Scanned Pico-Projectors: Seeing the Big Picture (with a Small Device)

Mark Freeman, Champion and Sid Madhavan

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Green SHG laser

Blue LD Pico-projectors are the latest technology to prove that big things often do come in small packages. These tiny projectors are embedded in mobile devices to provide large-screen displays that can be viewed from anywhere. The authors provide an overview of these technologies—which are poised PicoP Integrated Photonics MEMS to hit the market by next year—with a special focus on Module (IPM) scan engine. scanner

2D scanned laser projection. Miguel Khoury/Microvision

Scanned laser: A simple properties of the red, green, or blue projector design laser and relay it through the scanner With pico-projectors, and onto the screen with high efficiency The figure above shows the basic layout and image quality. The pixel profile is of the pico-projector being developed you can project a designed to provide high resolution by Microvision Inc., called PicoP. The and infinite focus with a smooth non- architecture is quite simple, consisting full-size image onto pixellated image. t’s amazing what we carry in our pockets these days. From cell phones to iPods to of one red, one green, and one blue laser, whatever is near at However, with the simple opto- PDAs, we have at our fingertips connectivity with friends and colleagues around each with a lens near the laser output Ithe world, libraries of text, music, photos, videos and more. mechanical design, the electronics that collects the light from the laser and hand, whether it be the Unfortunately, the displays that we use to view all this information are also small; design becomes more challenging. This provides a very low NA (numerical aper- wall, your shirt, or a is because some of the display complex- they are flat-panel screens with just a few square inches of display area. No wonder ture) beam at the output. The light from that projectors that display large images from within hand-held electronic devices— ity is moved to the electronics. This the three lasers is then combined with piece of paper. the so-called pico-projectors—are drawing so much attention in the tech world. approach requires that engineers be able dichroic elements into a single white With pico-projectors, you can project a full-size image onto whatever is near at hand, to accurately place pixels and to modu- beam. Using a beamsplitter or basic fold- whether it be the wall, your shirt, or a piece of paper. Pico-projectors represent a core late the laser at pixel rates. optics, the beam is relayed onto enabling technology for the future growth of portable devices. a biaxial MEMS scanning mirror that Infinite focus Moreover, scanned laser pico-projectors have an infinite focus that can be project- for each pixel. This produces brilliant scans the beam in a raster pattern. The ed onto surfaces with any 3D depth profile and remain in focus. The possible applica- images with the wide color gamut avail- In a raster-scanned laser projector, there projected image is created by modulating tions are various and many. Aside from having a larger format for typical mobile able from RGB lasers. is no projection lens. The projected beam applications, such as reading e-mail and sharing pictures, pico-projectors could be the three lasers synchronously with the Direct-driving of the lasers pixel-by- directly leaves the MEMS scanner and used for impromptu business presentations or perhaps scientific visualizations. position of the scanned beam. The com- pixel at just the levels required brings creates an image on whatever surface it From a business point of view, the size of the market for pico projectors is extremely plete projector engine, also known as the good power efficiency and inherently is shone upon. Because of the scanned Mark Freeman, Mark Champion large. In 2007, there were worldwide sales of more than 1 billion mobile phones, 200 Integrated Photonics Module, or IPM, is high contrast. The efficiency is maxi- single pixel design, light-collection just 7 mm in height and less than 5 cc in and Sid Madhavan million personal media players, 125 million digital cameras, and 25 million Nintendo mized, since the lasers are only on at efficiency is kept high by placing the DS handheld game devices. And it’s not just device manufacturers who stand to ben- total volume. the level needed for each pixel. The con- collection lenses near the output of the efit by offering a larger-screen viewing experience. Service providers would also have The essence of the design is that, trast is high because the lasers are com- lasers, while the output beam NA is very the opportunity to sell bandwidth, and content providers could sell more compelling with the exception of the scanner, the pletely off for black pixels rather than low. By design, the rate of expansion of and rich multi-media content. A handful of manufacturers, including Samsung and rest of the optics engine deals with using an SLM (spatial light modulator) the single-pixel beam is matched to the 3M, are already beginning to offer pico projectors this year as stand-alone accessories making a single pixel. All three lasers to deflect or absorb any excess intensity. rate that the scanned image size grows. that can be attached to other handheld devices. By 2010, pico-projectors that are are driven simultaneously at the levels The single-pixel collection optics are As a result, the projected image is always embedded into devices will start to become available. needed to create the proper color mix optimized to take the particular beam in focus.

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Miguel Khoury/Microvision OPN May 2009 | 29 [ Scanned vs. imaging projectors ] [ Bidirectional raster scan ] PICO-PROJECTOR TECHNOLOGIES Scanned laser focus advantage Horizontal trajectory: drive vs. time A number of companies around the world are actively pursuing pico-projector technology. One way to view the big 3.00 picture is to categorize the projector technologies based on the approaches being used to generate the projected Typical imaging projector spot size focused for 0.5 m projection distance images, including those that result from (1) scanning a pixel in two dimensions, (2) imaging a 2D array of pixels, and 2.50 (3) mixing imaging and scanning. 2.00 4number of pixels c Imaging pico projectors c Scanning pico projectors 1.50 These projectors follow the basic design used in small- Scanning pico-projectors directly utilize the narrow 1.00 PicoP pixel diameter business projectors. A small spatial light modulator divergence of laser beams, and some form of 2D scan- Display pixel size = 43° image size Spot size [mm] (SLM), which has an individually addressable modulator ning to “paint” an image, pixel by pixel. Usually the scan 0.50 for each pixel, is used to create the picture. The SLM is pattern is similar to the raster pattern used in traditional imaged by a projection lens onto the projection surface. television, albeit in this case it is photons, rather than 0.00 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Unlike business projectors, which sometimes use three electrons, being scanned. Some designs use separate SLM panels (one for each color), these projectors use scanners for the horizontal and vertical scanning direc- Distance from projector [m] just a single SLM to meet the pico form factor. tions, while others use a single biaxial scanner. The spe- Display Blanking Illumination in these systems comes from either cific beam trajectory also varies depending on the type “Infinite focus” property of the scanned laser projector compared with an Vertical trajectory: drive vs. time Generation of raster pattern lasers or LEDs. The light is collected and optically of scanner used. Since the scanner replaces an array of imaging-type projector. conditioned into a uniform beam that illuminates all the pixels and projection optics, these projectors can be very pixels in the SLM at once. The pixel modulators absorb small while remaining in focus at any projection distance. or deflect light from the illumination beam to create the Microvision’s PicoP falls into this category. modulated pixel gray levels. These projectors require The figure above shows the spot size [ PicoP projection display system ] created by running the horizontal axis focusing and have a potential power disadvantage, since as a function of projection distance at its resonant frequency—which is c Mixed imaging and scanning pico projectors the SLM illumination beam must always be on. for the PicoP. Notice that the spot size Safety typically about 18 KHz for the WVGA Mixed imaging and scanning pico projectors use a one- Frame buffer grows at a rate matched to the growth memory subsystem (Wide Video Graphics Array) scanner. Color sequential: Color sequential is the most common dimensional spatial light modulator to create a single approach to produce full color and requires a fast modu- column of pixels and a 1D scanner to sweep the column of a single pixel. For comparison, the The horizontal scan velocity varies sinu- lator technology capable of cycling through all colors figure also includes the estimated spot Video ASIC Laser drive soidally with position. The MEMS con- horizontally, thereby creating a 2D image. Like the imag- ASIC during a single video frame interval. SLM technologies ing-type projectors, a projection lens is used to image the growth for an imaging-type projector troller uses feedback from sensors on the that support color sequential include the DLP (Digital SLM onto the projection surface. that has been focused for a 0.5-m projec- System controller MEMS scanner to keep the system on Light Processing from Texas Instruments) and FLCOS The 1D SLM is based on the diffraction of laser light. tion distance. We assume a moderately and SW resonance and at fixed scan amplitude. (ferro-electric liquid crystal on silicon; made by Display- Each pixel consists of a set of reflective ribbons located fast F/4 projection lens and the focal The image is drawn in both direc- tech and others). Color sequential has the advantage of side-by-side that can be individually shifted in a direc- length chosen to give the same 43° rate tions as the scanner sweeps the beam good color gamut and one-third the pixel count of color tion perpendicular to their flat surface to create either a Beam shaping filter system with the same display resolution. Color of growth with projection distance for optics, combiner back and forth. This helps the system plane mirror or a diffraction grating. Modulation occurs by the projected image. The depth of focus efficiency in two ways. First, by run- breakup is an artifact associated with the color sequen- illuminating the SLM with a uniform beam and adjusting MEMS drive for an imaging-type projector is much ASIC ning on resonance, the power required tial approach that may be disturbing to some. the diffraction grating for each pixel to diffract the desired MEMS device reduced compared to the scanned laser. to drive the scan mirror is minimized. Color filter array: The color filter approach uses an array amount of light. of color filters built into the SLM. It is used with liquid- A gray level is achieved by allowing only the diffracted To the user, this means that the Second, bi-directional video maximizes crystal-on-silicon modulators based on the more com- (or the remaining undiffracted) light to pass through the imaging-type projector must be the laser use efficiency by minimizing monly used nematic liquid crystals. It sacrifices some projector and blocking the rest. One-dimensional SLMs refocused as the projection distance the video blanking interval. This results of the color gamut available with RGB light sources of this type include Sony’s GLV (grating light valve), is changed, and that portions of the consumer products. The electronics, in a brighter projector for any given (assuming white LED illumination), and requires a pixel Samsung’s SOM (spatial optical modulator), and Kodak’s image will be out of focus when one which can be integrated more straight- laser output power. count that is 3X the display resolution; however, it avoids GEMS (grating electro mechanical system). To the best of projects onto surfaces that present a forwardly into consumer products, take The vertical scan direction is driven color breakup. HiMax is one company that offers a our knowledge, of these three, only Samsung is actively range of projection distances within the over tasks that are done optically with with a standard sawtooth waveform to color-filter-based LCOS panel. pursuing this approach for a pico-projector. image—for example, projecting onto a other projector designs. Some of the provide constant velocity from the top flat surface at an angle or onto surfaces tasks that are shifted include pixel posi- to the bottom of the image and a rapid with a significant 3D profile. tioning, color alignment and brightness retrace back to the top to begin a new This special property comes from This can’t be achieved in more tradi- the lens aperture must be kept large to uniformity. With the PicoP, the video frame. This is also managed in closed- dividing the task of projecting an image tional projector designs, where projec- maximize the projector’s brightness. Shifting projector functions processor and MEMS controller have loop fashion by the MEMS controller into using a low NA single-pixel beam tion optics are used to image a spatial This dictates the need for a fast projec- from optics to electronics been implemented as custom ASICs that based on position feedback from the to establish the focus and a 2D scanner light modulator onto the projection tion lens, with F/2 lenses being typical. With the simplification of the opto- drive the IPM scan engine. MEMS scanner to maintain a smooth to paint the image. In fact, the MEMS screen due to conflicting constraints on Depth of focus is proportional to F-stop. mechanical projector engine design, a and linear trajectory. The frame rate scanner plays the role of fast projec- the projection lens. On the one hand, The trade-off for traditional projec- greater portion of the display emphasis MEMS Drive ASIC is typically 60 Hz for an 848 3 480 tion optics by producing an image that a short focal length lens is needed to tor designs balances the rate the image is shifted to the electronics. This allows The MEMS Drive ASIC drives the WVGA resolution; it can be increased expands with a 43-degree horizontal create an image that grows quickly with grows with distance, light efficiency and the physical size of the projector engine MEMS scanner under closed loop when the projector is used in lower projection angle. projection distance, while, on the other, depth of focus. to be minimized to fit with hand-held control. The horizontal scan motion is resolution applications.

OPN May 2009 | 31 32 | OPN May 2009 www.osa-opn.org Video ASIC also allows the pixel positions for each Components [ Bi-axial MEMS scanner ] range, the green wavelength should Biaxial MEMS scanner be chosen where it will be most useful The video processor accepts either RGB These super-small, color to be adjusted independently. This Vertical scan simplifies the manufacturing alignment The biaxial MEMS scanner is made Only two frame with for enhancing the color of the display. or YUV (NTSC/PAL) input. A video drive drive coils low-power projector lines Green lasers at 530 nm are a good choice buffer is provided to allow artifact-free of the IPM by relaxing the requirement using standard bulk silicon MEMS Piezo scan conversion of input video. Gamma that the three laser beams be perfectly fabrication processes. The WVGA resistive for maximizing the color gamut. systems will open up sensor correction and colorspace conversion aligned. The positions of the red, green pico-projector scanner has a scan mirror The ability to directly modulate the are applied to enable accurate mapping the display bottleneck and blue pixels can be adjusted elec- diameter of approximately 1 mm, and lasers is at the heart of the scanned laser pico-projector technology. Pixel times of input colors to the wide laser color for mobile devices, tronically to bring the video into perfect it produces an active video scan cone of gamut. A scaling engine is available alignment, even if the laser beams are 43.2° by 24.3°. at the center of the WVGA scanned Scan display are on the order of 20 ns. Lasers for upconverting lower resolution allowing information not. This capability can also be used to The scanner uses moving-coil actua- mirror video content. compensate for some types of chromatic tion with a single drive coil, which can therefore need modulation bandwidths A proprietary Virtual Pixel Synthesis to be accessed and aberration if the PicoP is used as an be seen on the vertical scan frame in the on the order of 100 MHz. (VPS) engine uses a high-resolution shared more easily from engine in a larger optical system. photo with just two drive lines as shown. green laser in the PicoP is based on interpolation to map the input pixels to Mapping from digital video coding to The single coil design simplifies the infrared lasers developed for the telecom A path forward industry, the other massive market for the sinusoidal horizontal trajectory. The portable devices. laser drive is performed by the Adaptive fabrication of the MEMS scanner and The first generation of pico-projectors laser technology. Robust near-infra-red VPS engine demonstrates how projector Laser Drive (ALD) system. The ALD is reduces the number of required inter- are being launched this year. These laser diodes with very high modula- functions have been shifted from the a closed-loop system that uses optical connects. The MEMS die is housed in a super-small, low-power projector systems tion bandwidths are combined with optics to the electronics in the scanned managed in the VPS engine by adjust- feedback from each laser to actively package with small magnets that provide will open up the display bottleneck for a frequency-doubling crystal, usually laser paradigm. It effectively maps the ing coefficients that control the overall compensate for changes in the laser a magnetic field oriented at approximately mobile devices, allowing information to periodically poled lithium niobate, to input pixels onto a high-resolution brightness map for the display. characteristics over temperature and 45° to the scan axes. A single composite be accessed and shared more easily from produce a green laser that can be directly virtual coordinate grid. Besides enabling Optical distortions—including key- aging. This ensures optimum brightness, drive signal is applied that contains the portable devices. With the high volumes modulated. With an eye toward the the repositioning of video information stone, parallelogram, and some types of color and grayscale performance. Unlike superposition of the fast-scan horizontal expected, there is great market oppor- burgeoning pico-projector industry, sev- with subpixel accuracy onto the sinusoi- pincushion distortion—can be compen- other display systems, optical feedback drive at the resonant frequency of the tunity for key components such as red, eral companies, including Corning and dal scan, the VPS engine optimizes the sated by using the VPS engine to adjust is incorporated to ensure optimum color horizontal mirror motion and the 60 Hz green and blue lasers. OSRAM, are ramping up production of image quality. Brightness uniformity is the pixel positions. The VPS engine balance and grayscale. vertical drive sawtooth waveform. The scanned laser projector paradigm suitable green lasers. The mechanical design of the MEMS provides a path forward to higher-reso- The choice of which wavelength to scanner allows motion along only the two lution projectors without growth in size. use for the lasers is based on two con- orthogonal scan directions. Mechanical Unlike SLM-based projector technol- REQUIREMENTS FOR PICO-PROJECTORS siderations. First is the response of the filtering, resulting from the different mass ogies—in which increased resolution In order to operate in a mobile format, pico-projectors will need to meet a human eye (the photopic response) to and flexure stiffness governing horizontal means growth in the number of pixels number of requirements related to the following dimensions. different wavelengths. This response is a and vertical motion, sorts the drive sig- in the SLM—the single-pixel, single- somewhat Gaussian-looking curve that nals by frequency content, inducing the scan-mirror nature of the engine remains Size: The height or thickness of the projector is its most use (in other words, the distance from the projector to the peaks in the green-wavelength region image will likely change often). 18 KHz resonant motion of the horizon- the same, even as the resolution of the important characteristic; the technology must be able to and falls off significantly in red and be embedded in thin handheld devices. Both height and tal axis and the 60 Hz sawtooth motion projected display increases. t Resolution: Resolution can be expected to continue to blue. The amount of red and blue power volume should be minimized. The initial products will have of the vertical axis. Piezo-resistive sensors grow as product technology matures. The wide screen for- projector engine sizes ranging in height from 7 to 14 mm provide scan mirror position feedback to needed to get a white-balanced display Visit OPN online (www.osa-opn.org) to mat is generally desirable for viewing video content. Initial and in overall volume from 5 to 10 cc. varies rapidly with wavelength. view videos that demonstrate the technical product offerings will typically offer resolutions from QVGA the MEMS controller ASIC to maintain closed loop accuracy of the desired scan For example, eye response increases concepts behind scanned laser projection and Brightness: The brighter the better. Brightness is limited (320 x 240) to WVGA (848 x 480). mirror motion. by a factor of two when the wavelength how the MEMS scanner works. by the available brightness of the light sources, either Color: Pico-projectors typically use either lasers or red, is changed from 650 nm (the wave- lasers or LEDs, the optical efficiency of the projector green and blue LEDs for light sources. In both cases, the Lasers length used for DVD drives) to 635 nm. Mark Freeman (mark_freeman@microvision. design, and by the need for low-power operation in order com) is a principal engineer in optics at result is large color gamuts that far exceed the usual color The technology for the red and blue This allows the required laser power Member to maximize battery life. Initial product offerings will be in Microvision Inc., in Redmond, Wash., range experience of TVs, monitors, and conference-room- to drop by the same factor, making a U.S.A. Mark Champion is a principal engineer the range of 5-10 lumens. lasers in the PicoP leverages the technol- type projectors. White LEDs used with color filters yield a ogy of similar lasers that are used for projector that is lower power. Similarly, in electronics and Sid Madhavan is a vice presi- reduced color gamut. dent of engineering at Microvision. Image size: In general, the shorter the distance it takes the optical disk storage industry. The the blue laser should be chosen to have for the projector to produce a large image, the better—at Contrast: The higher the better. Just as brightness is wavelength requirements are shifted as long a wavelength as possible. Cur- [ References and Resources ] least up to a point. A one-to-one distance/image-size ratio a measure of the absolute whiteness of pico- projec- somewhat, but the basic technology is rently, blue lasers in the range of 440 to (projection angle of 53 degrees) is probably a good target. >> R. Sprague et al. “Mobile Projectors tors, contrast is the measure of their absolute darkness. the same—GaAlInP red laser diodes 445 nm are the best practical choice. As Using Scanned beam Displays”, from The first round of products on the market will have projec- Contrast is the dynamic range of a display, making the and GaN blue laser diodes. the industry grows, longer wavelengths Mobile Displays, Technology and Appli- tion angles in the range of 30-45 degrees. cations, Bhowmik, Li, and Bos editors, difference between washed out images and crisp dramatic- Pico-projectors incorporate green in the range of 460 to 470 nm may looking images. John Wiley and Sons, Chapter 21, 2008. Focus-free operation: Focus-free operation is a very lasers as well. Prior to the push for become a better option. >> W. Davis et. al. “MEMS-Based Pico desirable attribute in mobile applications. It is like “point Battery life: A starting goal for mobile devices is that they laser projectors, green lasers had not The second consideration is color Projector Display”, Proceedings of IEEE/ and shoot” in a camera. Unlike typical projectors used in should last for the length of a complete movie. This puts been used for any similar high-volume gamut. Since the photopic response is LEOS Optical MEMS & Nanophotonics, Freiburg, Germany, 2008. business settings, pico-projectors are designed for mobile the lower limit at around 1.5 hours. applications. The technology for the peaked all through the green wavelength

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