[CEATEC] Advances Seen in Elemental Technologies for Ultra-small Projectors

Oct 13, 2009 18:35 Tsuneyuki Miyake, Nikkei Microdevices

A variety of ultra-small projectors were exhibited at CEATEC JAPAN 2009 by companies including Hitachi Ltd, Microvision Inc, Nippon Signal Co Ltd and 3M Co.

Especially, projectors using laser light sources showed remarkable advances and draw visitors' attention as well as green laser technologies that realize those projectors.

Ultra-small projectors function as projection-type displays that are embedded in battery-powered mobile devices or used as accessories for devices. They use LEDs or laser diodes as light sources and project images on a wall, etc by scanning of a MEMS mirror.

In past few years, several companies have announced that they are developing ultra-small projectors, and some of them hit the market in 2008.

Projectors using semiconductor laser diodes as light sources are seen as front runners because they have high image quality and do not require focal adjustment. However, they have a number of technical problems, and most of the products in the market use LEDs as light sources. At this year's CEATEC, 3M exhibited an LED-based ultra-small projector.

The image quality and power consumption (battery life) of projectors using semiconductor laser diodes are dependent on green laser diodes. Currently, the specifications of green laser diodes are not high enough for ultra-small projectors.

Specifically, direct oscillation devices with a green wavelength of 532nm need to be developed at commercial level. Therefore, some manufacturers use SHG (second harmonic generation) elements to halve the near-infrared wavelength of 1,064nm to 532nm for their products and prototypes.

Among the laser diodes used to emit the near-infrared light with a wavelength of 1,064nm, only a few can withstand high-speed modulation. Modulation is necessary because ultra-small projectors normally emit RGB laser at a mirror, and the screen is scanned by the reflected light to display video images.

The higher the resolution of video, the higher the modulation speed needs to be. And there have only been a few devices that meet that requirement.

Furthermore, the suppression of speckle noise, which is a major factor of image quality degradation, depends on the modulation speed of laser diodes. Speckle noise is a phenomenon that causes flickering problems by, for example, the interference of irradiated light and reflected light.

To reduce speckle noise, the interference can be suppressed by modulating laser light. But, because many of the laser diodes with the near-infrared wavelength of 1,064nm do not have a high enough modulation speed, speckle noise is often found in green light.

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