A Single-Mirror Laser-Based Scanning Display Engine

Myriad pico projectors are scheduled to become commercialized in 2009, and each one offers a different pathway to success. Here, Microvision details the technology and development decisions that have lead to its PicoP single-mirror laser-based scanning display engine.

by David Lashmet, David Baty, and Matt Nichols

THE RACE TO MARKET for pico projectors has never been more heated. Companies active in this sector have all taken somewhat different methods to developing these tiny projectors that will be mobile and lightweight while delivering a big-screen viewing experience from mobile devices. Following an aggressive 24-month design program, kicked off in 2006, Microvision is on the verge of commercializing its ultra-miniature laser-based display engine, which will first be integrated into a battery-operated handheld pico projector roughly the size of an iPhone and will follow as an embedded pico projector in mobile phones and other portable devices.

This article details Microvision's fundamental technology, reasons for its design approach, historical development, advantages, and future direction for the PicoP display engine.

Display-Engine Platform

Microvision's PicoP^™ display-engine platform provides a distinct display approach in the emerging pico-projector category. The foundation of the display engine is based on modulating light temporally and scanning spatially using a single tiny oscillating MEMS silicon mirror to produce an image. The tiny scanning mirror itself is less than approximately 1 mm² in area – about the size of the head of a pin. The MEMS scanning mirror is integrated with red, blue, and green lasers, video and MEMS drive electronics, and an optical combiner that enables the total display engine (which measures 42 mm x 20 x 7 mm and weighs 20 grams) (see Fig. 1). In an accessory device, this design could use as little as 3 W with a further expected reduction to 1.5 W or lower for implementation into embedded mobile-phone projectors.

Fig. 1: A single MEMS mirror oscillates biaxially to raster-scan a two-dimensional image – much like old TVs but with photons instead of electrons.

The RGB lasers are modulated on an 8-bit-per-color-channel basis to address each of the individual picture elements, or pixels, that comprise a digital image or frame of video. Thus, if one of the three colors is not needed due to the image content, this laser is down-modulated, which minimizes power consumption. All three colors are combined by optics into a single light path. The single scanning mirror then scans the combined beam of light in a raster-like fashion, one pixel at a time, at 60 frames per second (fps) to project large, high-resolution video images (Fig. 2).¹ The current PicoP display engine projects a WVGA (848 x 480) resolution image in a 16:9 format at a brightness of 10 lum, with a diagonal image size of about 1 m at a projection distance of 1.1 m.

Laser Advantages

In addition to the advantages of low power and enabling the small size of the display engine, laser light sources require no projection lens or expansion optics. This is because they generate a collimated beam of light, which is raster scanned by the single biaxial MEMS mirror. Furthermore, this allows Microvision's projector to be focus-free because each pixel is essentially a fast dwell-time dot from a laser pointer – and collectively, this re-creates the digital image across any brightly reflective surface at any viewing angle. In other words, a scanned-laser display eliminates the need for the movable optical lens associated with LED-based projectors that use physical arrays of pixels. Plus, it dispenses with the expansion lens needed for laser-lit physical arrays, whether LCOS or DLP. Lenses introduce losses, distortion, vignetting, and chromatic aberration. But the real issue is package size. Every cubic millimeter in a projector assembly is valued, especially in converged devices such as handsets, and size significantly affects adoption. In addition, reducing the size and complexity of the optical train reduces component and design costs.

Lasers also provide other advantages besides being focus-free, including a broader color gamut (greater than 100% of NTSC) and very high contrast ratios (>10,000:1), which leads to better gray scale. Lasers produce monochromatic red, green, and blue that, in the PicoP display-engine design, is directly transmitted through the system without having to pass through any filters or light-valve mechanism that can alter the emitted spectrum. For portable projection from handsets, this makes text and video images easier to see in a wide variety of lighting conditions.

Design Considerations: The Road to Commercialization

One or Two Mirrors? There are actually quite a few approaches to scanning light with MEMS mirrors; Microvision, Motorola, and Konica-Minolta have all shown commercial prototypes. The key differentiator between these is in the number of scanning mirrors: two mirrors; one for each axis of oscillation or one mirror that goes both back and forth and up and down bi-axially. In a two-mirror system, the second mirror has to catch the spread beam of the first mirror, which can cause image alignment or geometry issues, along with possible optical degradations. If either mirror is out of skew, the effects on the final image can be dramatic. In the case of a single-mirror system, only a single reflection of the modulated laser image is required and the registration between horizontal and vertical scanning is always ensured. Thus, we focused our efforts on a practical design with one MEMS mirror, not two.

Lasers in Scanned-Beam Pico Projectors

Building an ultra-miniature mobile laser projector, naturally enough, starts with small laser diodes. In terms of supporting an RGB laser system, red diodes are readily available, as they are found in consumer CD players. Blue laser diodes were first developed by Shugi Nakamura at Nichia in 1997 – today, these are used in Blue-ray DVD players. Blue laser diodes are also suitable for mobile projectors. True green laser diodes, however, are not yet available. But commercial plans for laser TVs as well as miniature projectors have encouraged both start-ups and companies such as Corning and OSRAM to develop frequency-doubled infrared laser diodes that yield green at a 530-nm wavelength.² Although each of these companies has a different approach to making green laser diodes, all have been successful, and they all are currently designing their production capabilities to support the high-volume requirements of the mobile marketplace beginning in 2009 and growing rapidly thereafter.

Form Factor: From Refrigerator Size to Cell-Phone Size in 14 Years

In order to deliver an optimized single-mirror scanned-laser display solution, numerous fundamental design considerations and quality goals have had to be addressed. Microvision's highest priorities have been on solidifying image quality, reducing power, and designing the engine for high-volume manufacturability.

Meeting all of these challenges has been the collective effort of 14 years of development, including 2 years focused exclusively on the PicoP display-engine product and miniaturizing the core scanning mirror technology. The point of entry for this project was the scanning mirror used in the Nomad^™ wearable head-up display. That product was a technical success, but a consumer-market failure because it was only powered by a red laser. Moving to RGB meant a rich color palette and a broad gray scale, with contrast ratios greater than 1000:1.

Fig. 2: A model PicoP-enabled projector system using a MEMS scanning mirror and three lasers requires no focus lens.

Nomad itself went through two iterations. The most notable was a change in mirror design from an electrostatic drive – requiring hermetic sealing and a 500-V transformer – to an electromagnetic drive that worked at 1 atm with a simple dust cover. Next, a military contract yielded the Spectrum 500 eyewear, which provided RGB (with chemical lasers) in a milk crate (> 10 liter) form factor. That is vastly smaller than Microvision's circa 1998 electronics suite, which included a rack of servers and could easily mimic a very warm refrigerator.

Thanks to Moore's Law and advances in solid-state laser technology, Microvision's first bench-top pico-projector prototype was realized prior to and demonstrated in 2006 at SID's Display Week. This demonstrator showcased the capabilities of an RGB single-mirror scanned-laser display, with greatly reduced electronics. Essentially, the electronics were the size of a pizza box, while the integrated photonics module (IPM) was well under 10 cc, suggesting today's smaller 5.6-cc IPM. This same benchtop demonstrator delivered bright 800 x 600 resolution at 60 Hz and 10 lum using wall power. But the core Nomad MEMS mirror only projected at a 22° horizontal angle (+/-11°), whereas the research-grade next-generation mirror offered twice the horizontal throw angle of 44° (+/-22°).

Given tremendous customer interest in a wider field of view, the second-generation MEMS mirror became Microvision's core focus, so this design was optimized for reliability, as its electronic controls were reduced to FPGAs. Thus, at Display Week 2007, Microvision demonstrated a benchtop PicoP display-engine projection system based on this wide-angle MEMS scanner, delivering "letterbox" WVGA (852 x 480) resolution in a 16:9 aspect ratio. The wide-angle MEMS mirror produced a full-color digital image with a viewable area four times larger than the 2006 prototype. The electronics were shrinking and speckle, a frequently cited somewhat objectionable optical artifact seen in most laser-based projection systems, was demonstrated to be largely undetectable to most observers of this demonstration, greatly mitigating this concern.

Further improvements in drive electronics and color control produced the battery-operated self-contained prototype in January 2008 for the Consumer Electronics Show in Las Vegas. Code-named the SHOW^™ prototype, this handheld device illustrated the concept of cell-phone projectors, given its compact integrated photonics module its wide field of view, bright colors, and high contrast ratio (>1000:1). The SHOW prototype supported content from a host of companion devices, including an iPod, a Nokia N95 cellular telephone, and a notebook computer.

By September 2008, the FPGAs in the electronics platform for Microvision's handheld pico projector were being converted to ASIC chips. This process produced the current third-generation (SCP-3) prototypes, which first were shown in public at the SID Mobile Displays Conference in September 2008. Compared to the SCP-1 SHOW prototypes, the SCP-3s exhibit much better small-font readability, with legible 10-point font text, along with a 200% color gamut compared to broadcast television, or roughly 100% more than NTSC. Technically speaking, the SCP-3s also normalized the color balance between the projectors for large-scale manufacturability. This established a reliable D65 white point for Microvosion's pico projectors and increased side-by-side contrast ratio on a checkerboard pattern to greater than 2000:1.

By commercial launch in 2009, the major drive electronics will be converted to ASIC chips, a process that significantly reduces the image noise introduced by prototype lasers. In parallel with this conversion of FPGAs to ASICs, a global team of supply-chain partners has been assembled to support requirements for initial and high-volume manufacturing of the integrated photonics module, as well as final product assembly. While many of the partners remain confidential, lead design assembly is being driven by Asia Optical, one of the largest assemblers of digital cameras in the world.

Debut, Debits, and a Brighter Future

Microvision is preparing for commercial product launch of an accessory pico projector in 2009. To support embedded designs, PicoP display-engine evaluation kits will also be made available. This 10-lum WVGA projector will be 5.6 cc in volume and only 7 mm in height, so it is optimized for incorporation into cellular handsets.

Microvision's pixel-by-pixel image creation method optimizes the power efficiency of its laser light sources. Currently, red and blue laser diodes offer double-digit electrical-to-optical efficiency, rivaling LED light sources, while producing a polarized, collimated beam. Frequency-doubled IR lasers – producing green laser light – are less efficient today, but pathways exist to improve these at least beyond incandescent lights. True green laser diodes have not yet been invented. But given the potential size of this market, we expect efforts by material scientists to solve this dilemma.

Also on the future roadmap is a higher resolution image, to 720p and beyond. Using Microvision's core technology, a higher resolution also means a broader angle of projection, and even further reduction in an already acceptable level of speckle. Other image-quality strategies are in advanced development: on the laser side, in the optical train, and with proprietary screening material. These continued improvements to image quality are likely to make pico projectors key supplements to mobile devices, ultimately becoming ubiquitous in smart phones, then in all phones. Eliminating the projection lenses and using lasers seems the most efficient way to accomplish this. Add focus-free capabilities, and it is easy to recognize the value of a single MEMS scanning mirror to end users.

Notes

PicoP, Nomad, and SHOW are trademarks of Microvision, Inc.

References

¹R. Sprague, M. Champion, M. Brown, D. Brown, M. Freeman, and M. Niesten, "Mobile Projectors Using Scanned Beam Displays," in Mobile Displays: Technology and Applications, A. K. Bhowmik (ed.) (Wiley & Sons, June 2008).

²M. Schmitt and U., Steegmüller, "Green laser meets mobile projection requirements," Optics.org/ole, 9 May 2008. •