Segmented silicon-micromachined microelectromechanical deformablemirrors for adaptive optics

Deformable mirrors improve optical efficiency of a system by correcting the wave front aberration caused by imperfections in the system components or by turbulent atmosphere in case of telescope optics. Micromachined mirror technology has the potential to substantially reduce the cost of adaptive optics systems. First, a brief review of the work in this field is presented with the goal of informing the reader of the challenges in the micromachined adaptive optics and the implementation tradeoffs including stress-induced curvature of multilayer mirrors. Then, recent results on the silicon micromachined, hybrid integrated microelectromechanical deformable mirrors for adaptive optics developed at the University of Colorado are presented. Various microfabrication processes including surface micromachining, bulk micromachining, and flip-chip assembly are implemented to fabricate high optical fill factor and large-stroke piston-type micromirror arrays. The achieved micromirror deflection for some designs is in the range of 2 to 3.5 μm, which results in the operating wavelength within infrared spectrum. Techniques to integrate microlenses on top of the micromirrors using self-aligned solder or transfer of ultrasmooth mirror plates on top of the micromirror actuators using flip-chip create high optical fill factor devices. Experimental results of aberration correction with such devices are presented     

Electrostatically operated micromirrors for Hadamard transformspectrometer

The paper presents the development of a linear micromirror array which can be used as a switchable entrance mask for a double-array Hadamard transform spectrometer. In addition to the detector array, the double-array spectrometer has a linear multislit array realized by independently switchable micromirrors at the entrance side. Two different switch positions of the electrostatically operated mirrors allow the reflection of light into or away from the spectrometer. With this arrangement (mirror array, concave grating, and array detector) and the use of the Hadamard transform principle it is possible to increase the signal-to-noise ratio and the resolution of the system compared to conventional spectrometers of the same size. In order to use a micromirror array as an entrance mask the dimensions of the switching elements must be similar to those of entrance slits and the mirror surface must have a high reflectivity in the desired spectral range. To obtain a high resolution a given pitch between the mirrors must be realized. Using thin-film technology we developed a linear array with 18 tiltable mirrors. The mirrors consist of a multilayer system of aluminum and silicon nitride deposited on a structured sacrificial layer of polyimide. The aluminum has a comparatively high reflectivity in the UV/VIS region and the silicon nitride yields good mechanical properties for realizing stress reduced mirrors and bending structures. After etching of the sacrificial layer, the mirrors can be switched electrostatically. The angle between on and off position is about 5°. Low switching voltages in the range of 15-45 V can be realized     

Development of piezoelectric actuators for active X-ray optics

Piezoelectric actuators are widely utilised in adaptive optics to enable mirrors having an actively controlled reflective surface for the purpose of the wavefront correction by reducing the effects of rapidly changing optical distortion. Two new prototype adaptive X-ray optical systems are under development with the aim of approaching the fundamental diffraction limit. One proposed technology is microstructured optical arrays (MOAs) involving two or four piezoelectric strips bonded to a silicon wafer to produce a micro-focused X-ray source for biological applications, and which uses grazing incidence reflection through consecutive aligned arrays of channels obtained using deep silicon etching. Another technology is large scale optics which uses a thin shell mirror bonded with 20?C40 piezoelectric actuators for the next generation of X-ray telescopes with an aim to achieve a resolution greater than that currently available by Chandra (0.5"). PZT-based piezoelectric actuators are being developed in this programme according to the design and implementation of the proposed mirror and array structures. Viscous plastic processing is chosen for the preparation of the materials system, which is subsequently formed and shaped into the suitable configurations. Precise controls on the thickness, surface finish and the curvature are the key factors to delivering satisfactory actuators. Unimorph type piezoelectric actuators have been proposed for the applications and results are presented regarding the fabrication and characterisation of such piezo-actuators, as well as the related design concepts and comparison to modelling work.     

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     

Arrays of High Tilt-Angle Micromirrors for Multiobject Spectroscopy

Micromirror arrays are promising components for generating reflective slit masks in future multiobject spectrographs. The micromirrors, 100 mum times200 mum in size, are etched in bulk single crystal silicon, whereas a hidden suspension is realized by surface micromachining. The micromirrors are actuated electrostatically by electrodes located on a second chip. The use of silicon on insulator (SOI) wafers for both mirror and electrode chip ensures thermal compatibility for cryogenic operation. A system of multiple landing beams has been developed, which latches the mirror at a well-defined tilt angle when actuated. Arrays of 5times5 micromirrors have been realized. The tilt angle obtained is 20deg at a pull-in voltage of 90 V. Measurements with an optical profiler showed that the tilt angle of the actuated and locked mirror is stable with a precision of 1 arcmin over a range of 15 V. This locking system makes the tilt angle independent from process variations across the wafer and, thus, provides uniform tilt angle over the whole array. The surface quality of the mirrors in actuated state is better than 10-nm peak to valley and the local roughness is about 1-nm root mean square     

Adaptive Array of Phase-Locked Fiber Collimators: Analysis and Experimental Demonstration

We discuss development and integration of a coherent fiber array system composed of densely packed fiber collimators with built-in capabilities for adaptive wavefront phase piston and tilt control at each fiber collimator. In this system, multi-channel fiber-integrated phase shifters are used for phase locking of seven fiber collimators and the precompensation of laboratory-generated turbulence-induced phase aberrations. Controllable x and y displacements of the fiber tips in the fiber collimator array provide additional adaptive compensation of the tip and tilt phase aberration components. An additional control system is utilized for equalization of the intensity of each of the fiber collimator beams. All three control systems are based on the stochastic parallel gradient descent optimization technique. The paper presents the first experimental results of adaptive dynamic phase distortion compensation with an adaptive phase-locked fiber collimator array system.     

Moving forward in retrodirective antenna arrays

Antenna arrays are individual radiating elements combined so that they function and perform like a single large antenna. Besides high radiating beam directionality, antenna arrays offer the capability to scan the main beam and control sidelobe radiation electronically. Retrodirective antenna arrays, as the name suggests, have a special feature. When receiving a signal from an unspecified direction, the array can automatically transmit a signal response to that same direction without any previous knowledge of the source direction. This function is performed automatically without the use of phase-shifters or digital circuitry. Compared to other array antennas that rely on digital signal processing (DSP) for beam direction control, this approach is much simpler and potentially faster because digital calculation is not needed. The automatic beam control nature of retrodirective arrays is well suited for RFID (radio frequency identification) and microwave tracking beacon applications. The article discusses the basic functionality as well as the ongoing research efforts and developments in retrodirective array architectures. Self-phasing attributes of different arrays are presented, as well as their applicability in practical communication systems.     

基于结构光测量技术的 DMD 自适应掩膜生成

为解决强反射表面物体测量过程中易发生局部镜面反射,影响测量精度的问题。将数字微镜添加到结构光测量光路中并设计搭建测量系统,包括数字微镜、CCD相机、投影仪等器件。完成了系统的匹配、相机和投影仪的参数标定以及相位计算,采用麻雀搜索算法优化BP神经网络的方法建立数字微镜单元和相机像素单元之间的坐标映射关系,映射误差为0.583 pixels。基于PID控制器提出一种自适应掩膜生成方法,并对具有强反射表面的量块进行了测量实验,实验表明该方法能有效降低过曝光区域的灰度,实现了高动态范围成像。提出的方法可为强反射表面的三维测量提供理论支撑。     

Single-Crystal-Silicon Continuous Membrane Deformable Mirror Array for Adaptive Optics in Space-Based Telescopes

In this paper, we present a single-crystal-silicon (SCS) continuous membrane deformable mirror (DM) as a corrective adaptive-optics (AO) element for space-based telescopes. In order to correct the polishing errors in large aperture (~8 m) primary mirrors, a separate high-quality surface DM array must be used. Up to 400000 elements and a mirror stroke of ~100 nm are required for the correction of these polishing errors. A continuous membrane mirror formed by the the SCS device layer of a silicon-on-insulator (SOI) wafer is used to achieve a high-quality optical surface and to minimize the additional diffractive effects in the optical system. To achieve substantial local deformation needed to correct high-order errors, we use a highly deformable silicon membrane of 300-nm thickness. This thin membrane is able to deform locally by 125 nm at an operating voltage of 100 V with a pixel pitch of 200 mum. The resonance frequency of a pixel is 25 kHz with a low Q-factor of 1.7 due to squeeze-film damping. The device is fabricated by processing the microelectromechanical system (MEMS) and electronic chips separately and then combining them by flip-chip bonding. This allows optimization of the MEMS and electronics separately and also allows the use of an SOI layer for the mirror by building the MEMS bottom up. A small prototype array of 5times5 pixels with 200-mum pitch is fabricated, and we demonstrate single pixel and multiple pixel actuation     

Mutual injection locking: a new architecture for high-power solid-state laser arrays

In this paper, bidirectional (mutual) injection locking is demonstrated with solid-state lasers, producing significant improvements over traditional single-direction injection locking. Each laser element shares part of its output with other elements in bidirectional locking, distinct from single-direction (traditional) injection locking where one master laser provides the locking signal for a number of slaves. In a phase-locked array, the individual laser outputs add coherently, and the brightness of the entire array scales with the square of the number of elements, as if the active material diameter were increasing. Benefits of bidirectional locking, when compared to traditional injection locking, include reduced laser threshold, better output beam quality, and improved scaling capability. Experiments using two Nd:YVO/sub 4/ lasers confirmed that mutual injection locking reduced lasing threshold by a factor of at least two and increased the output beam quality significantly. The injection-locking effects began with 0.03% coupling between lasers and full-phase locking for coupling exceeding 0.5%. The 0.5% requirement for full-phase locking is significantly lower than the requirement for traditional injection locking. The large coupling requirement limits traditional injection-locked arrays to fewer than 20 elements, whereas mutually injection-locked arrays have no such limit. Mutual injection locking of an array of lasers can lead to a new architecture for high-power laser systems.     

MTL‐based implementation of current‐mode CMOS RMS‐to‐DC converters

In this paper, systematic implementation of current‐mode RMS‐to‐DC converters based upon MOS translinear (MTL) principle, utilizing symmetric cascoded MTL cell (SCMC) is proposed. Theory of operation and mathematical analysis of both explicit (direct) and implicit (indirect) techniques for realization of SCMC‐based RMS‐to‐DC converters are discussed. The SCMC includes a folded MTL loop and realizes an MTL equation. MTL principle utilizes the square law characteristics of saturated MOS transistors to realize square‐root domain (SRD) functions. The SCMC is constructed by two connected cascoded current mirrors and has a compact, symmetric, and multi‐purpose structure, with capability of implementing the circuits into the programmable and configurable structures. The proposed RMS‐to‐DC converters utilize the SCMC along with a configurable current mirror array. The required squaring and square‐rooting functions are realized using the SCMC, after proper configuration of the current mirror array. The proposed circuits have been implemented using a reconfigurable architecture fabricated in a 0.5 µm CMOS technology. Copyright © 2014 John Wiley & Sons, Ltd.     

Monolithically integrated multiwavelength sampled grating DBRlasers for dense WDM applications

For accurate control of the channel spacing in fabricating multiwavelength laser arrays or discrete multicolor lasers, we proposed a novel approach that exploits sampled grating distributed Bragg reflector (DBR) mirrors to vary the laser wave length across the wafer. This approach can realize a set of lasers with a wavelength spacing that meets the ITU recommendations for dense wavelength-division multiplexing systems and a wavelength range that can cover up to 40 nm or more. The wavelength variation across an array is achieved by changing the sampling periods of the DBR mirrors from laser to laser. The accuracy on the channel spacing of sampled grating DBR laser arrays was shown to be the same as that of conventional distributed feedback or DBR laser arrays, but their wavelengths can be better controlled for the gratings are fabricated with single holographic exposure. Arrays of 21 lasers have been successfully fabricated and have around 0.8-nm wavelength spacing with a simple tuning mechanism     

Mathematical Modeling and Analysis of MEMS Deformable Mirror Actuated by Electrostatic Piston Array

In this report, we constructed a simple mathematical analysis model for a new MEMS deformable mirror using an electrostatic piston array. The deformable mirror has been developed as a wavefront compensation device in adaptive optics for retinal observation. The device realizes a large convex deformation with a low operation voltage because of moving bottom electrodes on the pistons. The constructed model is based on the plate theory and simple superposition of the actuation components. The calculated deformation analyzed using this model agrees well with that analyzed using the finite element method not only in deformation shape but the peak‐valley deformations. In addition, the calculation time is much shorter, so the model can be used for design optimization of the device.     

带支撑物圆阵波达方向估计实验研究

为了验证带支撑物圆阵波达方向估计算法的有效性,在介绍算法原理的基础上,利用设计的镶嵌在刚性球上的12元均匀圆阵,分别在消声室和外场进行了声源定向实验研究.定向结果显示消声室内平均估计误差不大于2°,均方根误差不大于2.2°,外场平均估计误差和均方根误差均不大于3°.实验表明基于带支撑物圆阵所推导的波达方向估计算法是有效的,能正确估计出声源的波达方向,且具有较高的精度,为利用带支撑物圆阵进行波达方向估计的工程应用奠定了基础.     

Materials,effects and components for tunable micro‐optics

An overview of recent activities in the area of tunable micro‐optical components is given. These include polymer‐based deformable mirrors for adaptive optics, tunable microlenses and arrays using fluids and membranes, pneumatically actuated scanning micromirrors and tunable Bragg filters and mirrors using swelling polymers. For each device, the structure, essential fabrication technology and optical characteristics, as well as a discussion of application areas are presented. Copyright © 2007 Institute of Electrical Engineers of Japan© 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

A Large-Area High-Reflectivity Broadband Monolithic Single-Crystal-Silicon Photonic Crystal Mirror MEMS Scanner With Low Dependence on Incident Angle and Polarization

In this paper, we introduce a single-axis resonant combdrive microelectromechanical systems (MEMS) scanner with a large-area highly reflective broadband monolithic single-crystal-silicon (SCS) photonic crystal (PC) mirror. PC mirrors can be made from a single monolithic piece of silicon through alternate steps of etching and oxidation. This process allows the fabrication of a stress-free PC reflector in SCS with better optical flatness than deposited films such as polysilicon slabs on low-index oxide. PC mirrors can be made in IR transparent dielectric material and can achieve high reflectivity over a broad wavelength range. PC reflectors have several advantages over other mirror technologies. They can tolerate much higher processing temperatures and higher incident optical powers as well as operate in more corrosive environments than metals. Compared to multilayer dielectric stacks, PC mirrors allow for simpler process integration, thus making them highly compatible with CMOS and MEMS processing. In this paper, we fabricate a PC mirror MEMS scanner in SCS without any deposited films. Our PC mirrors show broadband high reflectivity in the wavelength range from 1550 to 1600 nm, and very low angular and polarization dependence over this same range. The single-axis MEMS scanners are fabricated on silicon-on-insulator (SOI) wafers with the PC mirrors also fabricated in the SOI device layer. The scanners are actuated by electrostatic comb drives on resonance. Dynamic deflection measurements show that the scanners achieve 22deg total scan angle with an input square wave of 67 V and have a resonance frequency of 2.13 kHz.     

Current‐mode filters based on current mirror arrays

A technique is proposed for obtaining current‐mode filters based on current mirror arrays that operate as unity gain current amplifiers. These amplifiers by properly driving capacitors realize active lossless integrators which are the basic active elements for the derivation of filters according to the leapfrog method. Due to the fact that both the structure of the amplifiers and the adapted method for filter design are simple, the proposed technique is attractive for filter design and implementation. A design and the implementation of two third‐order low‐pass filters are presented. The array of the amplifiers has been implemented in a 0.8 µm CMOS technology. Copyright © 2007 John Wiley & Sons, Ltd.     

太阳能供电系统的多目标优化设计方法研究

为合理设计太阳能供电系统的蓄电池和太阳电池板阵列,提出一种综合考虑阵列经济性及供电可靠性的多目标优化设计方法。基于蓄电池充放电特性和太阳电池板发电特性,结合负载供电需求和蓄电池及太阳电池板成本,利用蓄电池温度修正系数、放电深度、充电效率和太阳能日辐射量及斜面修正系数,综合考虑两次最长连续阴雨天数及其最短间隔天数等,提出了评价阵列经济性的阵列总价和评价阵列供电可靠性的潜在故障概率,并进一步提出了综合评价阵列经济性和供电可靠性的阵列综合系数,从而确定经济可靠综合性能最优的蓄电池和太阳电池板阵列设计方案。     

Development of Deformable Mirror Composed of Piezoelectric Thin Films for Adaptive Optics

In this paper, we report a piezoelectric deformable mirror composed of piezoelectric thin films for low-voltage adaptive optics (AO). A 2-mum-thick piezoelectric Pb(Zr,Ti)O₃ (PZT) film was deposited on a Pt-coated silicon-on-insulator (SOI) substrate, and a diaphragm structure of 15 mm in diameter was fabricated by etching a Si handle wafer. A 19-element unimorph actuator array was produced on the PZT films with an Al reflective layer over the backside of the diaphragm. Measurements of the displacement profile using a laser Doppler vibrometer demonstrated that a large displacement of approximately 1 mum was obtained by applying a voltage of 10 V_pp on one actuator. To examine the application feasibility of the deformable mirror to AO, we reproduced low-order Zernike modes by calculating the voltage on each individual electrode using an influence function matrix. The measurements demonstrated that the deformable mirror could produce the Zernike modes up to the seventh term. Considering the low-voltage actuation as well as the capability for miniaturization of the electrode size, deformable mirrors (DMs) actuated by PZT films are desirable for low-cost AO