Microelectromechanical Systems Deformable Mirror Research Articles

MEMS Enabled Miniature Two-Photon Microscopy for Biomedical Imaging

Over the last decade, two-photon microscopy (TPM) has been the technique of choice for in vivo noninvasive optical brain imaging for neuroscientific study or intra-vital microendoscopic imaging for clinical diagnosis or surgical guidance because of its intrinsic capability of optical sectioning for imaging deeply below the tissue surface with sub-cellular resolution. However, most of these research activities and clinical applications are constrained by the bulky size of traditional TMP systems. An attractive solution is to develop miniaturized TPMs, but this is challenged by the difficulty of the integration of dynamically scanning optical and mechanical components into a small space. Fortunately, microelectromechanical systems (MEMS) technology, together with other emerging micro-optics techniques, has offered promising opportunities in enabling miniaturized TPMs. In this paper, the latest advancements in both lateral scan and axial scan techniques and the progress of miniaturized TPM imaging will be reviewed in detail. Miniature TPM probes with lateral 2D scanning mechanisms, including electrostatic, electromagnetic, and electrothermal actuation, are reviewed. Miniature TPM probes with axial scanning mechanisms, such as MEMS microlenses, remote-focus, liquid lenses, and deformable MEMS mirrors, are also reviewed.

Optical calibration and first light for the deformable mirror demonstration mission CubeSat (DeMi)

Microelectromechanical systems (MEMS) deformable mirrors (DMs) can provide high-precision wavefront control with a small form-factor, low power device. This makes them a key technology option for future space telescopes requiring adaptive optics for high-contrast imaging of exoplanets with a coronagraph instrument. The Deformable Mirror Demonstration Mission (DeMi) CubeSat payload is a miniature space telescope designed to demonstrate MEMS DM technology in space for the first time. The DeMi payload contains a 50-mm primary mirror, an internal calibration laser source, a 140-actuator MEMS DM from Boston Micromachines Corporation, an image plane wavefront sensor, and a Shack–Hartmann wavefront sensor (SHWFS). The key DeMi payload requirements are to measure individual actuator wavefront displacement contributions to a precision of 12 nm and correct both static and dynamic wavefront errors in space to less than 100-nm RMS error. The DeMi mission will raise the technology readiness level of MEMS DM technology from a five to at least a seven for future space telescope applications. We summarize the DeMi optical payload design, calibration, optical diffraction model, alignment, integration, environmental testing, and preliminary data from in-space operations. Ground testing data show that the DeMi SHWFS can measure individual actuator deflections on the MEMS DM to within 10 nm of interferometric calibration measurements and can meet the 12-nm precision mission requirement for actuator deflection voltages between 0 and 120 V. Payload data from throughout environmental testing show that the MEMS DM and DeMi payload survived environmental testing and provides a valuable baseline to compare with space data. Initial data from space operations show the MEMS DM actuating in space with a median agreement between individual actuator measurements from space and equivalent ground testing data of 12 nm.

MEMS Deformable Mirrors for Space-Based High-Contrast Imaging

Micro-Electro-Mechanical Systems (MEMS) Deformable Mirrors (DMs) enable precise wavefront control for optical systems. This technology can be used to meet the extreme wavefront control requirements for high contrast imaging of exoplanets with coronagraph instruments. MEMS DM technology is being demonstrated and developed in preparation for future exoplanet high contrast imaging space telescopes, including the Wide Field Infrared Survey Telescope (WFIRST) mission which supported the development of a 2040 actuator MEMS DM. In this paper, we discuss ground testing results and several projects which demonstrate the operation of MEMS DMs in the space environment. The missions include the Planet Imaging Concept Testbed Using a Recoverable Experiment (PICTURE) sounding rocket (launched 2011), the Planet Imaging Coronagraphic Technology Using a Reconfigurable Experimental Base (PICTURE-B) sounding rocket (launched 2015), the Planetary Imaging Concept Testbed Using a Recoverable Experiment - Coronagraph (PICTURE-C) high altitude balloon (expected launch 2019), the High Contrast Imaging Balloon System (HiCIBaS) high altitude balloon (launched 2018), and the Deformable Mirror Demonstration Mission (DeMi) CubeSat mission (expected launch late 2019). We summarize results from the previously flown missions and objectives for the missions that are next on the pad. PICTURE had technical difficulties with the sounding rocket telemetry system. PICTURE-B demonstrated functionality at >100 km altitude after the payload experienced 12-g RMS (Vehicle Level 2) test and sounding rocket launch loads. The PICTURE-C balloon aims to demonstrate contrast using a vector vortex coronagraph, image plane wavefront sensor, and a 952 actuator MEMS DM. The HiClBaS flight experienced a DM cabling issue, but the 37-segment hexagonal piston-tip-tilt DM is operational post-flight. The DeMi mission aims to demonstrate wavefront control to a precision of less than 100 nm RMS in space with a 140 actuator MEMS DM.

Development and characterization of a 140-element MEMS deformable mirror

For satisfying the boarder application requirement of adaptive optics (AO) and solving the problem of large volume and high cost of conventional deformable mirrors (DM), micro DM based on micro-electro-mechanical system (MEMS) technology is developed and measured. The developed DM has 140 hexagonal parallel plate capacitor electrostatic actuators. The actuators are arranged as a square array and the pitch is 400 μm. A DM prototype is fabricated by MEMS surface micromachining process and packaged by a ceramic pin grid array (CPGA). A miniaturization multi-channel high voltage driver for the DM is developed too. The measurement results show that the prototype has a surface PV value of 411 nm, RMS value of 78 nm, reflectivity of about 80% in 600 nm to 900 nm wavelength, stroke of 1.8 μm, actuator coupling of 15%, working bandwidth of 13 kHz and step response time of 23 μs. Thus the DM has the advantages of small volume, low cost and fast response. Besides the measurement of single element, the whole DM is controlled open loop to fit Zernike aberration and its fitting capability is demonstrated. Above results indicate that the DM prototype can satisfy initially the requirement of AO system.

Wavefront sensing in space: flight demonstration II of the PICTURE sounding rocket payload

A NASA sounding rocket for high-contrast imaging with a visible nulling coronagraph, the PICTURE payload, has made two suborbital attempts to observe the warm dust disk inferred around Epsilon Eridani. The first flight in 2011 demonstrated a 5 milliarcsecond fine pointing system in space. The reduced flight data from the second launch, on 25 November 2015, presented herein, demonstrate active sensing of wavefront phase in space. Despite several anomalies in flight, post-facto reduction phase stepping interferometer data provides insight into the wavefront sensing precision and the system stability for a portion of the pupil. These measurements show the actuation of a 32$\times$32-actuator microelectromechanical system deformable mirror. The wavefront sensor reached a median precision of 1.4 nanometers per pixel, with 95% of samples between 0.8 and 12.0 nanometers per pixel. The median system stability, including telescope and coronagraph wavefront errors other than tip, tilt, and piston, was 3.6 nanometers per pixel, with 95% of samples between 1.2 and 23.7 nanometers per pixel.

Surface micromachined MEMS deformable mirror based on hexagonal parallel-plate electrostatic actuator

Deformable mirrors (DM) based on microelectromechanical system (MEMS) technology are being applied in adaptive optics (AO) system for astronomical telescopes and human eyes more and more. In this paper a MEMS DM with hexagonal actuator is proposed and designed. The relationship between structural design and performance parameters, mainly actuator coupling, is analyzed carefully and calculated. The optimum value of actuator coupling is obtained. A 7-element DM prototype is fabricated using a commercial available standard three-layer polysilicon surface multi-user-MEMS-processes (PolyMUMPs). Some key performances, including surface figure and voltage-displacement curve, are measured through a 3D white light profiler. The measured performances are very consistent with the theoretical values. The proposed DM will benefit the miniaturization of AO systems and lower their cost.

Impact of large field angles on the requirements for deformable mirror in imaging satellites

For certain imaging satellite missions, a large aperture with wide field-of-view is needed. In order to achieve diffraction limited performance, the mirror surface Root Mean Square (RMS) error has to be less than 0.05 waves. In the case of visible light, it has to be less than 30 nm. This requirement is difficult to meet as the large aperture will need to be segmented in order to fit inside a launch vehicle shroud. To reduce this requirement and to compensate for the residual wavefront error, Micro-Electro-Mechanical System (MEMS) deformable mirrors can be considered in the aft optics of the optical system. MEMS deformable mirrors are affordable and consume low power, but are small in size. Due to the major reduction in pupil size for the deformable mirror, the effective field angle is magnified by the diameter ratio of the primary and deformable mirror. For wide field of view imaging, the required deformable mirror correction is field angle dependant, impacting the required parameters of a deformable mirror such as size, number of actuators, and actuator stroke. In this paper, a representative telescope and deformable mirror system model is developed and the deformable mirror correction is simulated to study the impact of the large field angles in correcting a wavefront error using a deformable mirror in the aft optics.

Dynamic performance of microelectromechanical systems deformable mirrors for use in an active/adaptive two-photon microscope.

Active optics such as deformable mirrors can be used to control both focal depth and aberrations during scanning laser microscopy. If the focal depth can be changed dynamically during scanning, then imaging of oblique surfaces becomes possible. If aberrations can be corrected dynamically during scanning, an image can be optimized throughout the field of view. Here, we characterize the speed and dynamic precision of a Boston Micromachines Corporation Multi-DM 140 element aberration correction mirror and a Revibro Optics 4-zone focus control mirror to assess suitability for use in an active and adaptive two-photon microscope. Tests for the multi-DM include both step response and sinusoidal frequency sweeps of specific Zernike modes (defocus, spherical aberration, coma, astigmatism, and trefoil). We find wavefront error settling times for mode amplitude steps as large as 400 nm to be less than 52???s, with 3 dB frequencies ranging from 6.5 to 10 kHz. The Revibro Optics mirror was tested for step response only, with wavefront error settling time less than 80???s for defocus steps up to 3000 nm, and less than 45???s for spherical aberration steps up to 600 nm. These response speeds are sufficient for intrascan correction at scan rates typical of two-photon microscopy.

Wavefront control in space with MEMS deformable mirrors for exoplanet direct imaging

To meet the high contrast requirement of 1×10 −10 to image an Earth-like planet around a sun-like star, space telescopes equipped with coronagraphs require wavefront control systems. Deformable mirrors (DMs) are a key element of a wavefront control system, as they correct for imperfections, thermal distortions, and diffraction that would otherwise corrupt the wavefront and ruin the contrast. The goal of the CubeSat DM technology demonstration mission is to test the ability of a microelectromechanical system (MEMS) DM to perform wavefront control on-orbit on a nanosatellite platform. We consider two approaches for an MEMS DM technology demonstration payload that will fit within the mass, power, and volume constraints of a CubeSat: (1) a Michelson interferometer and (2) a Shack-Hartmann wavefront sensor. We clarify the constraints on the payload based on the resources required for supporting CubeSat subsystems drawn from subsystems that we have developed for a different CubeSat flight project. We discuss results from payload laboratory prototypes and their utility in defining mission requirements.

High-stroke, high-order MEMS deformable mirrors

Monolithic fabrication of continuous facesheet high-aspect ratio gold microelectromechanical systems (MEMS) deformable mirrors (DMs) onto a thermally matched ceramic-glass substrate (WMS-15) has been performed. The monolithic process allows thick layer deposition (tens of microns) of sacrificial and structural materials thus allowing high-stroke actuation to be achieved. The fabrication process does not require wafer bonding to achieve high aspect ratio three-dimensional structures. A gold continuous facesheet mirror with 3.4 nm surface roughness has been deposited on a 16 × 16 array of X-beam actuators on a 1-mm pitch. A stroke of 6.4 μm was obtained when poking two neighboring actuators. Initial electrostatic actuation displacement results for a high-aspect ratio gold MEMS DM with a continuous facesheet will be discussed. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JMM.12.3.033012)

Controlling a MEMS Deformable Mirror in a Miniature Auto-Focusing Imaging System

Conventional auto-focusing camera modules adjust their optical focus by moving lenses. Although motors, such as stepping motors or voice coil motors, can change the lens position with great precision and thus vary the image sharpness, large volume and excess power consumption remain critical issues for mobile device applications. This paper discusses an auto-focusing camera module that adopts a microelectromechanical system (MEMS) deformable mirror (DM) with variable focusing capability to replace mechanical motors. The DM used in this paper can provide 20 diopters of focusing capability, which is one order of magnitude higher than silicon-based DMs, so the autofocus module can be compact enough for mobile devices. Because of the characteristics of MEMS DMs, we proposed a “percentage-drop” searching algorithm to find a sharp image based on a focus value calculation. The average auto-focusing time is 45% faster than the global searching method.

Single-Crystal PMN-PT MEMS Deformable Mirrors

This paper describes microelectromechanical systems deformable mirrors (DMs) fabricated from Pb(Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</sub> Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sub> )O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -PbTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single crystal (PMN-PT) for use in ocular adaptive optics. The DM is a piezoelectric unimorph with 35 actuators on a 13-mm circular membrane. Each actuator inside the 8-mm pupil achieves a static stroke of over 5 μm at 10 V. Dynamic measurements prove that the DM can be operated up to a 2.27-kHz bandwidth. The large stroke with low driving voltage and high operating bandwidth confirm that the DM is a promising candidate for use as a wavefront corrector in vision science applications. The measured piezoelectric properties of the PMN-PT are in close agreement with factory specifications, demonstrating that the piezoelectric properties of single-crystal PMN-PT are not degraded by the bonding and lapping process used here. The large 13-mm-diameter 30-μm-thick membrane is produced by constructing silicon rings to protect the membrane from the considerable compressive stress present in the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer of a silicon-on-insulator wafer. In the prototype mirror, residual stress in the electrode metal results in an initial peak-to-valley surface flatness of 3.3 μm which is reduced to 0.7 μm by iterative computation of the control voltages applied to the electrodes.

High-aspect-ratio MEMS deformable mirrors for next-generation telescopes

The next generation of extremely large telescopes (ELTs) that will be built on a scale of 30–50 meters in diameter necessarily require an increased ability to correct wavefront errors. Currently, wavefront correction is achieved through the use of a deformable mirror (DM). The degree and amount of correction depends on amechanical displacement of segments of themirror (stroke), and the number of actuators in the mirror surface (order). An effective ELTwill require a high-stroke (10microns) and high-order (100X100 actuators) mirror, a costly prospect using current technology.1 Lowering the cost while improving the performance of DMs is possible using microelectromechanical systems (MEMS) technology. However, current surface machining processes used to fabricate MEMS deformable mirrors, such as the Sandia ultra-planar multilevel MEMS technology (SUMMiT) and MEMSCAP’s PolyMUMPS process, limit a DM’s stroke due to the use of thin-film sacrificial layers (which are dissolved away after the etching process to free up micromechanical components in the actuators).2, 3 Commercially available surface micromachined MEMS mirrors are limited in stroke to approximately 5.5 microns, about half of the stroke required by ELTs. A high aspect ratio fabrication process capable of depositing thick layers will allow the DM’s to provide both high-stroke and highorder corrections, thus bypassing the need for a complex dual woofer (high stroke, low order)/tweeter (high order, low stroke) DM configuration.4 In our work, we tested different actuator designs with a bonded faceplate constructed using the LIGA (German acronym for lithography, electroplating, and molding) process, enabling multilayer fabrication of MEMS devices.5 Various types of high-stroke gold actuators consisting of folded springs with Figure 1. Diagram showing the final release of the monolithic fabricated structure created using our technique. The gap for the Xbeam electrostatic actuators is 20 microns, enabling a mirror stroke of 10 microns. WMS-15: specific glass-ceramic substrate. Not to scale.

Design and fabrication of a large-stroke MEMS deformable mirror for wavefrontcontrol

Adaptive optics is a technology that improves the performance of optical systems byreducing wavefront distortion. Currently, it is playing a more important role in astronomy,laser physics, nonlinear optics, medicine, vision and the defense industry. In this paper, wedemonstrate a microelectromechanical system (MEMS) deformable mirror that is made of a2.15 µm thick polyimide film and is actuated by electrostatic force. We made a large-strokeMEMS deformable mirror with a 20 mm diameter circular opening and 67hexagonal actuation electrodes. We also used modeling software, ANSYS,to simulate the deformation behavior of the membrane and discussed thedevice parameter tuning for versatile applications. The maximum stroke was39 µm as 195 V was applied. Because of the large stroke of the device, the resonant frequency wasapproximately 40 Hz. The resonant frequency can be increased by thickening the polyimidemembrane. The polymer deformable mirror is a strong candidate for active wavefrontcontrol, based on our experimental results.

High-aspect-ratio microelectromechanical systems deformable mirrors for adaptive optics

Adaptive optics (AO) applications in astronomy and vision science require deformable mirrors (DMs) with high-stroke, high-order packing density at a lower cost than the currently available technology. The required AO specifications are achievable with microelectromechanical systems (MEMS) devices fabricated with LIGA (lithographie galvanofomung abformung) high-aspect-ratio processing techniques. Different actuator designs and a bonded faceplate fabricated in a LIGA process, enabling multilayer fabrication of MEMS devices, are investigated. Various types of high-stroke gold actuators for AO consisting of folded springs with rectangular and circular membranes as well as x-beam actuators supported diagonally by fixed-guided springs are designed, simulated, and fabricated individually and as part of a continuous-face-sheet DM system. The actuators and DM displacement versus voltage are measured with an interferometer and the corresponding results are compared to finite element analysis simulations. Simulations and interferometer scans show the ability of the actuators to achieve displacements of greater than 1/3 of the initial gap. A stroke of ~9.4 µm is achieved, thus showing that this fabrication process holds promise in the manufacture of future MEMS DMs for the next generation of extremely large telescopes.

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).

Thin autofocus camera module by a large-stroke micromachined deformable mirror

The conventional auto-focus and zoom image systems were made by a set of motor-moved lenses. Because of mechanical moving parts, it is not easy to miniaturize their sizes. In this paper, we propose a thin autofocus system using a large stroke MEMS (micro-electro-mechanical systems) deformable mirror which has the potential to downscale the size and to minimize chromatic aberration. The large stroke MEMS deformable mirror is made by a polyimide membrane that has a maximum 12 microm displacement over a 3 mm aperture. The module size is 5.4 mm thick in optical design layout and 6.7 mm after packaging. This autofocus system is designed with the f-number=4.13, on-axis MTF=0.28 at full frequency of 230 cycles/mm, and incident light within+/-26 degree. The position of clear image can vary from 4 cm to 50 cm achieved by controlling the surface curvature of the MEMS deformable mirror.

静电排斥型微机电系统变形镜驱动器

A novel microelectromechanical systems (MEMS) deformable mirror actuator based on electrostatic repulsive force is presented. The design is based on the principle that asymmetric electric field can generate repulsive force. The actuator can eliminate the pull-in effect which limits the reliability of the conventional electrostatic MEMS deformable mirrors. The actuator consists of five pairs of electrodes, the largest of which is composed of a central mass and a bottom electrode right below it, and the other four of which are positioned on each side of the actuator. The central mass is supported by four L-shaped springs which respectively connecting four anchors on the corners. The resonant frequency and transient response time are simulated by finite element analysis, and they are 4 kHz an

Automation of Challenging Spatial-Temporal Biomedical Observations With the Adaptive Scanning Optical Microscope (ASOM)

Biological studies, drug discovery, and medical diagnostics benefit greatly from automated microscope platforms that can outperform even the most skilled human operators in certain tasks. However, the small field-of-view of a traditional microscope operating at high resolution poses a significant challenge in practice. The common approach of using a moving stage suffers from relatively low dynamic bandwidth and agitation to the specimen. This paper describes an automated microscope station based on the novel adaptive scanning optical microscope (ASOM), which combines a high-speed post-objective scanning mirror, a custom design scanner lens, and a microelectromechanical systems (MEMS) deformable mirror to achieve a greatly expanded field-of-view. After describing the layout and operating principle of the ASOM imaging subsystem, we present a system architecture for an automated microscope system suitable for the ASOM's unique wide field and high-speed imaging capabilities. We then describe a low-cost experimental prototype of the ASOM that demonstrates all critical optical characteristics of the instrument, including the calibration of the MEMS deformable mirror. Finally, we present initial biological (living nematode worms) imaging results obtained with the experimental apparatus and discuss the impact of the ASOM on biomedical imaging activities.

4096-element continuous face-sheet MEMS deformable mirror for high-contrast imaging

We present the progress in the development of a 4096-element microelectromechanical systems (MEMS) deformable mirror, fabricated using polysilicon surface micromachining manufacturing processes, with 4 µm of stroke, a surface finish of 5 kHz. The packaging and high-speed drive electronics for this device, capable of frame rates of 22 kHz, are also presented.