Laser display with single-mirror MEMS scanner Peter Schreiber Bernd Hoefer Andreas Braeuer Michael Scholles Abstract — While arrayed DMD and LCD microdisplays are well-established approaches for visuali- zation tasks, image-forming laser scanners are an emerging technology used to build miniaturized projection displays. A directly modulated RGB-laser module consisting of diode lasers for red and blue and a frequency-doubled semiconductor laser for green with color combining optics form the light source for the laser scanner have been developed. Subsequent beam-shaping optics suppresses unwanted stray light and enables optimum illumination of the scanning mirror. The MEMS device features a single scanning mirror oscillating in two directions in resonant mode. This requires appro- priate data delivery realized by a custom-made driving logic, which converts the pixel stream origi- nally arranged in rows and columns to the Lissajous-like spot trajectory on the screen. Additionally, the increased image brightness at the vertical and horizontal borders of the field of view (FOV) is also compensated by the modulation of laser power. Theoretical investigations of the resulting maximum achievable system transmission are presented. Different systems, such as an extremely miniaturized monochrome projection head with an integrated diode laser and a full-color projector have been realized. Important problems to be tackled are fast analog modulation of the laser power with high resolution and improved suppression of stray light and speckle. Keywords — Projection displays, scanning, MEMS. DOI # 10.1889/JSID17.7.591 1 Introduction With the development of LCD- and DLP-based projection displays, growing interest in the miniaturization of these devices with the long-term goal of integration into personal- information equipment such as mobile phones is growing. This interest, together with the advent of very fast MEMS single-mirror scanning devices and miniaturized modulated laser sources, enforces investigations on achievable resolu- tion, light flux, and overall size of full-color displays aimed for VGA resolution – or even higher. Contrary to the well-established miniature projection displays consisting of a incoherent light source (usually an LED), 1 a pixelated imager and projection optics, the main components of the scanning projector are a modulated sin- gle-mode RGB-laser source and a single-mirror MEMS scanner. 2 Thus, the main challenges of system miniaturiza- tion shift from shrinking imager and projection optics and minimizing size and étendue of the light source towards efficiency enhancement and miniaturization of high-speed modulated laser sources, development of MEMS scanners with optimum mirror diameter and tilt angles, mapping image information on the non-linear movement of the spot at the screen and tilt angle as well as miniaturization of laser- beam-shaping optics. Additional effort – which is outside the scope of this paper – is required to define reasonable specifications for pico-projector scanning devices, which account for the peculiarities of laser projection such as speckle and diffraction-determined pixel images as well as application-specific properties of the projection screen (structure, color, motion, tilt, warp, etc.) and effects of utili- zation such as tremor for handheld devices. 2 Design considerations The most interesting application-relevant optical specs of MEMS scanning displays are achievable resolution and optical flux. In the following, parameters influencing these proper- ties will be discussed. 2.1 Scanning By restricting the estimations for the one-dimensional case (Fig. 1), a mirror oscillating harmonically between maxi- P. Schreiber, B. Hoefer, and A. Braeuer are with the Fraunhofer Institute for Applied Optics and Precision Mechanics, Micro-Optical Systems, Albert-Einstein-Str. 7, Jena, Thuringia 07745, Germany; telephone +49-3641-807-430, fax –603, e-mail: peter.schreiber@iof.fraunhofer.de. M. Scholles is with the Fraunhofer Institute for Photonic Materials, Dresden, Germany. © Copyright 2009 Society for Information Display 1071-0922/09/1707-0591$1.00 FIGURE 1 — Basic scanning geometry. Journal of the SID 17/7, 2009 591