Micro deformable mirrors for next generation Adaptive Optics : Characterization and Simulation

Next generation Adaptive Optics systems will require deformable mirrors with a high number of actuators combined with a reduced inter-actuator spacing. Current technologies have shown their limitation and a new kind of components is requested. We are designing, modeling, realizing and characterizing micro-deformable mirrors based on Micro-Opto-Electro-Mechanical Systems (MOEMS) technology. The originality of our approach lies in the elaboration of a sacrificial layer and of a structural layer made of polymer materials in collaboration with LAAS. A dedicated characterization bench has been developed for the complete analysis. This Twyman-Green interferometer allows high in-plane resolution (3µm) or large field of view (20mm). Out-of-plane measurements are performed with phase-shifting interferometry showing a high resolution (standard deviation<1nm). Features such as optical quality or electro-mechanical behavior are extracted from these high precision three-dimensional component maps and finite-element models can be fitted. Dynamic analysis like vibration mode and cut-off frequency is realized with time-averaged interferometry. The first polymer piston-motion actuators have been processed in order to reach high strokes for low driving voltages: a 10µm thick mobile plate with four springs attached to the substrate, and with an air gap of 10µm exhibits a piston motion of 2µm for 30V. Actuators and a continuous-membrane micro deformable mirror (3*3 actuators, 600*600 µm2) have been designed in-house and processed in the Memscap foundry. The deformable mirror exhibits a 350nm stroke for 35 volts on the central actuator. A linearization of the electrostatic response is done electronically in order to fulfil the Adaptive Optics requirements. The limited stroke should be overcome by changing the components material. Preliminary comparisons with FEM models show good agreement and design of complete polymer-based micro-deformable mirrors looks possible.