Thomas G. Bifano

Microelectromechanical deformable mirrors

A new class of silicon-based deformable mirrors is described. These devices are capable of correcting time-varying aberrations in imaging or beam forming applications. Each mirror is composed of a flexible silicon membrane supported by an underlying array of electrostatic parallel plate actuators. All structural and electronic elements were fabricated through conventional surface micromachining using polycrystalline silicon thin films. A layout and fabrication design strategy for reducing nonplanar topography in multilayer micromachining was developed and used to achieve nearly flat membrane surfaces. Several deformable mirrors were characterized for their electromechanical performance. Real-time correction of optical aberrations was demonstrated using a single mirror segment connected to a closed-loop feedback control system. Undesirable mirror contours caused by residual stress gradients in the membrane were observed.

Continuous-membrane surface-micromachined silicon deformable mirror

The authors describe the development of a new type of micromachined device designed for use in correcting optical aberrations. A nine-element continuous deformable mirror was fabricated using surface micromachining. The electromechanical behavior of the deformable mirror was measured. A finite-difference model for predicting the mirror deflections was developed. In addition, novel fabrication techniques were developed to permit the production of nearly planar mirror surfaces.

Development of a MEMS microvalve array for fluid flow control

A microelectromechanical system (MEMS) microvalve array for fluid flow control is described. The device consists of a parallel array of surface-micromachined binary microvalves working cooperatively to achieve precision how control on a macroscopic level. Flow rate across the microvalve array is proportional to the number of microvalves open, yielding a scalable high-precision fluidic control system. Device design and fabrication, using a one-level polycrystalline silicon surface-micromachining process combined with a single anisotropic bulk etching process are detailed. Performance measurements on fabricated devices confirm feasibility of the fluidic control concept and robustness of the electromechanical design. Air-flow rates of 150 ml/min for a pressure differential of 10 kPa were demonstrated. Linear flow control was achieved over a wide range of operating flow rates. A continuum fluidic model based on incompressible low Reynolds number flow theory was implemented using a finite-difference approximation. The model accurately predicted the effect of microvalve diaphragm compliance on flow rate. Excellent agreement between theoretical predictions and experimental data was obtained over the entire range of flow conditions tested experimentally.

Elimination of stress-induced curvature in thin-film structures

Argon ion machining of released thin-film devices is shown to alter the contour shape of free-standing thin-film structures by affecting their through-thickness stress distributions. In experiments conducted on MEMS thin-film mirrors it is demonstrated that post-release out-of-plane deformation of such structures can be reduced using this ion beam machining method. In doing so optically flat surfaces (curvature <0.001 mm/sup -1/) are achieved on a number of 3 /spl mu/m-thick surface micromachined silicon structures, including mirrors with either initially positive curvature or initially negative curvature measuring up to 0.02 mm/sup -1/. An analytical model incorporating the relevant mechanics of the problem is formulated and used to provide an understanding of the mechanisms behind the planarization process based on ion machining. The principal mechanisms identified are 1) amorphization of a thin surface layer due to ion beam exposure and 2) gradual removal of stressed material by continued exposure to the ion beam. Curvature history predictions based on these mechanisms compare well with experimental observations.

Management of R&D projects under uncertainty: a multidimensional approach to managerial flexibility

In this paper, we describe the practical application of a flexibility-based management approach to new product development, highlighting advantages, and limitations of this methodology. The model is concerned with the resolution of uncertainty over the product development life cycle and deals with technical, market, and cost factors all together. To this end, we consider a real options model, which uses multidimensional decision trees, to assess the development process of a high-technology product, namely, the Adaptive Optics Scanning Laser Ophthalmoscope. Moreover, we show how this project could be managed by estimating its value and determining optimal managerial actions to be taken at each review stage of the new product development process. Finally, we draw conclusions about this models general utility and particular challenges associated with its use as a product development tool, and emphasize the need to consider a multidimensional model, instead of a single dimensional one.

Adaptive optic correction using microelectromechanical deformable mirrors

A micromachined deformable mirror (?-DM) for optical wavefront correction is described. Design and manufacturing approaches for ?-DMs are detailed. The ?-DM employs a flexible silicon membrane supported by mechanical attachments to an array of electrostatic parallel plate actuators. Devices are fabricated through surface micromachining using polycrystalline silicon thin films. ?-DM membranes measuring 2 mmx2 mmx2 ?m, supported by 100 actuators are described. Figures of merit include stroke of 2 ?m, resolution of 10 nm, and frequency bandwidth dc to 7 kHz in air. The devices are compact, inexpensive to fabricate, exhibit no hysteresis, and use only a small fraction of the power required for conventional DMs. Performance of an adaptive optics system using a ?-DM is characterized in a closed-loop control experiment. Significant reduction in quasistatic wavefront phase error is achieved. Advantages and limitations of ?-DMs are described in relation to conventional adaptive optics systems and to emerging applications of adaptive optics such as high-resolution correction, small-aperture systems, and optical communication.

Focusing through dynamic scattering media

We demonstrate steady-state focusing of coherent light through dynamic scattering media. The phase of an incident beam is controlled both spatially and temporally using a reflective, 1020-segment MEMS spatial light modulator, using a coordinate descent optimization technique. We achieve focal intensity enhancement of between 5 and 400 for dynamic media with speckle decorrelation time constants ranging from 0.4 seconds to 20 seconds. We show that this optimization approach combined with a fast spatial light modulator enables focusing through dynamic media. The capacity to enhance focal intensity despite transmission through dynamic scattering media could enable advancement in biological microscopy and imaging through turbid environments.

Contouring algorithm for ion figuring

Ion beam figuring provides a highly deterministic method for the final precision figuring of optical components with advantages over conventional methods. The ion-figuring process involves bombarding a component with a stable beam of accelerated particles, which selectively removes material from the surface. The specific figure corrections are achieved by rastering the fixed-current beam across the workpiece at varying velocities. Unlike conventional methods, ion figuring is a noncontact technique that avoids such problems inherent in traditional fabrication processes as edge roll-off effects, tool wear, and force loading of the work piece. Other researchers have demonstrated that ion beam figuring is effective for correcting of large optical components. This work is directed toward the development of the precision ion-machining system (PIMS) at NASAs Marshall Space Flight Center (MSFC). This system is designed for processing small (~ 10 cm diameter) optical components. The ion-figuring process involves obtaining an interferometric error map of the surface, choosing a raster pattern for the beam, and determining the velocities along that path. Because the material removed is the convolution of the fixed ion beam removal and the rastering velocity as a function of position, determining the appropriate velocities from the desired removal map and the known ion beam profile is a deconvolution process. A unique method of performing this deconvolution was developed for the project, which is also applicable to other mathematically similar processes, including computer-controlled polishing. This paper presents the deconvolution algorithm, a comparison of this technique with other methods, and a simulation analysis. Future research will implement this procedure at MSFC.

In vivo fluorescent imaging of the mouse retina using adaptive optics

In vivo imaging of the mouse retina using visible and near infrared wavelengths does not achieve diffraction-limited resolution due to wavefront aberrations induced by the eye. Considering the pupil size and axial dimension of the eye, it is expected that unaberrated imaging of the retina would have a transverse resolution of 2 microm. Higher-order aberrations in retinal imaging of human can be compensated for by using adaptive optics. We demonstrate an adaptive optics system for in vivo imaging of fluorescent structures in the retina of a mouse, using a microelectromechanical system membrane mirror and a Shack-Hartmann wavefront sensor that detects fluorescent wavefront.

Ion beam figuring of small optical components

Ion beam figuring provides a highly deterministic method for the final precision figuring of optical components with advantages over conventional methods. The process involves bombarding a component with a stable beam of accelerated particles that selectively removes material from the surface. Figure corrections are achieved by rastering the fixed-current beam across the workpiece at appropriate, time-varying yelocities. Unlike conventional methods, ion figuring is a noncontact technique and thus avoids such problems as edge rolloff effects, tool wear, and force loading of the workpiece. This work is directed toward the development of the precision ion machining system at NASAs Marshall Space Flight Center. This system is designed for processing small (≈ 10-cm diam) optical components. Initial experiments were successful in figuring 8-cm-diam fused silica and chemical-vapor-deposited SiC samples. The experiments, procedures, and results of figuring the sample workpieces to shallow spherical, parabolic (concave and convex), and non-axially-symmetric shapes are discussed. Several difficulties and limitations encountered with the current system are discussed. The use of a 1-cm aperture for making finer corrections on optical components is also reported.