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.
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.
2D-MEMS scanners for the deflection of Laser light in two directions are used to illuminate a measurement volume within 40° in horizontal and vertical direction. This solid angle of about 0.02 is scanned by a 658nm Laser beam with a maximum repetition rate of 350MHz digital pulses with an intensity of about 50mW. Reflected light is detected through an objective by an APD with a bandwidth of 80MHz. The phase difference between the scanned Laser light and the light reflected from an object is analyzed by sub-Nyquist sampling allowing the calculation of its distance and velocity. Presently, the achieved accuracy of the system is between 5mm and 10mm and the measurement range is about 2m. The experimental set-up of the Lidar system is presented in detail and first measurements demonstrating the capability of the system are discussed.
Electrostatic driven 2D MEMS scanners resonantly oscillate in both axes leading to Lissajous trajectories of a digitally modulated laser beam reflected from the micro mirror. A solid angle of about 0.02 is scanned by a 658nm laser beam with a maximum repetition rate of 350MHz digital pulses. Reflected light is detected by an APD with a bandwidth of 80MHz. The phase difference between the scanned laser light and the light reflected from an obstacle is analyzed by sub-Nyquist sampling. The FPGA-based electronics and software for the evaluation of distance and velocity of objects within the scanning range are presented. Furthermore, the measures to optimize the Lidar accuracy of about 1mm and the dynamic range of up to 2m are examined. First measurements demonstrating the capability of the system and the evaluation algorithms are discussed.
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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