Olga Blum Spahn

Large-stroke MEMS deformable mirrors for adaptive optics

Surface-micromachined deformable mirrors that exhibit greater than 10 /spl mu/m of stroke are presented. The segmented arrays described here consist of 61 and 85 hexagonal, piston/tip/tilt mirrors (three actuators each) with diameters of 500 and 430 /spl mu/m, respectively, and fill a 4 mm circular aperture. Devices were packaged in 208 and 256 pin-grid arrays and driven by a compact control board designed for turn-key operation. After metallization and packaging mirror bow is /spl sim/680 nm (/spl lambda//1), but a heat-treatment procedure is proposed for controlling mirror curvature to better than /spl lambda//10. An optical test bed was used to demonstrate basic beam splitting and open-loop aberration correction, the results of which are also presented.

Planar microoptomechanical waveguide switches

Planar micromechanical waveguide switches based on lateral deflection of a cantilever beam are presented. Two material systems have been used: a GaAs-AlGaAs structure with integrated waveguides and a silicon-on-insulator (SOI), with postprocessed polymeric waveguides. The switches are characterized by low actuation voltage (3-20 V), short switching times (32-200 /spl mu/s), and low crosstalk (< -30 dB).

GaAs-based microelectromechanical waveguide switch

We describe a 1/spl times/2 waveguide switch which is also a cantilever, fabricated in GaAs-based materials. This switch can be cascaded into 1/spl times/N structure. The layout and layer cross section of the waveguide are shown schematically. Actuation is accomplished by electrostatic means, by application of bias between the movable waveguide and static electrodes. This results in 4 /spl mu/m motion of the cantilevered waveguide in the plane of the wafer. The waveguide consists of 4 /spl mu/m thick GaAs/AlGaAs layer, while the release layer is composed of 2 /spl mu/m of Al/sub 0.7/Ga/sub 0.3/As. Metal contacts are deposited on a planar substrate prior to waveguide definition. Then 3 /spl mu/m wide waveguide is defined by RIBE. Photoresist is defined on the areas to be protected against release and sacrificial layer is removed by a HF-based wet etch. After photoresist removal, devices are sublimation dried. Fabrication issues, such as choice of materials, release chemistries and their implications are further discussed. Also, further details of device performance are given.

Crystallographic dependence of the lateral undercut wet etching rate of InGaP in HCl

The crystallographic dependence of the lateral etch rate in 12 M HCl of InGaP lattice matched to GaAs has been measured. The etch rate at 20 °C is found to have twofold rotational symmetry about [100] and varies between <0.01 μm/min for mesas oriented along 〈011〉 directions and ∼0.9 μm/min for mesas 55° and 125° from [011] towards [011]. Etch fronts consist of {111}A planes. The etch rate also depends on the direction of etch step flow, suggesting that reconstruction plays an important role during InGaP wet etching.

High-Speed MEMS Micromirror Switching

We report a high-speed MEMS micromirror that switches in 225 ns using 22 V. Switch repetition rates of up to 100 kHz have been demonstrated. These performance characteristics significantly extend the application space of micromirrors.

Active zoom imaging for operationally responsive space

Deployment costs of large aperture systems in space or near-space are directly related to the weight of the system. In order to minimize the weight of conventional primary mirrors and simultaneously achieve an agile system that is capable of a wider field-of-view (FOV) and true optical zoom without macroscopic moving parts, we are proposing a revolutionary alternative to conventional zoom systems where moving lenses/mirrors and gimbals are replaced with lightweight carbon fiber reinforced polymer (CFRP) variable radius-of-curvature mirrors (VRMs) and MEMS deformable mirrors (DMs). CFRP and MEMS DMs can provide a variable effective focal length, generating the flexibility in system magnification that is normally accomplished with mechanical motion. By adjusting the actuation of the CFRP VRM and MEMS DM in concert, the focal lengths of these adjustable elements, and thus the magnification of the whole system, can be changed without macroscopic moving parts on a millisecond time scale. In addition, adding optical tilt and higher order aberration correction will allow us to image off-axis, providing additional flexibility. Sandia National Laboratories, the Naval Research Laboratory, Narrascape, Inc., and Composite Mirror Applications, Inc. are at the forefront of active optics research, leading the development of active systems for foveated imaging, active optical zoom, phase diversity, and actively enhanced multi-spectral imaging. Integrating active elements into an imaging system can simultaneously reduce the size and weight of the system, while increasing capability and flexibility. In this paper, we present recent progress in developing active optical (aka nonmechanical) zoom and MEMS based foveated imaging for active imaging with a focus on the operationally responsive space application.

High-speed, sub-pull-in voltage MEMS switching.

We have proposed and demonstrated MEMS switching devices that take advantage of the dynamic behavior of the MEMS devices to provide lower voltage actuation and higher switching speeds. We have explored the theory behind these switching techniques and have demonstrated these techniques in a range of devices including MEMS micromirror devices and in-plane parallel plate MEMS switches. In both devices we have demonstrated switching speeds under one microsecond which has essentially been a firm limit in MEMS switching. We also developed low-loss silicon waveguide technology and the ability to incorporate high-permittivity dielectric materials with MEMS. The successful development of these technologies have generated a number of new projects and have increased both the MEMS switching and optics capabilities of Sandia National Laboratories.

Stress and curvature in MEMS mirrors

The goal of this study is to understand how to optimize the performance of micro-mirrors for a variety of optical microsystem applications. Our approach relies on a number of process variations and mirror designs to ultimately produce relatively large (500μm to mm-scale), smooth (for nm RMS), and flat mirrors (greater than 1m curvature). White-light interferometric measurements, and finite element models are discussed in support of these findings. Stress gradients and residual stresses have been measured for accurate modeling of micro-mirrors. Through this modeling study, we have identified relevant structural parameters that will optimize SUMMiT V MEMS mirrors for optical applications. Ways of mitigating surface topography, print-through effects, and RMS roughness are currently being investigated.

High optical power handling of pop-up microelectromechanical mirrors

Summary form only given. Several applications of microelectromechanical systems (MEMS) require handling of large optical powers. One specific example includes steering or switching of an optical beam onto a photovoltaic device. In this way MEMS can be remotely powered with an incident high power optical beam. Photovoltaic cells integrated with MEMS can then convert the optical energy into electrical energy necessary to power the MEMS. In this paper we describe the optical damage mechanisms, as well as means of extending the optical damage threshold in order to handle 1 W of continuous wave incident power in the near infrared regime.

Experimental and computational study on laser heating of surface micromachined cantilevers

Microsystems are potentially exposed to laser irradiation during processing, diagnostic measurements, and, in some cases, device operation. The behavior of the components in an optical MEMS device that are irradiated by a laser needs to be optimized for reliable operation. Utilizing numerical simulations facilitates design and optimization. This paper reports on experimental and numerical investigations of the thermomechanical response of polycrystalline silicon microcantilevers that are 250 μm wide, 500 μm long, and 2.25 μm thick when heated by an 808 nm laser. At laser powers above 400 mW significant deflection is observed during the laser pulse using a white light interferometer. Permanent deformation is detected at laser powers above 650 mW in the experiments. Numerical calculations using a coupled physics finite element code, Calagio, agree qualitatively with the experimental results. Both the experimental and numerical results reveal that the initial stress state is very significant. Microcantilevers deflect in the direction of their initial deformation upon irradiation with a laser.