Mirror Plate Research Articles

A 1×20 MEMS mirror array with large scan angle and low driving voltage for optical wavelength-selective switches

Wavelength-selective switches (WSSs) are the core devices for reconfigurable optical add-drop multiplexers (ROADMs) that are essential in all-optical networks. WSSs based on microelectromechanical (MEMS) mirror arrays exhibit high potential due to their high switching speed and large port counts, but there still exist challenges to achieve large optical scanning angle at low voltage. This paper presents a 1×20 MEMS mirror array (MMA) design that can scan over 10° in both axes at less than 5 V. The MMA is based on an unique electrothermal bimorph actuator design that can perform 2D scanning and its fill factor reaches 96 %. The 1×20 MMA design has been successfully fabricated using a combined bulk- and surface- micromachining process. The size of each mirror plate is 240 μm by 500 μm. The measured maximum optical scan angles in the x-axis and y-axis are 29.3° and 11.8°, respectively, at 3 V. The measured response time is about 3.7 ms. The thermal crosstalks between adjacent electrothermal actuators are about 10 %. Long-term stability tests show that the angular drifts of the individual mirrors are about 0.002°/h.

Extra Large Aperture FPCB Mirror Based Scanning LiDAR

This article presents an extra-large (25 × 50 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) flexible printed circuit board (FPCB) mirror to cover both the emitting and receiving lenses of a single-point LiDAR to form a biaxial scanning LiDAR with a compact structure and long measurement distance and low cost. The FPCB mirror is fabricated using the low cost and commercially available FPCB fabrication process. In addition, a widely available single–point LiDAR is used as the measurement unit. This article's novelty lies in the following two points: 1) integrating a large aperture scanning mirror and a single-point LiDAR (low cost and widely available) to construct a biaxial scanning LiDAR; 2) proposed a large aperture FPCB mirror using long existing FPCB process such as to achieve low cost. The scanning LiDAR is designed for applications in factory for robots and automated guided vehicles; navigation. The FPCB mirror consists of a mirror plate, two permanent magnets, and a FPCB structure, which includes two torsion beams, a middle seat, and a Flame retardant 4 (FR4) stiffener frame. Four-layer copper coils are embedded in the FPCB structure, which is fabricated using the low cost commercially available FPCB fabrication process. The mirror plate is diced from a thin silicon mirror plate with gold coating and then attached on top of the middle seat of the FPCB structure. With such large aperture FPCB scanning mirror, and a long measurement distance single-point LiDAR, a compact biaxial scanning LiDAR is constructed. The scanning LiDAR based on the FPCB mirror is constructed and tested. Achieved performances are: the field of view of 60°, measurement distance of 50 m, refresh rate of 20 Hz, 500 points for each scanning frame.

Axial Vibration Analysis of Thrust Support of Vertical Hydrogenerator Unit

Axial vibration is the main accident that affects the normal function of the vertical hydro-generator unit and endangers the use of the unit. There are many reasons for the axial vibration of the unit. This article only analyzes the mechanism of the axial vibration of the thrust bracket of the vertical unit caused by the connection of the thrust head and the mirror plate, and gives a real machine measurement example and treatment measures.

Low Power Electromagnetic Scanning Micromirror for LiDAR System

In this article, a low power and compact scanning micromirror for LiDAR (light detection and ranging) system is designed, fabricated, and characterized. A 5 mm-diameter scanning micromirror is electromagnetically actuated with multi-turn copper winding formed on the gimbal and magnet assembly formed under the silicon substrate. A unique magnetic circuit that generates an asymmetric radial magnetic field has been designed and analyzed. A series-connected double spring-mass system consisting of a gimbal and a mirror plate has been used to amplify the deflection angle. An analytic model of the system has been developed and verified with finite element analysis. Also, a reinforcement rim structure is utilized to reduce the dynamic deformation of the reflective surface down to 19.3 nm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> . An optical scan angle of 30 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> is obtained at 690 Hz, 17.4 mA <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> input, and corresponding power consumption is 7.8 mW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> . A prototype scanning system with 180 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> x 30 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> field-of-view has been demonstrated with fabricated scanning micromirror as the vertical scanner. The proposed approach provides a simple and compact design for a large diameter scanning micromirror, which can potentially be utilized in various LiDAR applications.

Temperature stability study of resonant angular scanning micromirrors with electrostatic comb-drive actuators

Resonant rotational scanning micromirrors with electrostatic comb-drive actuators are widely used in generating structured lights for 3D sensing, but their resonant scanning stability is affected considerably by the working temperature, which in turn compromises the sensing accuracy. The resonant scanning instability is mainly exhibited as the temperature drifts of the quality factor (Q) and the resonance frequency. In this paper, we focus on the Q variation due to the temperature dependence of air damping. The temperature dependence of the air damping in a comb-drive micromirror is investigated using both simplified analytical damping models and computational fluidic dynamic (CFD) models. The modeling results show that the temperature dependence of the viscous damping of the comb drive is opposite to that of the drag damping of the mirror plate, providing a means of minimizing the temperature dependence of Q by adjusting the structural parameters of the comb drive and the mirror plate. It has also been found from the modeling that the lengths of the comb-drive fingers and the mirror plate play the most significant role while the cavity height under the mirror plate has little effect if it is more than 10 % of the mirror plate length. Meanwhile, four micromirrors with different comb-finger or mirror-plate lengths are designed and fabricated. The testing results from these four fabricated mirrors have confirmed that the temperature coefficient of the Q value of an angular scanning comb-drive micromirror can be altered and ultimately compensated by changing the mirror-plate and/or comb-finger lengths.

A Silicon Optical Bench-Based Forward-View Two-Axis Scanner for Microendoscopy Applications.

Optical microendoscopy enabled by a microelectromechanical system (MEMS) scanning mirror offers great potential for in vivo diagnosis of early cancer inside the human body. However, an additional beam folding mirror is needed for a MEMS mirror to perform forward-view scanning, which drastically increases the diameter of the resultant MEMS endoscopic probe. This paper presents a new monolithic two-axis forward-view optical scanner that is composed of an electrothermally driven MEMS mirror and a beam folding mirror both vertically standing and integrated on a silicon substrate. The mirror plates of the two mirrors are parallel to each other with a small distance of 0.6 mm. The laser beam can be incident first on the MEMS mirror and then on the beam folding mirror, both at 45°. The MEMS scanner has been successfully fabricated. The measured optical scan angles of the MEMS mirror were 10.3° for the x axis and 10.2° for the y axis operated under only 3 V. The measured tip-tilt resonant frequencies of the MEMS mirror were 1590 Hz and 1850 Hz, respectively. With this compact MEMS design, a forward-view scanning endoscopic probe with an outer diameter as small as 2.5 mm can be made, which will enable such imaging probes to enter the subsegmental bronchi of an adult patient.

2D Scanning Micromirror with Large Scan Angle and Monolithically Integrated Angle Sensors Based on Piezoelectric Thin Film Aluminum Nitride.

A 2D scanning micromirror with piezoelectric thin film aluminum nitride (AlN), separately used as actuator and sensor material, is presented. For endoscopic applications, such as fluorescence microscopy, the devices have a mirror plate diameter of 0.7 mm with a 4 mm2 chip footprint. After an initial design optimization procedure, two micromirror designs were realized. Different spring parameters for x- and y-tilt were chosen to generate spiral (Design 1) or Lissajous (Design 2) scan patterns. An additional layout, with integrated tilt angle sensors, was introduced (Design 1-S) to enable a closed-loop control. The micromirror devices were monolithically fabricated in 150 mm silicon-on-insulator (SOI) technology. Si (111) was used as the device silicon layer to support a high C-axis oriented growth of AlN. The fabricated micromirror devices were characterized in terms of their scanning and sensor characteristics in air. A scan angle of 91.2° was reached for Design 1 at 13 834 Hz and 50 V. For Design 2 a scan angle of 92.4° at 12 060 Hz, and 123.9° at 13 145 Hz, was reached at 50 V for the x- and y-axis, respectively. The desired 2D scan patterns were successfully generated. A sensor angle sensitivity of 1.9 pC/° was achieved.

Silicon Direct Bonding for the European Space Agency's Next X-ray Observatory

Silicon direct bonding is a manufacturing process that is used in the fabrication of electronic, optical and mechanical microsystems. The chemical bonds are providing an intimate surface contact and strong structural stiffness that is for this reason also suitable for large assemblies. Together with the knowledge of the surface properties and of X-ray optics, we are making mirror imaging systems.The manufacturing process resorts to the highest-grade 12 inch silicon wafers with double Super Polished and near perfect plan parallel surfaces. With readily available semiconductor manufacturing and lithographic tools the wafers are processed into mirror plates with micrometer accuracy channel structures, yet maintaining the nanometer accurate figure of the silicon wafer. Multiple channeled X-ray mirrors shall subsequently be curved and stacked to form an optical component.We want to use such optical components for a ~3m diameter segmented optical system that would be part of the Athena observatory, European Space Agency's (ESA) next L-class mission, scheduled to launch in 2032. Athena will observe the hot and energetic Universe and answer two big scientific questions: How does ordinary matter assemble into the large-scale structures we see today? and how do black holes grow and shape the Universe?The scientific objectives of Athena require the largest X-ray optic ever flown. Silicon Pore Optics (SPO) is the selected technology that produces the building blocks for Athena's modular mirror. The segmented Athena optic will be made of about 700 X-ray mirror modules all co-aligned to a common focus, with radii of curvature varying from 0.25 m to 1.25 m. Furthermore, SPO has also shown to be a versatile technology that can be further developed for gamma-ray optics, medical applications and for material research.While considerable progress has been made on the performance of the optic, we are also upscaling towards an automated production line including mirror manufacturing, coating, cleaning, stacking and stack integration into mirror modules. The complete production line features many metrology steps to ensure quality and high yield. From the start, the development process has looked not just at the production of a single sample of high quality but at all the processes eventually required for the production of a complete system, and this in an effort to ensure that issues (mechanical, thermal, environment, supply chain etc) are identified early and addressed in a manner compatible with the core goal of producing high quality optics.In this paper we will discuss the challenges and present solutions that we have addressed. Cleanliness assessment, and bond strength enhancement are for instance two current topics of active development.

Silicon Direct Bonding for the European Space Agency's Next X-ray Observatory

Silicon direct bonding is a manufacturing process used in the fabrication of electronic, optical and mechanical microsystems. Chemical bonds are providing an intimate surface contact and strong structural stiffness that is, for this reason, also suitable for large assemblies. Based on the knowledge of the surface properties and of X-ray optics, we are making mirror imaging systems. The manufacturing process resorts to the highest-grade 12 inch silicon wafers with double-sided super polished and nearly perfect plan parallel surfaces. With readily available semiconductor manufacturing and lithographic tools, the wafers are processed into mirror plates with micrometer accuracy channel structures, yet maintaining the nanometer accurate figure of the silicon wafer. Multiple channeled X-ray mirrors shall subsequently be curved and stacked to form an optical component. We want to use such optical components for a ~3 m diameter segmented optical system that would be part of the Athena observatory.

Plasma etching for the compatibility of thin film metallic coatings and direct bonding of silicon pore optics

Silicon pore optics are a new type of high-performance x-ray optics designed to enable future space-borne x-ray observatories such as European Space Agency’s Athena. These optics will make it possible to build telescopes with effective areas of the order of a few square meters and angular resolutions better than 5 s of arc. During manufacturing of the optics, thin film metallic coatings are sputtered onto mirror plates to help achieve this large effective area. Then, these plates are stacked on top of each other using direct silicon bonding to achieve the shape of an approximate Wolter type-I telescope design. It is, therefore, necessary to verify the compatibility of the coating and bonding processes. We observe the unintentional removal of coatings on silicon pore optics plates after their wet chemical activation, a step required to make direct bonding possible. In this paper, we investigate plasma etching prior to thin film deposition as a solution to this problem. First, we ensure that plasma etching does not impact the low surface roughness required to achieve high imaging performance. Then, we demonstrate that plasma etching before thin film deposition prevents unintentional removal of the metallic coatings during the activation step, making coating deposition compatible with direct bonding of silicon pore optics plates.

Design and Development of a Double-Bridge Micromirror With Bending and Twisting Cantilevers for Multiobject Spectroscopy

This article presents the design, development, and characterization of a double-bridge electrostatically actuated micromirror for multiobject spectroscopy (MOS). The proposed structure is an improvement over single-bridge micromirrors in terms of aperture size, pull-in voltage, degrees of freedom, and fill factor. The two-axis symmetric rotation mechanism of the proposed micromirror is achieved by bending and twisting action of the suspended cantilevers which is contrary to the twisting cantilevers of a conventional one-axis torsional micromirror. The placement of anchor and cantilevers under the mirror plate results in a high fill factor when arranged in a 2-D array. The analytical modeling, design optimization, and static and dynamic analysis are done using finite-element method (FEM) in Coventorware. An optimized design is fabricated using a simple approach of surface micromachining and electroplating. For a micromirror of size $200\,\,\mu \text{m}\,\,\times 200\,\,\mu \text{m}$ and the actuation gap of $2.5~\mu \text{m}$ , the device exhibits a tilt angle of 1.5° at a pull-in voltage of 23.1 V, a switching time of $38~\mu \text{s}$ , and the resonance frequency of 35.23 kHz. A ${3} \times {3}$ array of the micromirror is demonstrated with a fill factor of more than 95%. The deflection range can be increased by simply increasing the thickness of the sacrificial layer and without any major process modification.

FPCB Masked One-Step Etching Large Aperture Mirror for LiDAR

This paper presents a Flexible Printed Circuit Board (FPCB) masked one-step etching large aperture mirror for Light Detection and Ranging (LiDAR). A FPCB structure is bonded on a thin silicon wafer ( $50\sim 200~\mu \text{m}$ thick) coated with a 100 nm thick metal film and one-step of DRIE is used to etch the wafer with the FPCB structure as the mask and the metal film as the stop layer. The simple fabrication process and 0.1 mm (instead of $ needed for microfabrication) resolution photolithography lead to a very low cost of a few dollars. Copper coils embedded in the FPCB structure are attached to the backside of the large aperture mirror plate, instead of beside the mirror plate as the case in MEMS magnetic mirrors, to generate a high magnetic force and achieve a relatively high frequency considering the large aperture and high thickness. Modeling and prototyping of two designs, Mirror_A and Mirror_B, are presented to verify the novel mirror technology. LiDAR applications of the novel mirror technology are demonstrated and tested. Achieved performances are: Mirror_A, aperture $12 \times 12$ mm, oscillation frequency of 510 Hz, field of view (FOV) of 30° and radius of curvature (ROC) of 10 m; Mirror_B aperture $24 \times 24$ mm, oscillation frequency of 160 Hz, FOV of 20° and ROC of 10 m. [2020-0085]

Mirror Angle Tuning of Electromagnetic Micro-Mirrors With Oscillation Compensation

Underdamped feature and hysteresis existing in electromagnetic scanning micro-mirrors (EMSMs) often lead to oscillation and undesired positioning error of deflection angle. In this paper, a model-based robust scheme is proposed for the angle tuning of mirror plate of the EMSM. To build a proper model for describing the behavior of EMSM, which is also convenient for the design of tuning strategy, a data-driven model composed of linear dynamic submodel as well as hysteresis submodel is developed. In the data-driven model, the deflection mechanism is described by a linear dynamic submodel while the hysteresis in EMSM is depicted by a nonsmooth hysteresis basis function submodel. Then, in order to compensate for the internal oscillation caused by both underdamping factor and hysteresis, a pseudo-feedforward oscillation compensator (PFOC) not relying on the inverse model is proposed. Then, the design procedure of the angle tuning strategy with the PFOC is presented. Afterward, the robust design of the angle tuning scheme is also studied to overcome the effect of compensation residual. Subsequently, the proposed scheme is applied to the angle tuning of EMSM.

A Monolithic Forward-View MEMS Laser Scanner With Decoupled Raster Scanning and Enlarged Scanning Angle for Micro LiDAR Applications

This paper reports the design, fabrication, and characterization of a forward-view optical scanner realized by two vertical 2-axis MEMS mirrors integrated on a Silicon Optical Bench (SiOB). The mirror plate sizes of the first and second MEMS mirrors are 0.6×0.7 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 1.2×1.4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. With the second MEMS mirror larger, the optical beam will not be truncated even during large-angle dynamic scanning. This dual-mirror configuration also enables multiple scanning modes, e.g., an enlarged 2D field of view (FoV) by combining the scans of both mirrors, and a decoupled raster scanning by actuating the two mirrors separately. Experiments show that the enlarged 2D FoV reached 28°(h)×31°(v) and the FoV of the decoupled raster scanning reached 38°(h)×8°(v). The measured first resonances of the two mirrors were 2.09 kHz and 0.85 kHz, respectively. The overall size of the MEMS scanner was only 5.3 × 5.8 × 3.0 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , and its weight was 20 mg. This 2-axis forward optical scanner is much smaller and lighter than other two-mirror scanning modules, giving it a great potential for applications in miniature LiDAR for micro-robotics.

AlScN based MEMS quasi-static mirror matrix with large tilting angle and high linearity

This paper presents a piezoelectrically (AlScN) driven 3 × 3 quasi-static mirror matrix, where each MEMS mirror utilizes a three-level-construction comprising a mirror plate (diameter = 0.8 mm), a pillar and four AlScN actuators hidden beneath the mirror plate, reducing the chip size to 1.1 × 1.3 mm2. AlScN as a high performance piezoelectric material is used to deliver large force enabling a mechanical tilting angle of ±14° at 150 VDC, in addition to great linearity, repeatability and long-term stability. For realizing the 3D construction three wafers for mirror plate, actuators and a TSV wafer for the vertical electrical contacts are applied. While first demonstrators have been manufactured by a chip-level hybrid assembly to mount the mirror plates onto the actuators, a BEOL triple-wafer-bonding process has been developed to integrate the mirror plates onto the actuators and TSV wafer. This paper will show the design and process efforts for improving the mechanical behavior of the MEMS mirrors, the process effort and current result of triple-wafer-bonding. It concludes by discussing the technological achievements, challenges and outlook for improved performance due to the material development of AlScN.

New point-like sources and a conducting surface in Maxwell\u2013Chern\u2013Simons electrodynamics

We investigate some aspects of the Maxwell–Chern–Simons electrodynamics focusing on physical effects produced by the presence of stationary sources and a perfectly conducting plate (mirror). Specifically, in addition to point charges, we propose two new types of point-like sources called topological source and Dirac point, and we also consider physical effects in various configurations that involve them. We show that the Dirac point is the source of the vortex field configurations. The propagator of the gauge field due to the presence of a conducting plate and the interaction forces between the plate and point-like sources are computed. It is shown that the image method is valid for the point-like charges as well as for Dirac points. For the topological source we show that the image method is not valid and the symmetry of spatial refection on the mirror is broken. In all setups considered, it is shown that the topological source leads to the emergence of torques.

Stability Study of an Electrothermally-Actuated MEMS Mirror with Al/SiO2 Bimorphs

Electrothermal actuation is one of the main actuation mechanisms and has been employed to make scanning microelectromechanical systems (MEMS) mirrors with large scan range, high fill factor, and low driving voltage, but there exist long-term drifting issues in electrothermal bimorph actuators whose causes are not well understood. In this paper, the stability of an bimorph electrothermal MEMS mirror operated in both static and dynamic scan mode has been studied. Particularly, the angular drifts of the MEMS mirror plate were measured over 90 h at different temperatures in the range of – °C. The experiments show that the temporal drift of the mirror plate orientation largely depends on the temperature of the electrothermal bimorph actuators. Interestingly, it is found that the angular drift changes from falling to rising as the temperature increases. An optimal operating temperature between °C to °C for the MEMS mirror is identified. At this temperature, the MEMS mirror exhibited stable scanning with an angular drift of less than °/h.

Tunable beam manipulation based on phase-change metasurfaces.

A metasurface combined with phase-change material Ge2Sb2Te5 (GST) is proposed to act as a switchable wave plate to adjust spin-orbit interactions (SOIs), so that the polarization and phase of the reflected light are simultaneously manipulated. A converter, which could act as a quarter-wave plate or three-quarter-wave plate when the GST layer is in the amorphous or crystalline state, and a switch, which could act as a mirror (corresponding to the "OFF" state of SOIs) or half-wave plate (corresponding to the "ON" state of SOIs) when the GST layer is in the amorphous or crystalline state, are designed, respectively. Consequently, a convertible vectorial beams converter, which could generate radial or azimuthal polarization, is designed when the GST layer is in the amorphous or crystalline state. In addition, a switchable vortex beam generator could realize orbital angular momentum with topological charge l=±2 when the GST layer changes from amorphous to the crystalline state. The designed metasurface could offer a promising route for high-efficiency reconfigurable devices and encrypted optical communications.

External Electromagnet FPCB Micromirror for Large Angle Laser Scanning.

An external electromagnet plus moving PM (permanent magnet) FPCB (flexible printed circuit board) micromirror is proposed in this paper that can overcome two limitations associated with the previous FPCB micromirror with a configuration of an external PM plus moving coil, i.e., (1) it reduces the overall width beyond the mirror plate, and (2) increases the maximum rotation angle. The micromirror has two external electromagnets underneath an FPCB structure (two torsion beams and a middle seat) with two moving PM discs attached to the back and a metal-coated mirror plate bonded to the front of the FPCB middle seat. Modeling and simulation were introduced, and the prototype was fabricated and tested to verify the design. The achieved performance was better than that of the previous design: a maximum resonant rotation angle of 62° (optical) at a driving voltage of ±3 V with a frequency of 191 Hz, the required extra width beyond the mirror plate was 6 mm, and an aperture of 8 mm × 5.5 mm with a roughness of <10 nm and a flatness of >10 m (ROC, radius of curvature). The previous FPCB micromirror’s performance was: strain limited maximum rotation angle was 40° (optical), the extra width beyond the mirror plate was 14.7 mm, and had an aperture of 4 mm × 4 mm with a similar roughness and flatness.

Large aperture surface-micromachined rotating micromirror with the majority of dimples removed

This paper presents a surface-micromachined repulsive-force driven 1D rotating micromirror with a large aperture (1 mm). Two rotating beams are located on the bottom side of the mirror plate, while two repulsive-force actuators are connected to two middle sides to push the mirror plate up for out-of-plane rotation. Thus, no complex self-assembly structure is required to raise the mirror plate for rotation as most conventional surface-micromachined rotating micromirrors do. In order to increase the yield rate for the large aperture micromirror and avoid dimples used for preventing stiction during fabrication, six post-fabrication melted supporting beams are used along the circumference to increase the stiffness in fabrication, which are electrically melted and broken after fabrication to restore the initial stiffness. Only 12 dimples in the center of mirror plate are used for preventing operational stiction. The mirror’s reflectivity is increased to 25% from 20% after removing the majority of dimples. 36 prototypes are fabricated and tested and none of them suffer from stiction problem. The 6.3° optical rotation is achieved with a settling time of 26 ms.