# Fabrication of Random Microlens Array for Laser Beam Homogenization with High Efficiency
## 1 Introduction  
Lasers have been widely applied in many fields, such as medical treatment [1; 2], laser illumination [3; 4], laser detection [5; 6] and satellite communication [7; 8]. However, the intensity distribution of a laser is usually in a Gaussian profile, which is always required to be converted into a flat-top profile so as to achieve the desired effect, e.g., laser welding [9; 10], an optical system of exposure machine [11], laser drilling [12; 13] and laser projection [14; 15]. There are some methods to achieve beam homogenization, such as an aspheric lens group [16; 17; 18], free-form lens [19; 20; 21], diffractive optical element (DOE) [22; 23; 24] and microlens array (MLA) [25; 26; 27]. For the aspheric lens group and free-form surface lens, the former has a high light energy utilization rate and high beam uniformity, which can be applied to high-power lasers, while the latter is a simple optical system with high design freedom which can be used to effectively achieve high uniformity. However, the above two methods require nanometer surface roughness, which increases the fabrication difficulty. In addition, because of the limitation of the sizes of the corresponding lenses, it is a challenge to realize the miniaturization and integration of the system. The DOE can be used to achieve the miniaturization and integration of the systems, and also has high design freedom. Nevertheless, the DOE is generally only suitable for a single wavelength, and cannot be used to shape the beam in abroad spectral range. Meanwhile, energy efficiency depends on the number of phase levels of the DOE; the greater the number of phase levels is, the higher the energy efficiency will be, but also the more complex the fabrication. In contrast, the MLA is widely applied to the field of multi-wavelength laser beam homogenization to achieve the miniaturization and integration of the systems, due to its merits of a high energy utilization rate, extremely simple structure and high beam uniformity. Many related research works of beam homogenization by means of using MLA have been reported before. In 2017, [PERSON] et al. (2018) used a monolithic periodic MLA to homogenize the laser beam. The collimated laser beam is subdivided into multiple beamlets by the MLA, and then the beamlets are superposed and recombined with each other to suppress the nonuniformity by a focusing lens. The uniformity of the obtained homogenized light spot is 91.1%. The energy utilization rate of the MLA with anti-reflection coating can reach 93%. In order to homogenize the incident laser beam with a certain divergence angle, a method on the basis of two periodic MLAs is proposed by [PERSON] et al. (2019) to obtain a homogenized light spot with high energy efficiency in 2017. However, when high-coherence lasers are shaped by their method, interference between beamlets will also occur due to the periodicity of the MLA, the resulting interference fringe appears in the obtained homogenized light spot and the uniformity of the homogenized light spot will be deteriorated. In order to eliminate the effect of interference on the homogenized light spot, [PERSON] et al. (2019) prepared a 16-level random phase fresnel lens array by means of setting the phase of sub lenses of the fresnel lens array to a random arrangement in 2019. This broke the coherence between the beamlets, and obtained a homogenized light spot with a uniformity of 83%. However, the fabrication of the 16-level random microlens array (rMLA) is complex. In 2019, [PERSON] et al. (2019) designed a continuous profile MLA with sub lenses of the random apertures and arrangements. It realized a breakthrough of the coherence of beamlets so that the uniformity obtained by simulation is able to reach 94.33%. However, they proposed to fabricate the rMLA by 3D printing technology, which is not yet mature. At present, the fabrication technology of continuous rMLA is not yet mature.