Scanned Laser Pico-Projectors: Seeing the Big Picture (with a Small Device) Mark Freeman, Mark Champion and Sid Madhavan www.osa-opn.org 28 | OPN May 2009 1047-6938/09/0??/0028/6-$15.00 ©OSA Red LD 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. mirror 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. 30 | OPN May 2009 www.osa-opn.org 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.