# Integrated Double-Sided Random Microlens Array Used for Laser Beam Homogenization
## 3 Fabrication of D-rMLA  
The fabrication methods of MLA mainly include thermal reflow technique [27], laser direct writing (LDW) [28], 3D printing [29], and gray-scale lithography [30]. Thermal reflow technique is an efficient way to produce MLA by preparing an array of photoresist polymer cylinders regularly distributed on a substrate and melting the cylinders into a hemispherical shape [27]. However, the filling factor of this preparation method cannot reach 100%. When it is used in beam homogenization, some lights will not be modulated resulting in a central bright spot, which reduces the quality of beam homogenization. LDW is to use the laser beam with variable intensity to expose the resist material on the surface of the substrate. After development, the required relief profile will be formed on the surface of the resist layer [28]. Two-photon polymerization (TPP) 3D printing can also achieve the fabrication of MLA with hundred-nanometer-scale or sub-micrometer-scale resolution [29]. Nevertheless, the LDW and TPP 3D printing techniques are based on point-by-point structural modification and require a long fabrication time for producing large-sized components. Thus, it is time-consuming and costly to fabricate large-scale components. Furthermore, gray-scale lithography is proposed to fabricate the MLA [30]. In the process of exposure, MLA with continuous surface is fabricated by moving the mask dynamically. The fabrication method is flexible and does not need expensive cost. However, for the fabrication of MLA with small apertures, the number of sampling and quantizing is limited, which will reduce the surface quality of the profile. Figure 2: The simulation of periodic MLA and D-rMLA: (**a**) the three-dimensional (3D) structure of the MLA; (**b**) the phase of periodic MLA; (**c**) the obtained homogenized spot; (**d**) The 3D structure of the rMLA; (**e**) the phase distribution of the rMLA; (**f**) the homogenized spot of D-rMLA. Although a lot of research work about the fabrication of MLA has been demonstrated, only few multi-focus MLA which consist of lens lets of different focal lengths were presented in literature. In 2012, [PERSON] et al. group proposed the pyroelectric continuous printing method to fabrication graded size MLA, which has different focal lengths [31; 32]. The technology limitation strongly depend on the viscosity of the polymeric material. The higher the viscosity, the higher the diameter of the lenses will be. It is difficult to achieve high fill factor. Moreover, the fabrication method which combining lithography and chemical etching has been proposed to prepare rMLA in the previous study [26]. It can realize the fabrication of randomly distributed MLA with small aperture and multi focal length at low cost. The pattern on the mask was transferred to the glass substrate by lithography technology, then the random microlens structure was etched on the glass substrate through the chemical etching method. The double-sided exposure technology is proposed on the basis of the fabrication of rMLA to realize the high precision alignment of the double-sided sub-lens unit structure on both sides of the substrate in this paper to obtain the integrated D-rMLA. In order to achieve the spatial alignment of the microstructures on both sides of the glass substrate, it is necessary to prepare two masks, M\({}_{1}\) and M\({}_{2}\), with random hole arrays distributed in mirror image, as shown in Figure 3. The diameter of the hole is 3.6 \(\upmu\)m. The interval of the central position in the holes vary from 25 \(\upmu\)m to 45 \(\upmu\)m, that is (30 \(-\) 5 \(\times\) rand (0, 1), 30 + 15 \(\times\) rand (0, 1)) \(\upmu\)m. Moreover, an alignment mark, which is a crosshair, was designed on each mask to align the two sides of the sub-lens at the process of exposure.