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A dual-layer rewritable optical-disk technology has been developed by Matsushita Electric Industrial Co. Ltd., one of the main technology leaders in digital video disks.

Using a set of violet lasers with a numerical aperture (NA) of 0.85 and a 0.1-mm cover layer, the company has developed disks that have a capacity of 50 Gbytes per side, which allows the recording of more than two hours of high-definition programs. Matsushita plans to present the technology Friday (Oct. 19) at the International Symposium on Optical Memory in Taipei, Taiwan.

Matsushita's announcement follows Hitachi Ltd.'s recent development of an optical pickup that aims for a capacity of 100 Gbytes per disk. Hitachi used the same numerical-aperture and cover-layer parameters as Matsushita.

This set of parameters was first used in the DVR-Blue disk recording system developed by Sony Corp. and Philips Electronics and demonstrated at last year's Ceatec, the largest electronics show in Japan. The departure from the current DVD format, however, was looked on with disapproval.

"It's not desirable for the industry to have split formats. The basic disk structure is almost the same as the one that Sony proposed. When a disk has a single layer, it would be quite similar to Sony's. We hope such a resemblance will work favorably to establish a unified format," said Shin-ichi Tanaka, director of Matsushita's optical-disk systems development center.

The numerical aperture controls these parameters, said engineers. To develop a high-capacity disk using a current optical system and a laser with a wavelength of about 400 nanometers, a large NA is essential, and 0.85 is within practical reach.

"Even if we stick to the same 0.6-mm-thick-per-side disk as DVD disks, it does not guarantee compatibility with DVDs. If a blue or violet laser is used, it becomes difficult to read current DVD dual-layer disks anyway," said Tanaka. "Of course, compatibility with the present DVD format is important, and we will guarantee the compatibility. But for the next-generation disk system, it should be desirable to standardize its format based on an NA of 0.85 and a 0.1-mm-thick cover layer."

The DVD Forum, which works on standards, has not made a decision about the next-generation disk system. "Lasers are not available from Nichia Corp. unless we enter into a nondisclosure agreement," said an engineer close to the DVD Forum. "We cannot discuss the format openly with a nondisclosure contract. In fact, light source availability is a hindrance to standardization work."

The 120-mm-diameter optical disk is a phase-change disk with dual recording layers on one side. With each layer having a 25-Gbyte rewrite capacity, a single-sided disk would have a 50-Gbyte capacity, and a doubled-sided disk would hold 100 Gbytes.

The layer closer to the laser is half-transparent. Matsushita engineers made the recording layer 6-nm thick to increase the transparency to 50 percent. Conventional dual-layer disks have a transparency of about 16 percent.

The second layer, which is about 30 microns away from the first, is 12-nm thick and has increased sensitivity. A backing layer made of aluminum works as a heat sink. The distance between the recording layer and the aluminum layer is extended to maintain heat for writing and reading operations.

Matsushita demonstrated the disk system using its home-grown second-harmonic-generation laser, which emits a 410-nm wavelength and outputs 30 milliwatts. The disk can record and play back at a data transfer rate of 33 Mbits/second, three times faster than conventional DVDs.

When recorded at 25 Mbits/s, the disk can store more than four hours of high-definition moving pictures. The dual-layer disk is made with a process similar to the one used for current DVD two-layer disks. When both sides have two layers, the disk will have four recording layers totaling 100 Gbytes.