# Integrated Double-Sided Random Microlens Array Used for Laser Beam Homogenization
## 1 Introduction
In view of the laser beam with high coherence, the interference will occur between the sub-beams due to the periodic structure of the MLA, resulting in interference fringes in the obtained homogenized spots. Therefore, the periodic lattice phenomenon will appear, reducing the homogeneity of the spot greatly. In order to eliminate the influence of interference on the homogenized spot, researchers propose to employ the random phase plate (RPP) or the multifocal MLA to modulate the laser beam. Both of these methods disturb the coherence between sub-beams by modulating the phase, but they are implemented in different ways. The RPP is a typical optical birefringent material by randomly etching a specific depth of the unit structure on the surface. Thus, a random phase shift is introduced to modulate the polarization direction of the incident beam to disturb the coherence condition of the beam. In addition, a smooth uniform spot can be obtained [23]. The implementation scheme of multifocal MLA is the focus of this paper. [PERSON] et al. [24] proposed to replace the first MLA with free-form MLA in the homogenizing system to improve the homogeneity in 2016. Each free-form surface in the MLA introduced an appropriate aberration in the wavefront to redistribute the irradiance of the beam. Compared to the traditional MLA homogenization system, the structure greatly reduced the diffraction effect and achieved a highly uniform beam profile. However, this method is only applicable to the laser beam with a larger diameter, since the aperture of the sub-lens is on the order of millimeter. To achieve the same uniformity for laser beams with small diameter, it is necessary to reduce the aperture of the MLA to the scale of micron and increase the number of MLA, which bringing in huge manufacturing difficulty of the MLA with free-form surface. Our research group have studied the laser beam homogenization using MLA. In 2016, [PERSON] et al. [25] proposed a laser beam homogenization method using central off-axis random MLA. The periodicity of the MLA was broken by the off-axis quantity of the center to eliminate the periodic lattice effect at the target surface. However, the fabrication will become difficult due to the sharp change of the surface between the sub-lens units. Meanwhile, it is hard to install and align the two microlenses during the practical application. In 2020, [PERSON] et al. [26] proposed a monolithic random MLA to homogenize the laser beam. In the process of beam homogenization, the coherence between sub-beams was completely broken, and the homogenized spot with high energy utilization rate was obtained. However, there is still a problem of adjustment and installation between the two plates when using the double MLA for beam homogenization. Considering the problems and shortages of the previous research, a high integrated D-rMLA used to laser beam homogenization is proposed, which can be used to homogenize the laser beam with small diameter. D-rMLA is fabricated on a single substrate by double exposure and chemical etching technique. In the process of beam homogenization, the interference fringes between sub-beams are disturbed to obtain homogenized spots with high uniformity and energy utilization rate. In this paper, the method is used to carry out laser beam homogenization experiments and its feasibility is verified. The main arrangements of this paper are as follows: Section 2 describes the principle and simulation analysis of the beam homogenization based on the proposed D-rMLA, while Section 3 demonstrates the fabrication process of D-rMLA. Section 4 shows experimental results and verifies the validity of our method, and Section 5 is a summary of the whole paper.