Ultraprecision machining based micro-hybrid lens fabrication for scanning device

摘要

In this research, refractive-diffractive hybrid design is proposed for micro-lens used in optical scanning system. For refractive design of the lens, optimization method with reducing transverse ray aberration is used for target focal length with minimized aberration. Diffractive efficiency considering depth and period of Fresnel edge pattern is calculated by RCWA, and then the proper depth and minimum period of the Fresnel edge pattern can be determined for high efficiency. In the lens geometry, machining limit of V-shaped diamond tool must be applied, because the tool determines limit of the micro-pattern size and possibility of the machining so that it determines the whole size of the lens. With the process, the refractive lens can be optimized for 300 µm focal length and minimum aberration, the depth of Fresnel edge is determined to 0.94 µm by the diffractive design of PMMA (index 1.4985) lens, and also the maximum diameter of the hybrid lens is determined to 423.4 µm when considering machining limit. This designed lens is fabricated by 5-axis ultraprecision machine with nanometer resolution for low surface roughness. Then, the lens is imprinted by hot embossing process. The result of measuring focal length is 336 µm and NA is calculated to 0.63. Also, imaging test of the lens is investigated, and the lens can be used in optical scanning system by the result.