Presented is a high-resolution AR micro display based on Laser Beam Scanning (LBS) applying a two-dimensional resonantly operated vacuum packaged MEMS mirror with large mirror diameter, high scan frequencies, high Q-factor and large field-of-view (FoV). The image is projected to the retina using a diffractive waveguide leading to a comfortably large eyebox. Advanced control algorithms and image processing methods are implemented to accurately drive, sense and control the biaxial resonant MEMS mirror as well as to optimize image projection quality. Due to a sufficiently large mirror diameter this micro display does not need any beam expansion optics between MEMS mirror and waveguide enabling an ultra-compact projection unit. Resonant operation of the MEMS mirror in both axes and exploiting the significant advantage of a hermetic vacuum package effectively reduces energy loss by damping and thus minimizes drive voltage and power consumption. The display setup demonstrates the successful realization of a small form factor high resolution micro projector that meets important requirements for enabling fashionable AR smartglasses.
Although external cavity diode lasers have become firmly established for their excellent properties for species detection, they have severe limitations in terms of high acquisition rates. In this paper, we present our new ECDL design based on a resonantly driven MEMS scanner. By using the MEMS technology, a defined frequency range can be tuned extremely fast and without mode-hops. This allows scanning frequencies in the high kHz range to be achieved. The results of the characterization of the spectral properties of the MEMS-based system and its use for rapid detection of trace gases are presented.
In this work, 2D MEMS quasi-static mirrors based on piezoelectric, non-ferroelectric AlScN/AlN actuators with three different mirror plates (diameters of 2 mm, 5 mm and 10 mm) using a design and manufacturing platform will be reported. While the AlN/AlScN driving actuators ensure high linearity and large tilting angles, the multiple-waferbonding technique via glass fritting enables 3D construction of the MEMS mirrors and hermetic sealing. Even though there is no request on vacuum package for quasi-static driving, hermitic sealing on wafer level with appropriate interior pressure level within the sealing improves the mechanical robustness of the MEMS components and protects them from the particles and humidity from the environment. Since the main design concept was adopted from the previous work and adapted for different aperture sizes, this paper will focus on reporting further simulation results on mechanical behaviors, especially shock survivability under very harsh environment, the technology efforts and results of utilizing such a design and manufacturing platform for AlScN/AlN driven MEMS mirrors.
In this work, 2D piezoelectrically driven MEMS circular scanners have been designed, fabricated and tested. These mirrors own large optical apertures of 7 mm, 10 mm and 20 mm for good beam shapes. Also HR-coating layers for 515 nm and 1050 nm reaching up to 99.99% reflexion and 0.1% transmission were applied onto the mirror surface for the suitability of high power laser, where the wavelengths were specified according to the laser source development demands. Based on piezoelectric position sensing elements integrated on the MEMS mirrors a closed-loop control was developed. In this paper the design efforts, realizing circular-scanning and eliminating non-linearity during mode superposition, and fabrication efforts will be reported. Characterization results focusing on mechanical behaviors, position sensing signal, HR-coating will be also important parts of this work.
Presented here is the world’s first resonant 1D MEMS mirror achieving mechanical scanning angles exceeding ±45° and thus providing a field of view of up to 180°. The MEMS scanner features a 2 mm x 4 mm ellipsoid mirror plate and oscillates at a scan frequency of about 1.5 kHz. Integrated sensors and closed-loop control allow for an accurate position detection below 0.1°. To achieve the scan angles as well as to guarantee long lifetime and reliability, the MEMS mirror is hermetically sealed on wafer level by a dedicated glass cover and operated in vacuum.
This paper presents a 2D MEMS mirror for smart headlights, combing high-Q vacuum package with AR (Anti Reflecting)-coating, piezoelectric driving and Lissajous scanning. While the vacuum package protects the MEMS device and the AR-coating suppresses parasite reflections from the glass lid, the AlN-based piezoelectric actuators are robust against shock and vibration in harsh environment, comparing to fragile capacitive finger structures. This gimbal-less MEMS mirror with a large circular aperture (diameter = 5.5 mm) utilizes Lissajous scanning possessing two perpendicular torsion modes with frequencies of fx = 2.26 kHz, fy = 2.30 kHz fulfilling high light density and large total optical scanning angles of 55°, 30° at ± 40 VAC. A 2D projection of 50° x 20° was realized, where the angle loss comparing to the 1D testing arose from pincushion distortion, whose effect was severely reduced by the redesign run. Due to the great long-term stability of AlN and protection of vacuum packages, the MEMS mirror also shows a good reliability. This paper will describe and discuss the design, fabrication and characterization results of this MEMS mirror.
One- and two-dimensional MEMS scanning mirrors for resonant or quasi-stationary beam deflection are primarily known as tiny micromirror devices with aperture sizes up to a few Millimeters and usually address low power applications in high volume markets, e.g. laser beam scanning pico-projectors or gesture recognition systems. In contrast, recently reported vacuum packaged MEMS scanners feature mirror diameters up to 20 mm and integrated high-reflectivity dielectric coatings. These mirrors enable MEMS based scanning for applications that require large apertures due to optical constraints like 3D sensing or microscopy as well as for high power laser applications like laser phosphor displays, automotive lighting and displays, 3D printing and general laser material processing. This work presents modelling, control design and experimental characterization of gimbal-less MEMS mirrors with large aperture size. As an example a resonant biaxial Quadpod scanner with 7 mm mirror diameter and four integrated PZT (lead zirconate titanate) actuators is analyzed. The finite element method (FEM) model developed and computed in COMSOL Multiphysics is used for calculating the eigenmodes of the mirror as well as for extracting a high order (n < 10000) state space representation of the mirror dynamics with actuation voltages as system inputs and scanner displacement as system output. By applying model order reduction techniques using MATLABR a compact state space system approximation of order n = 6 is computed. Based on this reduced order model feedforward control inputs for different, properly chosen scanner displacement trajectories are derived and tested using the original FEM model as well as the micromirror.
A higher achievable scan speed and the capability to integrate two scan axes in a very compact device are fundamental
advantages of MEMS scanning mirrors over conventional galvanometric scanners. There is a growing demand for
biaxial high speed scanning systems complementing the rapid progress of high power lasers for enabling the
development of new high throughput manufacturing processes. This paper presents concept, design, fabrication and test
of biaxial large aperture MEMS scanning mirrors (LAMM) with aperture sizes up to 20 mm for use in high-power laser
applications. To keep static and dynamic deformation of the mirror acceptably low all MEMS mirrors exhibit full
substrate thickness of 725 μm. The LAMM-scanners are being vacuum packaged on wafer-level based on a stack of 4
wafers. Scanners with aperture sizes up to 12 mm are designed as a 4-DOF-oscillator with amplitude magnification
applying electrostatic actuation for driving a motor-frame. As an example a 7-mm-scanner is presented that achieves an
optical scan angle of 32 degrees at 3.2 kHz. LAMM-scanners with apertures sizes of 20 mm are designed as passive
high-Q-resonators to be externally excited by low-cost electromagnetic or piezoelectric drives. Multi-layer dielectric
coatings with a reflectivity higher than 99.9 % have enabled to apply cw-laser power loads of more than 600 W without
damaging the MEMS mirror. Finally, a new excitation concept for resonant scanners is presented providing
advantageous shaping of intensity profiles of projected laser patterns without modulating the laser. This is of interest in
lighting applications such as automotive laser headlights.
Hermetic wafer level packaging of optical MEMS scanning mirrors is essential for mass-market applications. It is the
key to enable reliable low-cost mass producible scanning solutions. Vacuum packaging of resonant MEMS scanning
mirrors widens the parameter range specifically with respect to scan angle and scan frequency. It also allows extending
the utilizable range of mirror aperture size based on the fact that the energy of the high-Q oscillator can be effectively
conserved and accumulated. But there are also some drawbacks associated with vacuum packaging. This paper discusses
the different advantageous and disadvantageous aspects of vacuum packaging of MEMS scanning mirrors with respect to
laser projection displays. Improved MEMS scanning mirror designs are being presented which focus on overcoming
previous limitations. Finally an outlook is presented on the suitability of this technology for very large aperture scanning
mirrors to be used in high power laser applications.
Low-cost automotive laser scanners for environmental perception are needed to enable the integration of advanced driver assistant systems into all automotive vehicle segments, which is a key to reduce the number of traffic accidents on roads. Within the scope of the European-funded project MiniFaros, partners from five different countries have been cooperating in developing a small-sized low-cost time-of-flight-based range sensor. An omnidirectional 360-deg laser scanning concept has been developed based on the combination of an omnidirectional lens and a biaxial large aperture MEMS mirror. The concept, design, fabrication, and first measurement results of a resonant biaxial 7-mm gimbal-less MEMS mirror that is electrostatically actuated by stacked vertical comb drives is described. Identical resonant frequencies of the two orthogonal axes are necessary to enable the required circle scanning capability. A tripod suspension was chosen, since it minimizes the frequency splitting of the two resonant axes. Low-mirror curvature is achieved by a thickness of the mirror of more than 500 μm. Hermetic wafer-level vacuum packaging of such large mirrors based on multiple wafer bonding has been developed to enable a large mechanical tilt angle of ±6.5 deg in each axis. Due to the large targeted tilt angle of ±15 deg and because of the MEMS mirror actuator having a diameter of 10 mm, a cavity depth of about 1.6 mm has been realized.
Low-cost automotive laser scanners for environment perception are needed to enable the integration of advanced driver assistant systems (ADAS) into all automotive vehicle segments, a key to reducing the number of traffic accidents on roads. An omnidirectional 360 degree laser scanning concept has been developed based on combination of an omnidirectional lens and a biaxial large aperture MEMS mirror. This omnidirectional scanning concept is the core of a small sized low-cost time-of-flight based range sensor development. This paper describes concept, design, fabrication and first measurement results of a resonant biaxial 7mm gimbal-less MEMS mirror that is electrostatically actuated by stacked vertical comb drives. Identical frequencies of the two resonant axes are necessary to enable the required circle scanning capability. A tripod suspension was chosen since it allows minimizing the frequency splitting of the two resonant axes. Low mirror curvature is achieved by a thickness of the mirror of more than 500 μm. Hermetic wafer level vacuum packaging of such large mirrors based on multiple wafer bonding has been developed to enable to achieve a large mechanical tilt angle of +/- 6.5 degrees in each axis. The 7mm-MEMS mirror demonstrates large angle circular scanning at 1.5kHz.
Small size, low power consumption and the capability to produce sharp images without need of an objective make
MEMS scanning laser based pico-projectors an attractive solution for embedded cell-phone projection displays. To fulfil
the high image resolution demands the MEMS scanning mirror has to show large scan angles, a large mirror aperture
size and a high scan frequency. An additional important requirement in pico-projector applications is to minimize power
consumption of the MEMS scanner to enable a long video projection time. Typically high losses in power are caused by
gas damping. For that reason Fraunhofer ISIT has established a fabrication process for 2D-MEMS mirrors that includes
vacuum encapsulation on 8-inch wafers. Quality factors as high as 145,000 require dedicated closed loop phase control
electronics to enable stable image projection even at rapidly changing laser intensities. A capacitive feedback signal is
the basis for controlling the 2D MEMS oscillation and for synchronising the laser sources. This paper reports on
fabrication of two-axis wafer level vacuum packaged scanning micromirrors and its use in a compact laser projection
display. The paper presents different approaches of overcoming the well-known reflex problem of packaged MEMS
scanning mirrors.
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