Rapid fabrication of quasi-periodic aspherical concave microlens array with different focal lengths on fused silica by shaping picosecond laser-induced multi-step wet-etching for multi-layer optical imaging

Fused-silica-based microlens array (MLA) with different focal lengths is a significant optical element for multi-layer optical imaging. In this paper, we propose shaping picosecond laser-induced multi-step wet-etching to control the morphology of the MLA. A spatial light modulator (SLM) was employed to shape the Gaussian picosecond laser into multiple focal spots with different focal depths. The multiple focal spots induced the ablation regions with different depths inside the fused silica. As a result, the morphologies of the MLA could be controlled from the spherical surface to the aspheric surface. The diameter, sag height, vertex curvature radius, and cone coefficient of the aspheric surface could be readily controlled by the average pulse energy and the focal depths of each focal spot. The ratio between maximum vertex focal length and minimum vertex focal length could be adjusted between 1 and 9.52. Moreover, the patterned MLAs were used for multi-layer imaging and 3D code, which can be applied in optical storage or anti-fake. We believe that shaping picosecond laser induced multi-step wet-etching is a highly efficient and flexible approach to fabricate the micro-optical elements on fused silica.