effect of laser beam quality on homogenization and shaping was examined for the cylindrical microlens array system.

Based on the simulation result of Zemax software, the structural parameters of the microlens array are optimally designed with a sub-lens aperture size of 500 μm and a focal length of 5.4 mm. Compared with the use of one pair of cylindrical microlens arrays to homogenize and shape the horizontal and vertical directions, the experimental and theoretical results show that the Gaussian circular beam can be better homogenized with a more uniform energy distribution by utilizing two pairs of cylindrical microlens arrays placed orthogonally. As the size of the incident Gaussian beam increases, the uniformity of the homogenized spot increases and the sharpness of the spot edges decreases (Figs. 2 and 3). By controlling the interval distance between the microlens arrays, square beams with adjustable spot shapes and sizes and a near-flat-top distribution of light intensity can be obtained. With an incident spot diameter of 8 mm, homogenized output spots with adjustable beam aspect ratios such as 100 mm×100 mm squares, 100 mm×130 mm rectangles, and 130 mm×130 mm squares were successfully obtained (Fig. 4). In our case, the size of the spots increases with the transmission distance of the homogenized beams; however, the corresponding uniformity shows little change. The homogenization shaping system is flexible and versatile on a spatial scale, and it can better meet practical applications in scientific research and production. The microlens array system is insensitive to the beam quality of the incident laser, which makes it especially suitable for homogenizing and shaping excimer lasers, laser diode arrays, multimode light fields, or laser sources with irregular intensity distributions.

In this study, the physical mechanism and homogenization process of a cylindrical microlens array homogenization system are investigated in depth using a combination of theory and experiments. A discrete structure with two pairs of orthogonally placed cylindrical microlens arrays is designed for beam homogenization and shaping of a circular Gaussian beam in both the horizontal and vertical directions. By controlling the distance between the microlens arrays, a homogenized beam with an adjustable shape and size is obtained, and spot uniformity is maintained. These results open a novel way to realize a uniform square spot with an adjustable spot size and high flexibility in space utilization, which is suitable for practical applications in scientific research and industrial fields.