Julie A. Perreault

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.

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.

Differential capacitive position sensor for planar MEMS structures with vertical motion

A capacitive sensor has been developed for measuring the vertical deflection of bridge-type micro-electromechanical (MEMS) silicon actuators. The sensor requires no electrodes above the actuator surface and does not require the actuator diaphragm to be used as a signal electrode. Sets of interdigitated electrodes, one for ac signal injection and the other for signal sensing, are placed beneath the actuator membrane. As the actuator deflects, the capacitance between the interdigitated finger electrodes is altered, leading to a change in the time-varying charge induced on the sense fingers. This change in induced charge is monitored by a current-to-voltage converter, thereby providing a measure of actuator displacement in the direction perpendicular to the silicon substrate. Signal voltages on the order of 10 mV per 1 μm of deflection are observed for deflections in the 1-μm range.

Micromachined Deformable Mirrors for Adaptive Optics

Recent progress on deformable mirror systems made at Boston University and Boston Micromachines Corporation is described. The mirrors optical, electrical, and mechanical performance characteristics are summarized, and the effects of air damping on performance are described. Two applications that have employed the μDM in laser communications and retinal imaging are introduced.

REAL TIME OPTICAL CORRECTION USING ELECTROSTATICALLY ACTUATED MEMS DEVICES

Abstract This paper describes an optical correction system made from electrostatically actuated, surface machined micro-electromechanical (MEMS) mirrors. Such MEMS mirrors have applications in optical systems where they are used to correct wavefront aberrations and other image distortions. In our experiments, electrostatic actuators having a maximum surface-normal stroke of 2.5 μm control the individual orientations of each element in an array of 300-μm square mirror segments in the tip-tilt mode. Real time correction of random optical aberrations is demonstrated using a single mirror segment with four independently mounted corners and closed-loop feedback control.

Micromachined deformable mirror for optical wavefront compensation

A silicon micromachined deformable mirror ((mu) DM) has been developed by Boston University and Boston Micromachines Corporation (BMC). The (mu) DM employs a flexible silicon mirror supported by mechanical attachments to an array of electrostatic parallel plate actuators. The integrated system of mirror and actuators was fabricated by surface micromachining using polycrystalline silicon thin films. The mirror itself measures 3 mm X 3 mm X 3 micrometer, supported by a square array of 140 electrostatic parallel- electrode actuators through 140 attachment posts. Recently, this (mu) DM was characterized for its electro-mechanical and optical behavior and then integrated into two laboratory-scale adaptive optics systems as a wavefront correction device. Figures of merit for the system include stroke of 2 micrometer, resolution of 10 nm, and frequency bandwidth of 6.7 kHz. The device is compact, exhibits no hysteresis, and has good optical quality.

Adaptive optic correction using silicon-based deformable mirros

A micromachined deformable mirror ((mu) -DMs) for optical wavefront correction is described. Design and manufacturing approaches for (mu) -DMs are detailed. The (mu) -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. (mu) -DM membranes measuring 2 mm X 2 mm X 2 micrometers , supported by 100 actuators are described. Figures of merit include stroke of 2 micrometers , resolution of 10 nm, and frequency bandwidth DC - 7 kHz. 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 (mu) - DM was characterized in a closed-loop control experiment. Significant reduction in quasi-static wavefront phase error was achieved. Advantages and limitations of (mu) -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.

Manufacturing of an optical-quality mirror system for adaptive optics

Manufacturing of optical quality micromachined deformable mirrors for use in adaptive optic (AO) correction is described. Several non-standard manufacturing techniques have been developed to improve optical quality of surface micromachined mirrors. Two challenges to manufacturing optical quality micromachined mirrors are reducing surface roughness and increasing reflectivity. A chemo-mechanical polishing process has been used to improve surface quality of the mirrors, and a gold coating process has been developed to improve the reflectivity without introducing a significant amount of stress in the mirror membrane. Surface reflectivity and topography measurements of optically flat and smooth mirrors are presented. Based on these results, a new 1024 actuator mirror has been designed and is currently being fabricated. Design considerations and performance expectations for this mirror will be presented.

Compact adaptive optical compensation systems using continuous silicon deformable mirrors

Deformable mirrors have been fabricated using microelectromechanical system (MEMS) technology. The mirrors have been integrated into an optical test bed capable of generating static and dynamic aberrations in the beam path. It was found that the DM could be used to improve optical system resolution in the presence of static aberrations. Strehl ratio was measured for the optical system under four test conditions. A Strehl ratio of 0.81 was obtained for the case in which an introduced aberration was compensated by the DM, compared to a Strehl ratio of 0.45 for case in which the aberration was uncompensated and the DM was removed from the optical path. A parallel stochastic gradient descent approach was used for control.