A MEMS Optical Phased Array Based on Pitch Tunable Silicon Micromirrors for LiDAR Scanners

Abstract

This paper presents the design, modeling, fabrication, and testing of an optical phased array (OPA) based scanner utilizing microelectromechanical system (MEMS) micromirrors with in-plane pitch tuning capability for high-speed and high-resolution laser beam steering. A pair of lateral comb-drive actuators are located at both sides of the free-standing structure of the micromirror array and used to generate the force required for in-plane motion of the micromirrors. Identical folded-beam flexures on both sides of the moving part of each comb-drive actuator allow for the precisely guided lateral motion and reduction of the in-plane mechanical stiffness. The mirror-positioned flexures enable a symmetrical structure which helps to reduce side instability as well as levitation during lateral comb-drive actuation. Analytical models of the OPA structure are presented for estimations of the micro actuator performance as well as the optical characteristics. A prototype for the proposed OPA system with polysilicon as structural material and gold as optically reflective surface is fabricated using a surface micromachining process. The optical path difference is realized by slightly elevating every other micromirrors along the array in order to form the required optical phase shift. The aperture size of the OPA based scanner is about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$195\,\,\mu \text{m}\,\,\times 185\,\,\mu $ </tex-math></inline-formula> m and the fill factor of the array is 40-57%. Pitch tuning due to the electrostatic actuation results in an optical steering range of 0.06° with angular resolution of 0.002° at 30 V and a laser wavelength of 650 nm. [2021-0070]