Variable-focus SU-8 membrane mirror with enhanced stroke using feedback control

Abstract

We describe enhanced deflection of an electrostatic deformable mirror made from SU-8 epoxy. Using closed-loop feedback control, stable static deflection of a circular mirror to 75% of the air-gap has been achieved. IntroductionDeformable membrane mirrors are effective adaptive optical elements for focus and achromatic wavefront control. An inherent limitation for these membranes and other devices driven under electrostatic actuation is snapdown. The usable range of motion is restricted to approximately 44% of the air gap for circular deformable membranes [1]. Overcoming the snapdown limitation will provide increased stroke resulting in a greater range of variable optical power.We describe here the use of capacitive displacement sensing and closed-loop voltage control in order to increase the stable range of actuation. Specifically, this paper explores the performance of a feedback control scheme with a 2 mm diameter SU-8 2002 epoxy membrane mirror. We have observed stable static deflection to 75% of the air gap for this device. In addition we note a "softer" snapdown behavior with significantly reduced contact area and diminished likelihood of catastrophic stiction.