Basic study on a rotary focus-tunable freeform microlens fabricated using two-photon micro stereolithography

Abstract

Recent advancements in two-photon micro stereolithography have enabled the fabrication of complex micro optics with freeform surfaces. However, the design of freeform microlenses for optical MEMS (Micro Electro Mechanical System) applications remains underexplored. This paper presents a rotary focus-tunable freeform microlens, fabricated using two-photon micro stereolithography, capable of adjusting its focal length through rotational motion. The freeform microlens has a quasi-cylindrical freeform surface, and its curvature in the one axis direction of the incident light continuously changes with the rotational motion. A ray tracing simulation was employed to design a quasi-cylindrical freeform microlens with a diameter of 700 μm, achieving a focal length variation with a magnification ratio of more than 4 times. The quasi-cylindrical freeform microlens was fabricated using two-photon micro stereolithography, and its focal length tuning capability was experimentally tested using a bulk rotation stage. The focal length varied from 0.85 mm to 3.47 mm through the rotational motion, achieving a focal length variation ratio of 4.1 times. This experimentally demonstrated the validity of the proposed design of the quasi-cylindrical freeform microlens. Future integration of this freeform microlens with a MEMS rotary actuator is expected to enable the development of a MEMS-driven focus-tunable cylindrical microlens revolver at the microscale.