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Blue laser - the alpha and the omega

May 14, 2004

The future of DVD is blue. New, low-cost optical laser technology generates short-wavelength beams. At the other end of the beam are detector heads that will soon contain arrays of up to 25 sensors. Two Fraunhofer Institutes are taking the lead at both ends of the spectrum.

Man's appetite seems to be insatiable. In the dark ages, packing away enough provisions to last the winter was a matter of life and death. When it comes to digital storage, this squirrel mentality has largely been preserved. People are storing everything from text, images, music and films on CDs and DVDs until smoke pours from the burner. Waiting in the wings is Blu-ray Disc or simply Blue Disc, a technology in which all 41 Mozart symphonies could easily be recorded on a single high-quality DVD. With its estimated 50-gigabyte capacity, the Blue Disc - a name derived from the blue laser technology that can write and read data more densely than red laser light - can replace ten conventional DVDs.

Like all new technologies, the move to shorter-wavelength laser technology is not without its problems. As Professor Joachim Wagner from the Fraunhofer Institute for Applied Solid State Physics IAF explains, generating this blue light requires a new type of laser diode: "We're working with Osram to develop laser diodes made from gallium nitride, the best semiconductor material for application in the blue spectrum. The biggest problem at the moment is the material's complex aging mechanisms." Before Osram can shift into large-scale production of the blue laser, researchers must first find a reasonable compromise between beam intensity, quality and life span on the one hand and cost on the other.

Blue light reflected from the numerous tiny pits in the silver disc is transported in the form of a digital stream to a receiver, where an array of silicon photodiodes records the optical pulses and forwards them to the reading circuits as an electrical signal. The problem here is that position of the laser beam varies with respect to the disc, because the latter is not of a uniform thickness and thus has a tendency to wobble. Until now, it was possible to compensate for this error by combining four diodes in a square array. Before the beam moves too far out of focus or threatens to carom out of view of the detector, tiny mechanical control elements correct the positioning of the diodes. The Blue Disc, however, places far greater demands on the optomechanics due to the much finer local resolution. "To keep the error correction circuitry from being overtaxed, which leads to annoying dropouts, better detector heads are needed," says Armin Kemna from the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg. "Together with our industrial partner Thomson Multimedia, we have designed integrated circuits containing arrays of up to 5x5 sensors, which are manufactured using a CMOS process."

Fraunhofer-Gesellschaft

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