Hitachi and Asahi use a single objective lens, an essential technology to realize two or more recording layers on a disk. The structure also opens the door to lower-cost optical heads and to mobile, high-definition disk player/recorder applications, the companies said.

The partners will present the technology at the International Symposium on Optical Memory 2001 next week in Taipei, Taiwan.

A disk with two recording layers on each side could attain a 100-Gbyte capacity consisting of four 25-Gbyte layers. Such a product could store more than eight hours of high-definition video content.

Asahi Optical, known for its Pentax-brand cameras, has developed precision glass-molding technology to fabricate a single objective lens with a numerical aperture of 0.85 — the spot where most prototypes of next-generation optical-disk systems are converging. The precision level required is nearly 10 times that of current DVD products' lenses, said Takashi Yamanouchi, general manager of Asahi Optical's project group.

Alignment challenge

A two-element lens requires less accuracy in molding, but aligning the lens pair is difficult. Hence, "we are studying a single lens from the beginning," said Yamanouchi. The outside diameter of the lens is 5 mm and the effective diameter is 4 mm. "The working distance between lens and disk of this single lens is 0.7 mm, which is a much larger margin compared with a two-element lens," he said.

Sony's DVR-Blue system is built around a two-element objective lens. The two lenses, with 0.85 numerical apertures, squeeze the 405-nanometer laser beam spot in two steps to focus it on a recording layer roughly 0.1 mm from the disk surface. The working distance between the disk surface and the lens is just 150 microns. With this configuration, the recorder can read and record in a 0.3-micron pitch track with a density of 0.13 micron per bit, less than half that of current DVD disks.

"A disk and head could easily collide with each other in the 150-nm working distance," said Hirofumi Sukeda, senior researcher at Hitachi's information-storage research department.

Hitachi developed an aberration-compensating system for the optical head using the Asahi lens. A detector picks up aberrations in a laser-beam spot on the recording layer and feeds them to an LCD, which changes its pattern to adjust the laser beam and cancel the aberration. The prototype head can adjust any unevenness in the layer between the surface and the recording layer, up to ±12 micron. "Theoretically, this technology can cover not only two layers but also multiple layers, which some companies are studying," said Sukeda.

Asahi Optical is developing smaller-diameter lenses. A slim type for notebook PCs will be 3 mm in diameter with a working distance of 0.57 mm.

"Even a 1-mm-diameter lens for mobile equipment will have a working distance of about 0.2 mm, which has a larger margin than the two-element lens," said Yamanouchi.