Design and development of a 331-segment tip–tilt–piston mirror array for space-based adaptive optics

We report on the development of a micromachined silicon deformable mirror (DM) system for the hyper-contrast visible nulling coronagraph architecture designed by the Jet Propulsion Laboratory for NASA's Terrestrial Planet Finding (TPF) mission. The new DM is designed to achieve unprecedented optical quality and mechanical positioning precision as required by the nulling coronagraph. It consists of 331 close-packed hexagonal segments on a 600 μm pitch. Each segment has three adjustable degrees of freedom controlling out-of-plane rotation (in two axes) and surface normal motion over a continuous range of ±3 mrad and 1 μm, respectively. The coronagraph implementation specifies that each segment maintains an overall flatness better than 10 nm RMS, irrespective of segment angle. To achieve this objective, two design concepts were developed in parallel. The first design uses thick mirror segments consisting of epitaxial grown polysilicon (epipoly) to provide the mirror with high rigidity, allowing it to tilt without bending. The epipoly layer can also be highly polished to better than 1nm RMS local surface roughness. The second design uses thinner, more compliant mirror segments, but supports them with flexure-based electrostatic actuators that decouple tilt motion from mirror segment bending moments. The thinner mirror segment has reduced optical quality due to print-through effects, but offers greater ease in post-processing mirror curvature caused by residual stress gradients. DM segments fabricated with the flexure design experience a maximum mirror bend of 2.8 nm RMS at 4.3 mrad of tilt. Both DM designs and associated microfabrication processes are presented. Optical and electromechanical characterization results are also presented.