Laser displays
Projection displays based on diode-pumped-laser technology provide saturated colors, bright images, and high-definition-television (HDTV) quality-even with image sizes of 4 x 6 m. Applications expected to benefit from such enhanced imaging-which could generate brilliant, life-size pictures completely filling the spatial acceptance range of the human eye-range from flight simulators to home cinema.
Unlike conventional display technology, diode-pumped-laser-based projectors direct-write bright images with high contrast and spatial resolution. The light is collimated, so the depth of focus is almost unlimited, and the images are sharp regardless of the distance between the projector and screen. There are no aberration and chromaticity errors, and image color is enhanced by additive superposition of modulated red, green, and blue (RGB) laser light. The result is a color spectrum significantly broader than that of a conventional cathode-ray TV tube (see Fig. 1).
Mechanical deflections
In a diode-pumped laser-projection system such as that demonstrated by Laser Display Technologie GmbH (Gera, Germany), three visible laser beams with the appropriate RGB wavelengths are generated, with the power of each laser beam modulated by an electro-optic modulator that is driven by the electronic video signal. Modulated beams are then spatially superimposed and transmitted by a multimode optical fiber to a deflection unit. This unit combines a galvanometer mirror and a fast-spinning polygon mirror with up to 25 facets. The first mirror deflects the beam vertically. The second mirror deflects it horizontally, with rotation adjustable up to 2 kHz. The frequency varies according to the line-deflection frequency of different video standards such as Phase Alternation by Line (PAL), National Television System Committee (NTSC), or high-definition television.
Because the laser beam is collimated and the depth of focus is almost unlimited, the screen surface can be flat, cylindrical, or even spherical (see Fig. 2). The image size will depend on the beam-deflection angle and the distance between the scanning unit and projection surface. Image dimensions can be continuously changed by varying the distance between scanning unit and projection screen or by adding zoom optics in front of the beam scanner; this allows a factor-of-six variation in size without reduced image contrast or resolution.
Laser sources for displays
While the last few years have seen several advances in the basic components for laser projection displays, development of an appropriate laser source has remained a challenge until just recently. One reason involves the narrow wavelength bands required for the RGB laser light-620 to 630 nm (R), 510 to 540 nm (G), and 430 to 460 nm (B). If the wavelength of each color is within the given range, then the color gamut of the generated RGB light is large, and the powers of the three lasers for the generation of white light can be almost equal.
Another critical laser specification is the spatial quality of the beam, because it is the main determinant of the spatial resolution in the generated image. The divergence should, therefore, be close to the diffraction limit-with a beam product of less than 0.75 mm · mrad. Also necessary are a long-term stability of less than 2% variation over eight hours, noise of less than 1% rms, and linear polarization exceeding 100:1.
In addition, a short coherence length is necessary to minimize laser speckle so it is not noticeable. For the typical laser-projection screen, the coherence length of the laser will need to be on the order of a millimeter-a requirement that generally cannot be met by visible-output gas lasers such as argon- or krypton-ion types or by frequency-doubled, continuous-wave, infrared (IR) solid-state lasers.
These specifications are only possible when diode lasers are the primary light source for the projection system. Currently, though, visible-light-emitting diode lasers do not provide multiple-watt output powers in a diffraction-limited beam. An alternative developed by the Zentrum fur Lasermesstechnik und Diagnostik (Kaiserslautern, Germany) pumps IR-output solid-state lasers with high-power IR laser diodes (see Fig. 3). Subsequent nonlinear frequency conversion of the IR laser output then generates visible light with the power, color, and beam properties required for large-screen laser displays.
Diodes pump up power
The Universitat Kaiserslautern RGB laser source includes a diode-pumped, neodymium-doped yttrium vanadate (Nd:YVO4) oscillator that is passively modelocked with an antiresonant Fabry-Perot saturable absorber. When pumped longitudinally with the 10-W output of an 808-nm aluminum gallium arsenide (GaAs) diode laser, the oscillator produces 7-ps-long pulses with a repetition rate of 80 MHz and an average output power of 4.2 W.
This output is then amplified to a power level of 42 W by a system including one double-pass and three single-pass Nd:YVO4 amplifiers. In each, the Nd:YVO4 crystal is longitudinally pumped at both ends by fiber-coupled diode bars with a total power of 25 W. The total pump power of the laser system is 112 W, corresponding to an optical efficiency of 38%. The laser output has high spatial quality (M2 < 1.2).
The high peak power of the ultrashort IR pulses (up to 75 kW) allows efficient nonlinear conversion of the light into the visible region. A key component is a noncritically phase-matched potassium titanyl arsenate (KTA) optical parametric oscillator (OPO) that is synchronously pumped. This generates a signal wave at 1535 nm and a mid-IR idler wave at 3470 nm. With an appropriate signal-resonant cavity, the OPO converts laser light into signal and idler output with a total efficiency that can reach 75%.
Frequency-doubling part of the IR laser light in a lithium triborate crystal generates green (532 nm) light. Red (629 nm) output results from single-pass sum-frequency mixing (SFM) of the 1535-nm OPO signal wave and 1064-nm laser radiation in a KTA crystal. Another single-pass SFM process mixes the red light with residual OPO signal radiation to create a blue (446 nm) beam.
Adjusting the power ratio of the simultaneously generated RGB beams produces white laser light with a total power of 19 W. In relation to the power of the laser pump diodes, the visible RGB output corresponds to an overall efficiency of 17%. The visible light beams are almost diffraction-limited (M2 < 1.3), and the polarization is linear with a ratio exceeding 600:1.
Not only does this setup produce images with high spatial resolution, the spectral width of the ultrashort pulses is sufficient to suppress any disturbing laser speckle. Most important, the high peak power of the ultrashort pulses allows efficient conversion of the IR laser radiation into the visible. In this way, optimized RGB systems will output almost 20 W with a total electrical input of less than 1 kW and will meet the specifications required for a high-quality large-screen laser-projection display.
ACHIM NEBEL is a researcher, BORIS RUFFING is a Ph.D. student, and RICHARD WALLENSTEIN is a professor at the Zentrum für Lasermesstechnik und Diagnostik, Universität Kaiserslautern, Erwin-Schrodinger-Str. 46, D-67653 Kaiserslautern, Germany; e-mail: [email protected].