Light and electricity are said to be the all purpose tools for the next decades. Photonic Microsystems combine this tools in an ideal manner: They are electronically addressable devices with an optical functionality allowing to modulate light temporally and/or spatially. Further, they take advantage of high integration density, high reliability, high bandwidth and low cost fabrication for serial production. While in some cases Photonic Microsystem Technology is focused on the replacement of conventional devices, the majority of developments uses the unique potential of this technology to create devices based on novel principles with extended or even new functionality for advanced applications. Products based on Photonic Microsystem Technology have already entered or are only a few steps away from entering the market in various fields e.g. in information and communication technology, medicine, biology and metrology. This paper gives an overview of the Photonic Microsystems development activities with special emphasis on devices for light deflection and light modulation. Single micro mirrors e.g. for scanning or laser beam positioning are as well presented and discussed as micro mirror arrays and membrane mirrors for image generation and phase modulation. Technology trends are derived from the current development activities and an outlook to future work is given.
This paper presents a demonstrator of a low cost image projection device that has been developed at the Fraunhofer Institute of Photonic Microsystems. The image projection is not based on the common line by line raster scanning of the image. Instead, a resonant 2-dimensional micro scanning mirror is used for the deflection of a modulated laser beam. The mirror is operated at a low ratio of horizontal and vertical oscillation frequency. In particular, a ratio with a small shift from an integer value is used to enable a scan of the whole projection screen with a Lissajous pattern. The control circuit performs an excitation of both mirror axes by driving them with fixed frequency according to the response curves of the actuator. Programmable counters are used to generate the driving frequencies and to determine the actual beam position during the scanning process. That enables a very simple and low cost control circuit. A micro scanning mirror, fabricated at Fraunhofer IPMS, was used in the demonstrator set up. It is operated at oscillation frequencies of 1.4 kHz (slow axis) and 9.4 kHz (fast axis). The control circuit was realized and successfully tested with a FPGA implementation. The image resolution provided by the control circuit is 256 x 256 pixels.
The micro scanning mirror with lateral out-of-plane comb drives is on its way to volume fabrication. This article reviews the development and highlights the most important activities and decisions that are representative for many MEMS devices that are supposed to go the same way. Careful analysis of the product requirements, design for reliability, design for testability, design for packaging, a mature process, and automated testing preferably on wafer-level have been identified as keys to volume fabrication of MEMS.
We report on a conceptual design and feasibility demonstration for a scanned beam endoscope, with advantages over present CCD imaging technology in image resolution and quality, light source power, and package diameter. Theoretical calculations were made by optical modeling and finite element analysis of the performance projected for a design meeting size constraints. To verify the design target of 5 mm for the endoscope diameter, we conducted a design study of the deformation and resolution characteristics of a scan mirror small enough to fit within a 2.5 mm capsule within the endoscope. The results show that performance similar to the test system can be achieved. A functional prototype was then built and tested to validate the theory used. The test system consisted of a photonics module with red (635 nm), green (532 nm) and blue (473 nm) lasers, combined by dichroic mirrors and launched to a single mode fiber. The light emerging from the fiber is formed into a beam and reflected from a commercially available bi-axial MEMS scanner with a 1.56 mm square mirror, and a scan angle of 6 degrees zero to peak mechanical, at a frequency of 19.7 kHz. Scanned beam power from 1 to 3 mw impinges the test object at a range from 10 to 100 mm, and the scattered light is collected by several 3 mm diameter multimode fibers and conducted one-meter to detectors. The detected light was digitized and then reconstructed to form an image of the test object, with 800 by 600 output pixels. Several such images will be presented.
The Grating Light Valve (GLV) is a diffractive MOEMS spatial light modulator capable of very high-speed modulation of light combined with fine gray-scale attenuation. GLV-based products are field-proven in a variety of applications. In this paper, we describe the GLV device, its structure, theory of operation, and optical performance. The versatility and speed of the GLV device are described. We explain how the GLV device is integrated into an optical write engine to create a complete digital imaging system. In addition to the MOEMS die and drive electronics, the light engine also comprises illumination optics, Fourier filter, and imaging optics. We present current applications of the GLV device for high-resolution displays, and computer-to-plate printing, as well as future plans for digital imaging applications opened up by the unique properties of this diffractive MOEMS technology.
Eastman Kodak Company has developed a diffractive-MEMS spatial-light modulator for use in printing and display applications, the grating electromechanical system (GEMS). This modulator contains a linear array of pixels capable of high-speed digital operation, high optical contrast, and good efficiency. The device operation is based on deflection of electromechanical ribbons suspended above a silicon substrate by a series of intermediate supports. When electrostatically actuated, the ribbons conform to the supporting substructure to produce a surface-relief phase grating over a wide active region. The device is designed to be binary, switching between a reflective mirror state having suspended ribbons and a diffractive grating state having ribbons in contact with substrate features. Switching times of less than 50 nanoseconds with sub-nanosecond jitter are made possible by reliable contact-mode operation. The GEMS device can be used as a high-speed digital-optical modulator for a laser-projection display system by collecting the diffracted orders and taking advantage of the low jitter. A color channel is created using a linear array of individually addressable GEMS pixels. A two-dimensional image is produced by sweeping the line image of the array, created by the projection optics, across the display screen. Gray levels in the image are formed using pulse-width modulation (PWM). A high-resolution projection display was developed using three 1080-pixel devices illuminated by red, green, and blue laser-color primaries. The result is an HDTV-format display capable of producing stunning still and motion images with very wide color gamut.
The Grating Light Valve diffractive MOEMS device has been successfully used in imaging applications (lithography and display) requiring image data-rates of 1-5 giga-bits per second (Gb/s). However, new applications such as maskless photolithography and high performance displays require larger pixel counts and finer control of gray-scale. This paper discusses the suitability of the GLV device for high data-rate applications. It discusses the factors governing GLV device switching speed and illustrates how these properties are optimized relative to other requirements of the imaging system.
A new class of commercial platesetters, called computer-to-plate (CtP) systems, has been introduced in the graphic arts industry in the past few years. As opposed to conventional systems that use an analog mask, these CtP systems employ direct digital imaging methods to generate patterns on metal plates used for offset printing. By eliminating this intermediate mask step, CtP platesetters enable users to transfer images directly from the computer to the plate, thus reducing cost and cycle time. The Grating Light Valve (GLV) is a diffractive MOEMS device that has been successfully implemented as the spatial light modulator in a commercial CtP platesetter for the graphic arts industry. The combination of high power handling capability, fast modulation rate, and large number of pixels on the GLV allows for increased printing speed. Properties of the GLV, such as analog gray scale, and pulse width modulation can be used to increase print quality. In order to create images on the plates, infrared laser illumination is focused onto the GLV device, which reflects the beam with controlled intensity onto a photo-thermal medium. While offering advantages in quality and throughput, the high pixel count and form of the GLV presents some challenges in illumination and projection onto the printing plate. This paper will describe the system architecture and method of operation for a GLV-based optical write engine, and show performance and results.
A micro grating interferometer has been fabricated to use in measuring the dynamic performance of MEMS devices. The system uses a phase sensitive diffraction grating for interferometric axial resolution and a microfabricated lens for improved lateral response. Early experimental results using a non-deformable grating interferometer show that both the transient and steady state vibration of MEMS devices can be measured and mapped using the micro interferometer. These initial results also reveal vibrational noise and sample alignment problems. To avoid these obstacles and to maximize the sensitivity of the interferometer, a PID control unit is introduced. Analysis has been performed on the interferometer system to improve the controller design. A deformable grating interferometer has also been fabricated using microfabrication techniques and tested to show proper range of actuation under DC bias. This grating also demonstrates the ability to maintain a high sensitivity during operation.
In this paper a new approach for the realisation of a passive matrix image projection display consisting of electrostatic actuated Fabry-Perot filters for digital wavelength switching is presented. The switches either may be working by illumination with polychromatic or with monochromatic light, e.g. by a laser. In the first case the output light has to be filtered at the desired wavelength. In order to define the interferometric properties of the dielectric layers and thus the switching wavelength optical parameters like thickness and refractive index have to be adjusted carefully. The display switches can be adapted either to reflection or transmission mode, depending on whether silicon or quartz is used as substrate material. Especially hexagonal shaped pixel membranes for working either in reflection at a wavelength of 536 nm or in transmission for 500 nm are described. The assembly is arranged matrix-like in rows and columns, where at each intersection point a pixel is located. The switching of a pixel into the 'on'-state is achieved by applying a voltage on the corresponding row and column contact lines of the display. The resulting intersection potential deflects the addressed pixel membrane whereas adjacent pixels are nearly not affected. Actual measurements allow high switching frequencies of about 2 kHz at voltages in the range of 2 - 60 V, depending on the pixel design. The switching contrast maximum is aobut 80%, the contrast beteeen addressed and non-addressed adjacent pixels is 75%.
The Fraunhofer IPMS and Micronic Laser Systems AB have developed a technology for the maskless DUV microlithography using spatial light modulation (SLM). This technology uses an array of micromirrors as a pro-programable mask, which allows writing up to 1 million pixels with a framerate of up to 2 kHz. The SLM is fabricated at the IPMS using its high-voltage CMOS process. The mirrors are fabricated by surface micromachining using a polymer as sacrificial layer. The mirrors are operated in an analog mode to allow sub-pixel placement of the features.
Vanadium dioxide (VO2) thin film undergoes a semiconductor-to-metal phase transition at about 68°C, which is accompanied with abrupt changes in its optical properties. A light modulator array has been developed by surface micromachining based on this thermally induced optical switching. The good thermal isolation and the small thermal mass of the micromachined pixels prevent thermal cross talk and provide advantages of low power consumption as well as high switching speed. The VO2 pixel design was optimized by thermal and optical simulations. Active VO2 thin film was fabricated by evaporation of vanadium film followed by thermal oxidation. The light modulator array has been realized in 64 × 64 format by surface micromachining using polyimide sacrificial layer. Preliminary characterization and testing result will be presented.
Microelectromechanical Systems ("MEMS") and optics are a natural match. There are several reasons: MEMS devices have dimensions and achievable actuation distances comparable to the wavelength of light; smooth-surfaced dielectrics, semiconductors, and metals can be used in various combinations; and, photons don't weigh anything, so relatively feeble MEMS actuators can easily manipulate them. Many optical MEMS devices are based on mirror arrays that can be tilted using electrostatic actuation. This paper, however, focuses on programmable diffraction gratings and their uses for projection displays, spectroscopy, and wavelength management in modern optical telecommunication systems.
We present an overview of the results of our recent research in the field of adaptive optical components based on silicon microtechnologies, including membrane deformable mirrors, spatial light modulators, liquid-crystal correctors, wavefront sensors, and both spherical and aspherical micro-optical components. We aim at the realization of adaptive optical systems using standard-technology solutions.
This paper presents experimental data illustrating the dynamic behavior of micromachined deformable mirrors operated as cyclical focus control elements. The mirrors used for this study are circular silicon nitride membranes 1 mm in diameter with a fundamental resonance at 144 kHz and a quality factor in air of 0.86. Electrostatic actuation of the mirrors is provided by two concentric electrodes to control focus and spherical aberration. Mirror displacement has been characterized in terms of frequency response and dynamic surface figure. Surface shape was examined for variation with frequency over a range from 1 kHz to 180 kHz for small mirror deflection. Intra-cycle variation of both quartic and quadratic surface curvature terms was also measured at 60 kHz. Surface figure data are presented showing less than λ/10 spherical aberration for a mirror operating at 10 kHz with a focal length varying cyclically from ∞ to 32 mm.
Adaptive optics techniques have been proved in both laboratory and field tests to the satisfaction especially of the astronomical and surveillance communities. Such success have sparked interests in other fields, however, to increase efficiency and lower costs new technologies have to be brought to fruition. MEMs are becoming a very important player in this arena. In this paper we describe a portable adaptive optics (AO) system that has been tested in both laboratory and field experiments. Results of these tests will be discussed. Capabilities and shortcomings of this technology will be discussed. A look at future applications and trends will be given.
Accurate prediction of the dynamic behavior of comb-driven MEMS microscanners is important to optimize the actuator and structure design. In this paper, a numerical and an analytical model for the dynamic analysis of comb-driven microscanners under different excitation schemes are presented. The numerical model is based on a second order nonlinear differential equation. Due to the nature of the torque function, this governing equation of motion is a parametric nonlinear ODE, which exhibits hysteretic frequency domain behavior and subharmonic oscillations. Experimental results and approximate analytical expressions for this nonlinear torque function of the comb-drive are presented. Amplitude and phase relationship between the excitation signal and the resultant oscillations at different excitation frequencies are measured and we show that they are in close agreement with the numerical simulations. Analytical model uses perturbation methods to reach approximate close-form expressions for the dynamic behavior of the device in the first parametric resonance region. It is also utilized to predict the stability regions on the frequency-excitation voltage plane, where the device exhibit hysterical characteristics. Analytical and numerical modeling approaches proposed in this paper provides a simple yet powerful way to analyze the nonlinear frequency response of comb-driven actuators and simplify the design process for a microscanner based system.
A 1500-μm-diameter silicon/silicon nitride 3D scan mirror has been built using MEMS technology. It is capable of static and dynamic beam scanning achieved with a bi-axial gimbal. A gold-coated deformable membrane at the center of the device provides both focus control (z-axis) and spherical aberration correction. This architecture is able to move the focus of a laser beam throughout a three-dimensional space with a single optical surface, and is referred as a 3D scan mirror. This mirror will be incorporated into a miniature confocal laser scanning microscope for biomedical in-situ imaging applications. In this paper we describe the 3D scan mirror design, fabrication and characterization as well as its target application.