Feedback Stabilized Deformable Membrane Mirrors for Focus Control

摘要

This paper describes a method to extend the range of motion of a deformable, continuous membrane mirror beyond the limit of open-loop electrostatic instability through the use of a feedback control scheme. The feedback scheme is based on capacitive sensing of the mirror. The sensing is achieved by coupling the actuation electrodes to a ring oscillator. We use a differential technique, where the frequency of the coupled oscillator is compared to a reference ring oscillator. Analysis of the system using a simplified parallel-plate model shows that the range of stable deflection depends on the dynamics of the device and control circuitry, and suggests that stable full-gap displacement can be achieved under certain conditions. Experimental results are provided, showing stable closed-loop deflection of our silicon nitride test device to 61% of the air gap, consistent with the predictions of our model.