The term Adaptive Optics (AO) describes the active control of an optical device to remove distortions caused by aberrations in an optical beam path. An AO system enables beam forming and image correction in the presence of distortions and atmospheric effects. Major obstacles in imaging through the atmosphere include extended source/target anisoplanatism, distributed strong turbulence, scintillation, and branch points. Many applications have requirements for which the generation of a wavefront sensing source via the projection of a laser is undesirable or unfeasible. A variety of AO compensation techniques exist and have been demonstrated in the field, each with specific merits and disadvantages. A survey of the many types of AO control is presented. Common AO techniques include Classic Adaptive Optics, Multi-Conjugate Adaptive Optics (MCAO), and Extended Source AO (also known as correlation wavefront sensing). More recent applications include Stochastic Parallel Gradient Descent control (SPGD) and a Holographic Phase Conjugate Engine that were developed to advance the state of the art AO control. Innovative variations on the Stochastic Parallel Gradient Descent AO and Extended Source (scene-based) AO algorithms hold significant promise for the future of AO.
The design, manufacture, and testing of optical quality surface micromachined deformable mirrors (DMs) is described. With such mirrors, the shape of the reflective surface can be modified dynami-cally to compensate for optical aberrations and thereby improve image resolution in telescopes or microscopes. Over several years, we have developed microelectromechanical system (MEMS) processing technologies that allow production of optical quality of surface micromachined mirrors. These process steps have been integrated with a commercial foundry process to produce deformable mirrors of unprecedented quality. The devices employ 140 electrostatic actuators. Measurements of their performance detailed in this paper include 2µm of useful stroke, 3nm position repeatability, >90% reflectivity, and flatness better than 20nm RMS. 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 mem-brane. An ion bombardment technique has been developed to flatten mirrors. These silicon based deformable mirrors have the potential to modulate spatial and temporal features of an optical wave-front, and have applications in imaging, beam-forming, and optical communication systems. Design considerations and performance evaluation of recently fabricated DMs are presented.
A large-scale, high speed, high resolution, phase-only microelectromechanical system (MEMS) spatial light modulator (SLM) has been fabricated. Using polysilicon thin film technology, the micro mirror array offers significant improvement in SLM speed in comparison to alternative modulator technologies. Pixel opto-electromechanical characterization has been quantified experimentally on large scale arrays of micro mirrors and results are reported.
Recent progress on deformable mirror systems made at Boston University and Boston Micromachines Corporation is described. The mirror's 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.
Design, microfabrication, and integration of a micromachined spatial light modulator ((mu) SLM) device are described. A large array of electrostatically actuated, piston-motion MEMS mirror segments make up the optical surface of the (mu) SLM. Each mirror segment is capable of altering the phase of reflected light by up to one wavelength for infrared illumination ((lambda) equals 1.5 micrometers ), with 4-bit resolution. The device is directly integrated with complementary metal- oxide semiconductor (CMOS) electronics, for control of spatial optical wavefront. Integration with electronics is achieved through direct fabrication of MEMS actuators and mirror structures on planarized foundry-type CMOS electronics. Technical approaches to two significant challenges associated with manufacturing the (mu) SLM is discussed: integration of the MEMS array with the electronic driver array and production of optical-quality mirror elements using a metal-polymer surface micromachining process.
KEYWORDS: Spatial light modulators, Mirrors, Actuators, Electrodes, Microelectromechanical systems, Etching, Deep reactive ion etching, Analog electronics, Semiconducting wafers, Thin films
This paper presents a high-speed resolution phase-only microelectromechanical system (MEMS) spatial light modulator (SLM), integrated with driver electronics, using through- wafer vias and solder bump bonding. It employs a polysilicon thin film MEMS technology that is well suited to micromirror array fabrication and offers significant improvement in SLM speed in comparison to alternative modulator technologies. Vertical through-wafer interconnections offer scalability required to achieve 1M-pixel array size. The design, development, fabrication and characterization of a scalable driver integrated SLM is discussed. Pixel opto- electromechanical performance has been quantified experimentally on prototypes, and results are reported.
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.
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.
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.
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.
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