Jason B. Stewart

Polymer microcantilevers fabricated via multiphoton absorption polymerization

We have used multiphoton absorption polymerization to fabricate a series of microscale polymer cantilevers. Atomic force microscopy has been used to characterize the mechanical properties of microcantilevers with spring constants that were found to span more than four decades. From these data, we extracted a Young’s modulus of E=0.44GPa for these microscale cantilevers. The wide stiffness range and relatively low elastic modulus of the microstructures make them attractive candidates for a range of microcantilever applications, including measurements on soft matter.

MEMS Deformable Mirrors for Astronomical Adaptive Optics

We report on the development of high actuator count, micro-electromechanical (MEMS) deformable mirrors designed for high order wavefront correction in ground and space-based astronomical adaptive optics instruments. The design of these polysilicon, surface-micromachined MEMS deformable mirrors builds on technology that has been used extensively to correct for ocular aberrations in retinal imaging systems and for compensation of atmospheric turbulence in free-space laser communication. These light-weight, low power deformable mirrors have an active aperture of up to 25.2mm consisting of a thin silicon membrane mirror supported by an array of 140 to 4092 electrostatic actuators which exhibit no hysteresis and have sub-nanometer repeatability making them well suited for open-loop control applications such as Multi-Object Adaptive Optics (MOAO). The continuous membrane deformable mirrors, coated with a highly reflective metal film, are capable of up to 6μm of stroke, have a surface finish of <10nm RMS with a fill factor of 99.8%. Presented in this paper are device characteristics and performance test results, as well as reliability test data and device lifetime predictions that show that trillions of actuator cycles can be achieved without failures.

High-speed wavefront control using MEMS micromirrors

Over the past decade, a number of electrostatically-actuated MEMS deformable mirror devices have been used for adaptive control in beam-forming and imaging applications. One architecture that has been widely used is the silicon device developed by Boston University, consisting of a continuous or segmented mirror supported by post attachments to an array of parallel plate electrostatic actuators. MEMS deformable mirrors and segmented mirrors with up to 1024 of these actuators have been used in open loop and closed loop control systems to control wavefront errors. Frame rates as high as 11kHz have been demonstrated. Mechanically, the actuators used in this device exhibit a first-mode resonant frequency that is in the range of many tens of kilohertz up to a few hundred kilohertz. Viscous air damping has been found to limit operation at such high frequencies in air at standard pressure. Some applications in high-speed tracking and beam-forming could benefit from increased speed. In this paper, several approaches to achieving critically-damped performance with such MEMS DMs are detailed, and theoretical and experimental results are presented. One approach is to seal the MEMS DM in a full or partial vacuum environment, thereby affecting air damping. After vacuum sealing the devices predicted resonant behavior at tens of kilohertz was observed. In vacuum, the actuators intrinsic material damping is quite small, resulting in considerable oscillation in step response. To alleviate this problem, a two-step actuation algorithm was employed. Precise control of a single actuator frequencies up to 100kHz without overshoot was demonstrated using this approach. Another approach to increasing actuation speed was to design actuators that reduce air damping effects. This is also demonstrated in the paper.

Open-loop shape control for continuous microelectromechanical system deformable mirror

We characterize the errors associated with open-loop control of a microelectromechanical system deformable mirror (DM) using an approach that combines sparse calibration of the electrostatic actuator state space with an elastic plate model of the mirror facesheet. We quantify sources of measurement error and modeling error and demonstrate that the DM can be shaped in a single step to a tolerance of ∼8nm of that achievable with iterative feedback-based closed-loop control. Zernike polynomials with up to 2.5μm amplitude were made with this approach and yielded a shape error of <25nm rms in most cases. Residual errors were shown to be due primarily to spatial resolution limits inherent in the DM (e.g., uncontrollable errors).

Path length control in a nulling coronagraph with a MEMS deformable mirror and a calibration interferometer.

We report progress on a nulling coronagraph intended for direct imaging of extrasolar planets. White light is suppressed in an interferometer, and phase errors are measured by a second interferometer. A 1020-pixel MEMS deformable mirror in the first interferometer adjusts the path length across the pupil. A feedback control system reduces deflections of the deformable mirror to order of 1 nm rms.

Secure optical communication system utilizing deformable MEMS mirrors

An optical communication system suitable for voice, data retrieval from remote sensors and identification is described. The system design allows operation at ranges of several hundred meters. The heart of the system is a modulated MEMS mirror that is electrostatically actuated and changes between a flat reflective state and a corrugated diffractive state. A process for mass producing these mirrors at low cost was developed and is described. The mirror was incorporated as a facet in a hollow retro-reflector, allowing temporal modulation of an interrogating beam and the return of the modulated beam to the interrogator. This system thus consists of a low power, small and light communication node with large (about 60°) angular extent. The systems range and pointing are determined by the interrogator /detector/demodulator (Transceiver) unit. The transceiver is comprised of an optical channel to establish line of sight communication, an interrogating laser at 1550nm, an avalanche photo diode to detect the return signal and electronics to drive the laser and demodulate the detected signal and convert it to an audio signal. A functional prototype system was built using a modified compact optical sight as the transceiver. Voice communication in free space was demonstrated. The design and test of major components and the complete system are discussed.

Polymorphic optical zoom with MEMS DMs

A prototype optical system for compact, high-speed zooming is described. The system is enabled by a pair of MEMS deformable mirrors (DMs), and is capable of high-speed optical zoom without translation of components. We describe experiments conducted with the zoom system integrated with an optical microscope, demonstrating 2.5× zoom capability. Zoom is achieved by simultaneously adjusting focal lengths of the two DMs, which are inserted between an infinity-corrected microscope objective and a tube lens. In addition to zoom, the test system is demonstrated to be capable of automated fine focus control and adaptive aberration compensation. Image quality is measured using contrast modulation, and performance of the system is quantified.

Ultra-low-power multiplexed electronic driver for high resolution deformable mirror systems

We present a new multiplexed high-voltage driver architecture that departs from previous MEMS deformable-mirror drivers. Just one D/A converter and one high-voltage amplifier module drive the entire actuator array through a row-column addressing scheme. This approach reduces operational power consumption of a multiple-channel deformable-mirror driver by two orders of magnitude. It can provide for the integration of the deformable mirror and driver into a compact package, reducing driver volume by an order of magnitude. Both of these system modifications are essential for the implementation of MEMS deformable mirrors into space-based adaptive optics systems and other applications.

Through-wafer interconnects for high degree of freedom MEMS deformable mirrors

The development of an assembly and packaging process for MEMS deformable mirrors (DMs) with through wafer via (TWV) interconnects is presented. The approach consists of attaching a DM die with high-density TWV electrostatic actuator interconnects to an interposer substrate that fans out these connections for interfacing to conventional packaging technology.

Design and development of a 329-segment tip-tilt piston mirror array for space-based adaptive optics

We report on the development of a new MEMS deformable mirror (DM) system for the hyper-contrast visible nulling coronagraph architecture designed by the Jet Propulsion Laboratory for NASAs Terrestrial Planet Finding (TPF) mission. The new DM is based largely upon existing lightweight, low power MEMS DM technology at Boston University (BU), tailored to the rigorous optical and mechanical requirements of the nulling coronagraph. It consists of 329-hexagonal segments on a 600μm pitch, each with tip/tilt and piston degrees of freedom. The mirror segments have 1μm of stroke, a tip/tilt range of 600 arc-seconds, and maintain their figure to within 2nm RMS under actuation. The polished polycrystalline silicon mirror segments have a surface roughness of 5nm RMS and an average curvature of 270mm. Designing a mirror segment that maintains its figure during actuation was a very significant challenge faced during DM development. Two design concepts were pursued in parallel to address this challenge. The first design uses a thick, epitaxial grown polysilicon mirror layer to add rigidity to the mirror segment. The second design reduces mirror surface bending by decoupling actuator diaphragm motion from the mirror surface motion. This is done using flexure cuts around the mirror post in the actuator diaphragm. Both DM architectures and their polysilicon microfabrication process are presented. Recent optical and electromechanical characterization results will also be discussed, in addition to plans for further improvement of DM figure to satisfy nulling coronagraph optical requirements.