www.osa-opn.org28 | OPN May 20091047-6938/09/0??/0028/6-$15.00 ©OSA

Mark Freeman, Mark Champion and Sid Madhavan

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

OPN May 2009 | 29

Mark Freeman, Mark Champion and Sid Madhavan

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 to hit the market by next year—with a special focus on 2D scanned laser projection.

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t’s amazing what we carry in our pockets these days. From cell phones to iPods to PDAs, we have at our fingertips connectivity with friends and colleagues around

the world, libraries of text, music, photos, videos and more. Unfortunately, the displays that we use to view all this information are also small;

they are flat-panel screens with just a few square inches of display area. No wonder that projectors that display large images from within hand-held electronic devices—the so-called pico-projectors—are drawing so much attention in the tech world. With pico-projectors, you can project a full-size image onto whatever is near at hand, whether it be the wall, your shirt, or a piece of paper. Pico-projectors represent a core enabling technology for the future growth of portable devices.

Moreover, scanned laser pico-projectors have an infinite focus that can be project-ed onto surfaces with any 3D depth profile and remain in focus. The possible applica-tions are various and many. Aside from having a larger format for typical mobile applications, such as reading e-mail and sharing pictures, pico-projectors could be used for impromptu business presentations or perhaps scientific visualizations.

From a business point of view, the size of the market for pico projectors is extremely large. In 2007, there were worldwide sales of more than 1 billion mobile phones, 200 million personal media players, 125 million digital cameras, and 25 million Nintendo DS handheld game devices. And it’s not just device manufacturers who stand to ben-efit by offering a larger-screen viewing experience. Service providers would also have the opportunity to sell bandwidth, and content providers could sell more compelling and rich multi-media content. A handful of manufacturers, including Samsung and 3M, are already beginning to offer pico projectors this year as stand-alone accessories that can be attached to other handheld devices. By 2010, pico-projectors that are embedded into devices will start to become available.

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properties of the red, green, or blue laser and relay it through the scanner and onto the screen with high efficiency and image quality. The pixel profile is designed to provide high resolution and infinite focus with a smooth non-pixellated image.

However, with the simple opto-mechanical design, the electronics design becomes more challenging. This is because some of the display complex-ity is moved to the electronics. This approach requires that engineers be able to accurately place pixels and to modu-late the laser at pixel rates.

Infinite focusIn a raster-scanned laser projector, there is no projection lens. The projected beam directly leaves the MEMS scanner and creates an image on whatever surface it is shone upon. Because of the scanned single pixel design, light-collection efficiency is kept high by placing the collection lenses near the output of the lasers, while the output beam NA is very low. By design, the rate of expansion of the single-pixel beam is matched to the rate that the scanned image size grows. As a result, the projected image is always in focus.

With pico-projectors, you can project a

full-size image onto whatever is near at

hand, whether it be the wall, your shirt, or a

piece of paper.

PicoP Integrated Photonics Module (IPM) scan engine.

MEMS scanner

Green SHG laser

Blue LD

Red LD

Scanned laser: A simple projector design

The figure above shows the basic layout of the pico-projector being developed by Microvision Inc., called PicoP. The architecture is quite simple, consisting of one red, one green, and one blue laser, each with a lens near the laser output that collects the light from the laser and provides a very low NA (numerical aper-ture) beam at the output. The light from the three lasers is then combined with dichroic elements into a single white beam. Using a beamsplitter or basic fold-mirror optics, the beam is relayed onto a biaxial MEMS scanning mirror that scans the beam in a raster pattern. The projected image is created by modulating the three lasers synchronously with the position of the scanned beam. The com-plete projector engine, also known as the Integrated Photonics Module, or IPM, is just 7 mm in height and less than 5 cc in total volume.

The essence of the design is that, with the exception of the scanner, the rest of the optics engine deals with making a single pixel. All three lasers are driven simultaneously at the levels needed to create the proper color mix

for each pixel. This produces brilliant images with the wide color gamut avail-able from RGB lasers.

Direct-driving of the lasers pixel-by-pixel at just the levels required brings good power efficiency and inherently high contrast. The efficiency is maxi-mized, since the lasers are only on at the level needed for each pixel. The con-trast is high because the lasers are com-pletely off for black pixels rather than using an SLM (spatial light modulator) to deflect or absorb any excess intensity. The single-pixel collection optics are optimized to take the particular beam

Miguel Khoury/Microvision

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c Mixed imaging and scanning pico projectorsMixed imaging and scanning pico projectors use a one-dimensional spatial light modulator to create a single column of pixels and a 1D scanner to sweep the column horizontally, thereby creating a 2D image. Like the imag-ing-type projectors, a projection lens is used to image the SLM onto the projection surface.

The 1D SLM is based on the diffraction of laser light. Each pixel consists of a set of reflective ribbons located side-by-side that can be individually shifted in a direc-tion perpendicular to their flat surface to create either a plane mirror or a diffraction grating. Modulation occurs by illuminating the SLM with a uniform beam and adjusting the diffraction grating for each pixel to diffract the desired amount of light.

A gray level is achieved by allowing only the diffracted (or the remaining undiffracted) light to pass through the projector and blocking the rest. One-dimensional SLMs of this type include Sony’s GLV (grating light valve), Samsung’s SOM (spatial optical modulator), and Kodak’s GEMS (grating electro mechanical system). To the best of our knowledge, of these three, only Samsung is actively pursuing this approach for a pico-projector.

PICO-PROJECTOR TECHNOLOGIES

A number of companies around the world are actively pursuing pico-projector technology. One way to view the big picture is to categorize the projector technologies based on the approaches being used to generate the projected images, including those that result from (1) scanning a pixel in two dimensions, (2) imaging a 2D array of pixels, and (3) mixing imaging and scanning.

c Scanning pico projectorsScanning pico-projectors directly utilize the narrow divergence of laser beams, and some form of 2D scan-ning to “paint” an image, pixel by pixel. Usually the scan pattern is similar to the raster pattern used in traditional television, albeit in this case it is photons, rather than electrons, being scanned. Some designs use separate scanners for the horizontal and vertical scanning direc-tions, while others use a single biaxial scanner. The spe-cific beam trajectory also varies depending on the type of scanner used. Since the scanner replaces an array of pixels and projection optics, these projectors can be very small while remaining in focus at any projection distance. Microvision’s PicoP falls into this category.

c Imaging pico projectors These projectors follow the basic design used in small-business projectors. A small spatial light modulator (SLM), which has an individually addressable modulator for each pixel, is used to create the picture. The SLM is imaged by a projection lens onto the projection surface. Unlike business projectors, which sometimes use three SLM panels (one for each color), these projectors use just a single SLM to meet the pico form factor.

Illumination in these systems comes from either lasers or LEDs. The light is collected and optically conditioned into a uniform beam that illuminates all the pixels in the SLM at once. The pixel modulators absorb or deflect light from the illumination beam to create the modulated pixel gray levels. These projectors require focusing and have a potential power disadvantage, since the SLM illumination beam must always be on.

Color sequential: Color sequential is the most common approach to produce full color and requires a fast modu-lator technology capable of cycling through all colors during a single video frame interval. SLM technologies that support color sequential include the DLP (Digital Light Processing from Texas Instruments) and FLCOS (ferro-electric liquid crystal on silicon; made by Display-tech and others). Color sequential has the advantage of good color gamut and one-third the pixel count of color filter system with the same display resolution. Color breakup is an artifact associated with the color sequen-tial approach that may be disturbing to some.

Color filter array: The color filter approach uses an array of color filters built into the SLM. It is used with liquid-crystal-on-silicon modulators based on the more com-monly used nematic liquid crystals. It sacrifices some of the color gamut available with RGB light sources (assuming white LED illumination), and requires a pixel count that is 3X the display resolution; however, it avoids color breakup. HiMax is one company that offers a color-filter-based LCOS panel.

This special property comes from dividing the task of projecting an image into using a low NA single-pixel beam to establish the focus and a 2D scanner to paint the image. In fact, the MEMS scanner plays the role of fast projec-tion optics by producing an image that expands with a 43-degree horizontal projection angle.

This can’t be achieved in more tradi-tional projector designs, where projec-tion optics are used to image a spatial light modulator onto the projection screen due to conflicting constraints on the projection lens. On the one hand, a short focal length lens is needed to create an image that grows quickly with projection distance, while, on the other,

the lens aperture must be kept large to maximize the projector’s brightness. This dictates the need for a fast projec-tion lens, with F/2 lenses being typical. Depth of focus is proportional to F-stop. The trade-off for traditional projec-tor designs balances the rate the image grows with distance, light efficiency and depth of focus.

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The figure above shows the spot size as a function of projection distance for the PicoP. Notice that the spot size grows at a rate matched to the growth of a single pixel. For comparison, the figure also includes the estimated spot growth for an imaging-type projector that has been focused for a 0.5-m projec-tion distance. We assume a moderately fast F/4 projection lens and the focal length chosen to give the same 43° rate of growth with projection distance for the projected image. The depth of focus for an imaging-type projector is much reduced compared to the scanned laser.

To the user, this means that the imaging-type projector must be refocused as the projection distance is changed, and that portions of the image will be out of focus when one projects onto surfaces that present a range of projection distances within the image—for example, projecting onto a flat surface at an angle or onto surfaces with a significant 3D profile.

Shifting projector functions from optics to electronicsWith the simplification of the opto-mechanical projector engine design, a greater portion of the display emphasis is shifted to the electronics. This allows the physical size of the projector engine to be minimized to fit with hand-held

[ Bidirectional raster scan ][ Scanned vs. imaging projectors ]

[ PicoP projection display system ]

“Infinite focus” property of the scanned laser projector compared with an imaging-type projector.

Distance from projector [m]

Scanned laser focus advantage

Generation of raster pattern

BlankingDisplay

Frame buffer memory

System controller and SW

Video ASIC

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MEMS device

MEMS drive ASIC

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consumer products. The electronics, which can be integrated more straight-forwardly into consumer products, take over tasks that are done optically with other projector designs. Some of the tasks that are shifted include pixel posi-tioning, color alignment and brightness uniformity. With the PicoP, the video processor and MEMS controller have been implemented as custom ASICs that drive the IPM scan engine.

MEMS Drive ASICThe MEMS Drive ASIC drives the MEMS scanner under closed loop control. The horizontal scan motion is

created by running the horizontal axis at its resonant frequency—which is typically about 18 KHz for the WVGA (Wide Video Graphics Array) scanner. The horizontal scan velocity varies sinu-soidally with position. The MEMS con-troller uses feedback from sensors on the MEMS scanner to keep the system on resonance and at fixed scan amplitude.

The image is drawn in both direc-tions as the scanner sweeps the beam back and forth. This helps the system efficiency in two ways. First, by run-ning on resonance, the power required to drive the scan mirror is minimized. Second, bi-directional video maximizes the laser use efficiency by minimizing the video blanking interval. This results in a brighter projector for any given laser output power.

The vertical scan direction is driven with a standard sawtooth waveform to provide constant velocity from the top to the bottom of the image and a rapid retrace back to the top to begin a new frame. This is also managed in closed-loop fashion by the MEMS controller based on position feedback from the MEMS scanner to maintain a smooth and linear trajectory. The frame rate is typically 60 Hz for an 848 3 480 WVGA resolution; it can be increased when the projector is used in lower resolution applications.

PicoP pixel diameter

Display pixel size = 43° image size4number of pixels

Typical imaging projector spot size focused for 0.5 m projection distance

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Size: The height or thickness of the projector is its most important characteristic; the technology must be able to be embedded in thin handheld devices. Both height and volume should be minimized. The initial products will have projector engine sizes ranging in height from 7 to 14 mm and in overall volume from 5 to 10 cc.

Brightness: The brighter the better. Brightness is limited by the available brightness of the light sources, either lasers or LEDs, the optical efficiency of the projector design, and by the need for low-power operation in order to maximize battery life. Initial product offerings will be in the range of 5-10 lumens.

Image size: In general, the shorter the distance it takes for the projector to produce a large image, the better—at least up to a point. A one-to-one distance/image-size ratio (projection angle of 53 degrees) is probably a good target. The first round of products on the market will have projec-tion angles in the range of 30-45 degrees.

Focus-free operation: Focus-free operation is a very desirable attribute in mobile applications. It is like “point and shoot” in a camera. Unlike typical projectors used in business settings, pico-projectors are designed for mobile

Video ASICThe video processor accepts either RGB or YUV (NTSC/PAL) input. A video buffer is provided to allow artifact-free scan conversion of input video. Gamma correction and colorspace conversion are applied to enable accurate mapping of input colors to the wide laser color gamut. A scaling engine is available for upconverting lower resolution video content.

A proprietary Virtual Pixel Synthesis (VPS) engine uses a high-resolution interpolation to map the input pixels to the sinusoidal horizontal trajectory. The VPS engine demonstrates how projector functions have been shifted from the optics to the electronics in the scanned laser paradigm. It effectively maps the input pixels onto a high-resolution virtual coordinate grid. Besides enabling the repositioning of video information with subpixel accuracy onto the sinusoi-dal scan, the VPS engine optimizes the image quality. Brightness uniformity is

These super-small, low-power projector systems will open up the display bottleneck

for mobile devices, allowing information to be accessed and

shared more easily from portable devices.

use (in other words, the distance from the projector to the image will likely change often).

Resolution: Resolution can be expected to continue to grow as product technology matures. The wide screen for-mat is generally desirable for viewing video content. Initial product offerings will typically offer resolutions from QVGA (320 x 240) to WVGA (848 x 480).

Color: Pico-projectors typically use either lasers or red, green and blue LEDs for light sources. In both cases, the result is large color gamuts that far exceed the usual color range experience of TVs, monitors, and conference-room-type projectors. White LEDs used with color filters yield a reduced color gamut.

Contrast: The higher the better. Just as brightness is a measure of the absolute whiteness of pico- projec-tors, contrast is the measure of their absolute darkness. Contrast is the dynamic range of a display, making the difference between washed out images and crisp dramatic-looking images.

Battery life: A starting goal for mobile devices is that they should last for the length of a complete movie. This puts the lower limit at around 1.5 hours.

REQUIREMENTS FOR PICO-PROJECTORSIn order to operate in a mobile format, pico-projectors will need to meet a number of requirements related to the following dimensions.

managed in the VPS engine by adjust-ing coefficients that control the overall brightness map for the display.

Optical distortions—including key-stone, parallelogram, and some types of pincushion distortion—can be compen-sated by using the VPS engine to adjust the pixel positions. The VPS engine

also allows the pixel positions for each color to be adjusted independently. This simplifies the manufacturing alignment of the IPM by relaxing the requirement that the three laser beams be perfectly aligned. The positions of the red, green and blue pixels can be adjusted elec-tronically to bring the video into perfect alignment, even if the laser beams are not. This capability can also be used to compensate for some types of chromatic aberration if the PicoP is used as an engine in a larger optical system.

Mapping from digital video coding to laser drive is performed by the Adaptive Laser Drive (ALD) system. The ALD is a closed-loop system that uses optical feedback from each laser to actively compensate for changes in the laser characteristics over temperature and aging. This ensures optimum brightness, color and grayscale performance. Unlike other display systems, optical feedback is incorporated to ensure optimum color balance and grayscale.

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[ References and Resources ]>> R. Sprague et al. “Mobile Projectors

Using Scanned beam Displays”, from Mobile Displays, Technology and Appli-cations, Bhowmik, Li, and Bos editors, John Wiley and Sons, Chapter 21, 2008.

>> W. Davis et. al. “MEMS-Based Pico Projector Display”, Proceedings of IEEE/LEOS Optical MEMS & Nanophotonics, Freiburg, Germany, 2008.

ComponentsBiaxial MEMS scanner The biaxial MEMS scanner is made using standard bulk silicon MEMS fabrication processes. The WVGA pico-projector scanner has a scan mirror diameter of approximately 1 mm, and it produces an active video scan cone of 43.2° by 24.3°.

The scanner uses moving-coil actua-tion with a single drive coil, which can be seen on the vertical scan frame in the photo with just two drive lines as shown. The single coil design simplifies the fabrication of the MEMS scanner and reduces the number of required inter-connects. The MEMS die is housed in a package with small magnets that provide a magnetic field oriented at approximately 45° to the scan axes. A single composite drive signal is applied that contains the superposition of the fast-scan horizontal drive at the resonant frequency of the horizontal mirror motion and the 60 Hz vertical drive sawtooth waveform.

The mechanical design of the MEMS scanner allows motion along only the two orthogonal scan directions. Mechanical filtering, resulting from the different mass and flexure stiffness governing horizontal and vertical motion, sorts the drive sig-nals by frequency content, inducing the 18 KHz resonant motion of the horizon-tal axis and the 60 Hz sawtooth motion of the vertical axis. Piezo-resistive sensors provide scan mirror position feedback to the MEMS controller ASIC to maintain closed loop accuracy of the desired scan mirror motion.

LasersThe technology for the red and blue lasers in the PicoP leverages the technol-ogy of similar lasers that are used for the optical disk storage industry. The wavelength requirements are shifted somewhat, but the basic technology is the same—GaAlInP red laser diodes and GaN blue laser diodes.

Pico-projectors incorporate green lasers as well. Prior to the push for laser projectors, green lasers had not been used for any similar high-volume applications. The technology for the

range, the green wavelength should be chosen where it will be most useful for enhancing the color of the display. Green lasers at 530 nm are a good choice for maximizing the color gamut.

The ability to directly modulate the lasers is at the heart of the scanned laser pico-projector technology. Pixel times at the center of the WVGA scanned display are on the order of 20 ns. Lasers therefore need modulation bandwidths on the order of 100 MHz.

A path forwardThe first generation of pico-projectors are being launched this year. These super-small, low-power projector systems will open up the display bottleneck for mobile devices, allowing information to be accessed and shared more easily from portable devices. With the high volumes expected, there is great market oppor-tunity for key components such as red, green and blue lasers.

The scanned laser projector paradigm provides a path forward to higher-reso-lution projectors without growth in size. Unlike SLM-based projector technol-ogies—in which increased resolution means growth in the number of pixels in the SLM—the single-pixel, single-scan-mirror nature of the engine remains the same, even as the resolution of the projected display increases. t

Visit OPN online (www.osa-opn.org) to view videos that demonstrate the technical concepts behind scanned laser projection and how the MEMS scanner works.

Mark Freeman (mark_freeman@microvision.com) is a principal engineer in optics at Microvision Inc., in Redmond, Wash.,

U.S.A. Mark Champion is a principal engineer in electronics and Sid Madhavan is a vice presi-dent of engineering at Microvision.

[ Bi-axial MEMS scanner ]

green laser in the PicoP is based on infrared lasers developed for the telecom industry, the other massive market for laser technology. Robust near-infra-red laser diodes with very high modula-tion bandwidths are combined with a frequency-doubling crystal, usually periodically poled lithium niobate, to produce a green laser that can be directly modulated. With an eye toward the burgeoning pico-projector industry, sev-eral companies, including Corning and OSRAM, are ramping up production of suitable green lasers.

The choice of which wavelength to use for the lasers is based on two con-siderations. First is the response of the human eye (the photopic response) to different wavelengths. This response is a somewhat Gaussian-looking curve that peaks in the green-wavelength region and falls off significantly in red and blue. The amount of red and blue power needed to get a white-balanced display varies rapidly with wavelength.

For example, eye response increases by a factor of two when the wavelength is changed from 650 nm (the wave-length used for DVD drives) to 635 nm. This allows the required laser power to drop by the same factor, making a projector that is lower power. Similarly, the blue laser should be chosen to have as long a wavelength as possible. Cur-rently, blue lasers in the range of 440 to 445 nm are the best practical choice. As the industry grows, longer wavelengths in the range of 460 to 470 nm may become a better option.

The second consideration is color gamut. Since the photopic response is peaked all through the green wavelength

Member

Only two drive lines

Vertical scan frame with drive coils

Piezo resistive sensor

Scan mirror