Here is the abstract of the seminar:
Making Light Work – MEMS Based Laser Display technology
Tucked away in the mountains of Jackson Hole, WY is a research lab with a unique next-generation laser projection technology. At Alces Technology, Chris Arrasmith and Matthew Leone (both recent MSU alumni) work to develop innovative laser and MEMS (micro-electro mechanical system) microdisplay technologies. This talk will reveal some of Alces’ unique laser and MEMS microdisplay technologies and provide a unique look into the field of next-generation laser display.
Alces Technology, Inc.Alces Technology is a unique research and development lab located in Jackson, WY. The images here highlight the unique work environment inside the Alces facilities; an open-office collaborative workspace, well-equipped lab facilities which include a number of engineering and demonstration systems such as a 4K-wide laser projector and near-eye color display, and a modest clean room space for managing the back-end process of the Alces MEMS microdisplay.
Alces’ Display TechnologyAlces’ Laser Projection Display technology is a uniquely scalable and high performance display system. A illustration of the display, which may be described as a scanned linear array architecture, is shown alongside a block diagram of the key subsystems: RGB laser illumination, polarization based optical core, Alces’ MEMS microdisplay, and custom video electronics. Alces has assembled several benchtop display systems to demonstrate this architecture and its unique abilities.
MEMS MicrodisplayAt the core of display system is the Alces’ MEMS microdisplay containing thousands of reflective micro-ribbons. When paired with Alces’ optical core, this devices is transformed into a novel spatial light modulator capable of unmatched performance and scalability. Shown to the right are images of a packaged MEMS prototype. This is an 8mm array with over 2000 MEMS ribbons tied to approximately 256 drive channels. The images to the bottom show the MEMS micro-ribbons at three different magnifications and their length-scales.
MEMS Design and FabricationThrough Alces research and development programs the MEMS design and CMOS-compatible fabrication sequence were optimized for low-voltage and high-speed operation. Shown to the left, the thin film stack and process flow were engineered to minimize the operating voltage at roughly 12V (however recently new discoveries have further reduced the maximum drive voltage to digital logic levels <5V) and rise and fall times to roughly 250ns. The plots show examples these key metrics (voltage and performance) and how they were adjusted to improve performance. Devices are currently being fabricated at Stanford Nanofabrication Facility, released at the Alces facilities in Jackson, WY and packaged through microelectronics vendors in the San Jose region.
MEMS ActuationBy applying a voltage across the gap between the upper ribbon surface and lower electrode an electrostatic field is generated and the MEMS ribbons are displaced. The image in this slide shows two regions of actuated ribbons as seen through Alces’ optical system. When all ribbons are undisplaced and positioned in the same vertical plane, the microdisplay is dark, however, when ribbons are actuated, a 1D array of pixels are formed at the very center of the MEMS ribbon. The “bright” regions(appearing as two grey rectangles) show two blocks of ribbons with alternating ribbons displaced slightly beyond the quarter wavelength of the illumination source, or in this case approximately (533nm/4 = 133nm). For a single pixel in the display only a single ribbon must be displaced but for metrology purposes the ribbons in this image have been hardwired together and actuated as a block of ribbons in order to characterize the uniformity across the array.
Alces has developed a unique method to generate pixels from this purely reflective MEMS light modulator; this method is referred to as “Edge-E”, and fundamentally, Edge-E is what enables Alces to convert the phase modulation of the reflected illumination into amplitude modulated pixels seen on the display. The following slides present the details of this Edge-E method.
Poynting Vector Walk-offTo create Edge-E pixels, Alces uses the unique property of birefringent crystals called Poynting Vector Walk-off. This phenomenon spatially offsets a light ray into two orthorgonally polarized rays. Shown here is a piece of calcite creating a “double image” of the letter ‘A’ via the phenomenon of Poynting Vector Walk-off. The images are perpendicularly polarized.
Polarization Displacement DeviceTo incorporate Poynting Vector Walk-off, Alces developed a unique component referred to as a “Polarization Displacement Device” or PDD, which is in the same family as a Savart plate or Wollaston Prism. Two thin pieces of quartz are sandwiched together with their optical axis rotated 90 degrees. The diagram in the slide shows that when a vertically polarized input light beam is passed through the PDD, two beams with orthogonal polarizations are produced with an offset equivalent to “p”, which in Alces’ system is equivalent to the pixel pitch/MEMS ribbon pitch of 4um. When these to beams are passed through an “Analyzing Polarizer” the output is two spatially displaced, horizontally polarized light beams that are 180 degrees out of phase. This novel optical device provides the “light valve” functionality of an Edge-E display and is often referred to as the “discriminator,” because it discriminates between the polarization states of the input light and either passes it or blocks it. Through the combination of this optical light valve and MEMS microdisplay an Edge-E display can be created; the follow slides describe how this two components work in conjuction to create Edge-E pixels.
Theory of OperationThis slide presents a basic walkthrough of the Edge-E theory of operation; in essence it shows how to create a single pixel using the 1D array of MEMS ribbons via the creation of an edge or step in the phase of the reflected light.
In the top row, a 1D cross-section of the ribbons are shown with pitch of “p” and the leftmost ribbons displaced vertically by a quarter wavelength of illumination (λ/4). This displacement creates a phase-delay in the reflected light proportional to half the wavelength (λ/2), whereby the reflected electric field amplitude can be described as proportional to ejφ and in this case simplified to +1 and –1. In the next row, the impulse function of the optical system can be described as a pair of delta functions, spatially offset, and 180 degrees out of phase. This is the mathematical equivalent of output of the PDD shown in the previous slide.
When the reflected light off the MEMS ribbons is passed through the PDD and Analyzing Polarizer (this can be mathematically represented as the convolution of row b) and row c) or the convolution of the electric field of the reflected light and the impulse function of the optical core), the output light is spatially defined as a pulse with width “p”. This rectangular pulse is an ideal pixel in the display and centered on the edge created in the MEMS array; hence the name “Edge-E”.
The final line shows how this “ideal” rectangular pixel is transformed into a near-Gaussian shape after passing through the finite numerical aperture of the optical system, which is equivalent to a low-pass filtering operation.
To summarize, Edge-E is a method of converting the phase-delay of light (or shift in the polarization state), into the spatial amplitude modulation of a pixel via the creation of a edge in the MEMS ribbon array. More details are discussed in the final slide: Encoding Pixels using Edges.
Fourier Space DescriptionEdge-E can also be described in the dual of position-space; angle-space or k-space. In fact, for those familiar with Fourier Optics, the Edge-E method is particularly elegant due to the simplicity and succinctness of the mathematical description.
In order to describe Edge-E in angle-space several well-known Fourier transforms should be examined first. Shown on the right side are three Fourier pairs: the sinusoid, the step-function, and the sinc function. The top-most diagram shows the Fourier transform pair of a sinusoid, where in position space is represented as two offset delta functions, 180 degrees out of phase, and in angle space as the pure sinuosoid whose angular frequency is proportional to the spatial offset of the delta functions. The second row shows the Fourier transformof a step function in position space and its equivalent 1/k function in angle space. The third row shows the Fourier transform of a sinc function to a rectangular-pulse function whose pulse-width is proportional to the width of the of the sinc function.
The three Fourier transform pairs are incorporated into the main diagram showing the linear-system analysis of an Alces’ Edge-E method in angle-space. The impulse or transfer function of the optical system is described as H(k) and represented as the blue sinusoid. In other words this is the Fourier transform of the delta functions created by the PDD and Analyzing polarizer shown in the slide: Polarization Displacement Device. The input function is described as R(k) and represented by the red 1/k function, which is the Fourier transform of the step-function and created by imparting a phase delay on the reflected illumination through the displacement of MEMS ribbons. The output of the system in angle space is defined by the multiplication of the input function R(k) and transfer function H(k) and equivalent to sin(k)/k or sinc(k) and shown in the green plot. To convert from angle-space to position-space the inverse Fourier transform must be performed, which as previously mentioned, for the case of a sinc function, transforms to a rectangular pulse. This means the sinc function describes a single idea pixel in angle-space and this sinc function can be generated by creating an input function defined by a step and a transfer function defined by a sinusoid.
Encoding Pixels using EdgesMoving beyond just a single pixel, Alces developed a method to generalize the formation of Edge-E pixels across the MEMS array using concepts from digital communications. Non-return to zero inverted encoding or NRZi describes a method of creating a binary signal using transitions to define the logical value. In other words, a logical ‘0’ is defined as no-transition in the signal level and a logical ‘1’ is defined as a transition in the signal level, regardless of direction.
For an Alces Edge-E display system, NRZi can be applied spatially rather than temporally and in an analogue fashion rather than just binary. Pixels are thus centered over the gap between the MEMS ribbons and the displacement amplitude or edge height created between adjacent ribbons controls the amount of light or brightness of pixel.
The diagram in the bottom of the slide shows an example of how this type of NRZi-like encoding can be applied to the MEMS ribbon array to generate a 1D pattern of pixels. A cross-sectional view of the ribbon position is plotted along the top in blue. The “intensity” plot shows the amplitude of the spatially-varying pixels generated from the Edge-E method. The bottom “greyscale intensity” shows how this might look displayed as a 1D column of pixels.
Thus, a 1D column of greyscale pixels can be created by positioning the MEMS ribbons in such a way as to create step-height variations between adjacent MEMS ribbons matched to an input signal. For instance, to create a 100% white pixel then the step height between adjacent ribbons should be equivalent to the quarter wavelength of light, and to create a black pixel then the step height between ribbons should be zero. Using custom algorithms, Alces has devised a control system that can take in input video signal (a signal column of pixels) and transform those values into voltages which are applied to the MEMS ribbons and create the appropriate step height variations.
Final remarksHopefully this was an enlightening description of Alces novel Edge-E technology. For more details on Edge-E, be sure to read Alces’ patent 7,940,448 Display System. Further descriptions will follow as one post cannot fully encapsulate all the details, qualities and features of this system.
While this description provides the basics of the Alces Edge-E technology, it does not provide the full scope of Alces’ development efforts. Alces has gone beyond just the mathematical modeling and theory shown here and built physical display systems incorporating these concepts. These systems can be seen at Alces’ and are capable of generating full monochromatic and color images. And so Edge-E not only represents a unique example of a new kind of MEMS display technology but also exemplifies Alces’ ability to innovate, develop, and advance new display technologies.
For more details or to discuss Edge-E please get contact us.