07 May 2008
A lack of suitable green lasers has slowed the integration of scanning projectors into mobile devices. Now Michael Schmitt and Ulrich Steegmüller from OSRAM Opto Semiconductors claim that they have a device that meets all the requirements for this challenging application.
With most hand-held devices there is a mismatch between the display size and the device's ability to handle high-resolution content. Mobile phones, personal media players, digital cameras and many other hand-held devices can display high-resolution images, but usually only have a display with a diagonal of less than 2.5 inches and a resolution of maximum QVGA (320 x 240 pixel). One way of solving this issue is to integrate a projector into the hand-held device.
Such projectors can be realized either by micro-display or laser-scanning technology. If a micro-display approach is used, the imager is illuminated by an appropriate light source and the image is subsequently projected through an optical system. There are several advantages and disadvantages to this approach. Superior image quality on one side comes with lower efficiency, complex optics and a fairly large overall projector size. To enable the integration of projectors into hand-held devices, these disadvantages can be overcome by using a laser-scanning projection display.
In a laser-scanning or "flying spot" projector, the picture is written on a screen, using three lasers emitting red, green and blue light. The light is combined into one beam and a mirror system is used to scan over the screen, which can be an arbitrary surface. Each single pixel is written by a pulse of green, blue and red laser light with a pulse length in the tens of nanosecond range, depending on the image resolution. The intensity of each pixel is defined by the amplitude of the pulse. The advantages of laser-scanning projectors are that they have a large depth of focus; the ability to be projected on any arbitrarily formed surface; and they have a high colour brilliance due to the saturated colours of the laser light sources. Also the simple optics and the small form factor of the lasers are key items for realizing an integrated projector. Since lasers are coherent light sources, scanned beam images can contain a substantial amount of speckle contrast, which may reduce perceived image quality. Appropriate designs of the optical system can help to reduce the speckle content.
A laser-scanning projector puts certain requirements on the lasers. The lasers must be efficient to enable battery-powered operation and have sufficient optical output power to provide the required on-screen brightness. They must have a small form factor; be capable of handling high modulation speeds for high-resolution images; have high beam quality to provide small spots on the screen for sharp images; and have low noise to enable brightness control.
Due to their mature technology red and blue lasers already fulfil these requirements and are available as edge-emitting laser diodes. A green laser for this application, however, has been more elusive.
In the green spectral range a directly emitting semiconductor material is not available at present. An infrared optically pumped semiconductor disk laser (OPS) together with a frequency doubling nonlinear crystal is therefore used to generate green light. The configuration of the laser in principle is: an 808 nm infrared laser optically pumps the OPS chip that emits 1060 nm. Inside an external cavity the frequency doubling takes place and leads to 530 nm green light emission. The OPS technology is chosen because it can be highly integrated, provides excellent beam quality, possesses a large modulation bandwidth and allows power scaling.
In more detail, the OPS chip is pumped by a standard edge-emitting 808 nm GaAlAs laser diode. It was optimized for low threshold and provides a wall-plug efficiency of more than 50%. The divergent emission of the 808_nm diode is pre-collimated and focused on the OPS chip surface by a lens system.
The OPS chip is designed for an emission of 1060 nm and is grown on a 4-inch GaAs substrate using MOVPE technology. After epitaxial growth the wafers are coated and further processed to prepare for mounting, tested and finally separated into individual dies. The size is approximately 1 x 1 mm. Since the OPS chip includes a Bragg reflector, it forms one side of the resonant cavity and the cavity end mirror forms the other side. The small aperture cavity mirror is aligned and assembled to achieve high output power at fundamental mode (TEM00) operation. All optical surfaces of the cavity elements have been designed to minimize losses at 1060 nm to keep high conversion efficiency. The physical length of the cavity is short, enabling fast laser build-up and high modulation bandwidth.
To convert the 1060 nm light into 530 nm, quasi-phasematched commercially available MgO:PPLN crystals are used inside the cavity. The use of short and simple bulk crystals gives a significant advantage in terms of cost, since the material cost of green lasers is strongly influenced by the frequency doubling material.
Additional elements within the cavity serve as wavelength stabilization, temperature adjustment of the crystal and output coupler for the green light while maintaining high reflectivity for the fundamental 1060 nm wave.
All of the components of the green laser are assembled on a substrate that is mounted onto a metal carrier, which acts as an additional heat sink and provides the mechanical interface to the outside. A flexible printed circuit provides the electrical connection from the outside to the laser. For mechanical shelter and dust protection a metal cover is mounted over the laser system, which leads to an overall size of the green laser package (without the flex interface) of only 13 x 6.5 x 4.8 mm.
Regarding the optical output power, both threshold current and overall efficiency are important. The threshold current is determined by the 808 nm pump diode and the 1060 nm OPS chip. Both add up to a typical threshold of about 150 mA. With an efficiency of about 7% a typical optical output at 530 nm of about 70 mW at 500 mA can be achieved. The beam has a circular Gaussian profile with an M2 value close to 1. Theoretical modelling, as well as first high-speed measurements, confirm the laser's capability for high modulation speeds. Also no significant green noise can be observed, and the laser shows stable operation over a large range of amplitudes and pulse widths.
With these impressive performance characteristics, green lasers are now able to meet all the requirements that are mandatory for integration into a scanning-beam projector for mobile applications. Prototype scanning projectors for mobile applications are now starting to appear on the market thanks to the long-awaited availability of suitable green-laser sources.
• This article originally appeared in the May 2008 issue of Optics & Laser Europe magazine.
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