2. Guangxi Key Laboratory of Automatic Detecting Technology and Instruments, Guilin University of Electronic Technology, Guilin, Guangxi 541004, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China
Overview: Adaptive optics (AO) technology is an important method to compensate optical wavefront aberration caused by atmosphere turbulence and deformable mirror (DM), whose surface can be tuned dynamically, is one of its key components. For satisfying the boarder application requirement of AO technology and solving the problem of large volume and high cost of conventional piezoelectric DMs, micro DM based on micro-electro-mechanical system (MEMS) technology is developed and measured. Due to its low power consumption, zero hysteresis, and good integrated circuit (IC) compatibility, parallel plate capacitor electrostatic actuator is adopted to actuate the MEMS DM. The developed DM has 140 hexagonal parallel plate actuators, which has a larger stroke and higher frequency when compared to square parallel plate actuators. The stroke, pull-in voltage, one order natural frequency, and Euler critical stress of the actuator are simulated and compared by finite element analysis (FEA). The actuators are arranged as a 12×12 square array and the pitch is 400 μm. The facesheet of the DM is a continuous square membrane with an area of 4.4 mm×4.4 mm. A DM prototype is fabricated by MEMS surface micromachining process. The material for structural layer and sacrificial layer of the process are polysilicon and phosphosilicate glass (PSG), respectively. The whole process includes three structural layers and two sacrificial layers. The three structural layers are the lower electrode, the upper electrode and the facesheet, respectively. The fabricated DM prototype is packaged by a ceramic pin grid array (CPGA) and a quartz glass lid. A miniaturization multichannel high voltage driver for the DM is developed at the same time. The optical and electromechanical, including static and dynamic, performances of the prototype are measured by a Zygo NewView 7300 noncontact white light profiler. The measurement results show that the prototype has a surface PV value of 411 nm, RMS value of 78 nm, reflectivity of about 80% in 600 nm to 900 nm wavelength, stroke of 1.8 μm, actuator coupling of 15%, working bandwidth of 13 kHz and step response time of 23 μs. Thus the DM has the advantages of small volume, low cost and fast response. Besides the measurement of single element, the whole DM is controlled open loop to fit Zernike aberration and its fitting capability is demonstrated. Above results indicate that the DM prototype can satisfy preliminarily the requirement of AO system and promote its applications in civil market, such as ophthalmoscope and free space optical communication.