Preliminary results of large-actuator-count MEMS DM development

Abstract

Compensating for atmospheric turbulence in meter-class telescopes and for free-space communications can require deformable mirrors (DM) with hundreds of actuators. Advances in high-contrast imaging techniques and increased telescope sizes require DMs mirrors with thousands of actuators [1]- [3]. In response to these needs, Iris AO has been developing a nearly 500-actuator DM and is conducting pathfinding research into 3000-actuator class DMs. This paper begins with an overview of the segmented DM design and describes improvements made to the DM over the prior year in the areas of speed, high-quality dielectric coatings, and snap-in prevention structures. The paper then describes the next-generation PTT489 DM design and fabrication process. Failure modes encountered during fabrication are presented as well as test methods to detect the failure modes. Preliminary yield data are presented for the fabrication process as well. The paper concludes with a view to the future showing pathfinding research into 3000-actuator DMs. PTT SEGMENTED DM DESIGN OVERVIEWThe Iris AO piston/tip/tilt (PTT) segmented deformable mirror (DM) is based on the PTT segment schematic diagram shown in Figure 1a. Many of these three degree of freedom segments are tiled in a hexagonally close-packed pattern to form larger arrays. A photograph of a packaged PTT111-5 DM (111-actuators, 37 PTT segments, 5 µm stroke) is shown in Figure 1b. All of the DMs manufactured by Iris AO use the same basic segment design. Therefore, all of the knowledge gained from developing the PTT111 DM design transfers to larger mirrors described herein.The MEMS DM is a hybrid of surface micromachining and bulk micromachining technologies [4] [5]. Conventional polysilicon surface micromachining technology, in this case the SUMMiT IV process, creates the actuator platform, electrodes, and underlying electrical interconnect (not shown in the schematic) shown in Figure 1a. The top layer of the bimorph flexure is added by Iris AO in order to lift the structure 20-40 µm above the substrate after release. The bimorph layer was chosen such that the coefficient of thermal expansion (CTE) is matched as close to the polysilicon as possible. * michael.helmbrecht@irisao.com, phone (510) 849-2375, www.irisao.com Electrodes Tempera ture Insensitive Bimorph Flexure Bondsites Rigid High-Qua lity Mirror Segment Actua tor Pla tform Figure 1: a) Schematic diagram of a 700 µm diameter (vertex-to-vertex) mirror segment. Scaling is highly exaggerated in the vertical direction. b) Die photograph of a 111-actuator 37-piston/tip/tilt-segment DM with 3.5 mm inscribed aperture (Product name: PTT111-5).

Analytical decomposition of Zernike and hexagonal modes over a hexagonal segmented optical aperture

“…where the terms appearing before the sums in Equation 13 and Equation 14 are the ratios between the area of one segment and the normalization coefficients of the Zernike or hexagonal polynomials. The comparison between the theoretical fitting errors and the values obtained by means of simulations are presented in Fig.…”

“…Control authority for the wavefront is largely assigned to a second stage of optical system and thus on active or adaptive optics element (i.e., small-size deformable mirrors.) In adaptive optics, advanced wavefront control devices such as small-size segmented deformable mirrors [12][13][14][15] made of hexagonal segments are intensively used in astronomy [16][17][18], laser shaping [19], retinal imaging systems [7,20], microscopy [21,22] or laser communication [23].…”

“…This solution, which naturally scales with the size, positions, rotation and number of segments comprising the mirror, is versatile per se. On the contrary to numerical solutions, the analytical expressions are independent from the physical size of the pupil and can be applied on small deformable mirrors [12,13] as well as on future large optical telescopes [10,11] without loss of precision in the decomposition.. While numerical methods require computational and memory cost which can be impressive, our analytical solution allows extremely quick results whatever the pupil complexity.…”