Development and applications of an electrowetting-based non-mechanical beam steering device

Abstract

Beam steering changes the beam direction to various angles, and it is used in drones, autonomous vehicles, laser marking, 3D printer, robot, microscopy, and AR/VR, and substantial research effort has been made to achieve beam steering. Beam steering is implemented through a beam steering device, and the driving method of the beam steering device is classified into a mechanical method and a non-mechanical method. The mechanical method includes gimbal, galvo mirror, Risley prism, and a microelectromechanical systems (MEMS) mirror. The gimbal is a structure that allows objects to rotate around an axis. It is mainly used for imaging applications, but it is bulky and heavy. The Galvo mirror is fast and accurate, but bulky and achieves only one-dimensional scan, which means two mirrors are required for two-dimensional beam steering. Risley prism scans by rotating two or more prisms. It has high resolution, large field of view, but low scan rate. The MEMS mirror is fast, lightweight, and compact, but it has a low fill factor and reflective property, which causes volume problem. The non-mechanical method includes a liquid crystal and silicon-based optical phased array (OPA), electrowetting-based liquid lens, and electrowetting-based liquid prism. Since the non-mechanical beam steering method does not use mechanical motion, it has low power consumption and little noise, but these methods are still in the research stage. The optical phased array using liquid crystal has a wide beam steering angle but it has low operating speed and only 1D scan is possible. The silicon-based optical phased array is compact and has low power consumption, but it requires an additional device for 2D scan. The method using the electrowetting-based liquid lens is capable of 2D beam steering, but the beam is distorted. Electrowetting-based liquid prism is also capable of 2D beam steering, but additional research about the liquid-liquid interface and the chamber structure-related stability is required. In this dissertation, a liquid prism with a new form factor was developed by analyzing the operating principle and liquid prism structure of the previously proposed electrowetting-based beam steering. The sidewalls of the liquid prism and the liquid-liquid interface of the liquid prism were mathematically analyzed using vectors. Using this analyzed information, the relationship between the position information of the target object to be beam steered and the tilting angle of the liquid prism was presented. A high-performance liquid prism was implemented through new fabrication processes such as chamber material, substrate cutting method, liquid selection, and sealing process. A solution to the oil isolation problem inherent in the liquid prism structure was also presented. The relationship between the influence of gravity and the chamber size of the liquid prism was presented, and the flatness of the two liquids was quantitatively measured. It was proved that the liquid-liquid interface was almost flat through a low RMS values and a large RoC values, and the response time was measured for various chamber sizes. Beam steering angle of ±10. 5 °, transmissivity of 85 %, and a resolution of 14.25 lp/mm were implemented. Using the developed liquid-based beam steering device, the possibility to apply to image stitching, LiDAR, and confocal microscopy was presented. Two or more pictures that overlap a certain part were taken by adjusting the beam steering angle of the liquid prism, and image stitching was achieved using a keypoint matching algorithm. In addition, through the optical design combined with a wide-angle lens, the beam steering angle of the liquid prism has been improved up to ±68 °, realizing the specifications to be applied to the LiDAR. Finally, the liquid prism was combined with an electrowetting-based focus-tunable liquid lens, and the possibility of using it in a confocal microscopy was presented by demonstrating three dimensional beam steering without any mechanical operation.