IBM Scientists Demonstrate Multilevel Optical Disks: Could Increase Optical Data Capacities 10 Fold or More

May 12, 1994

SAN JOSE, Calif., May 12, 1994 -- Scientists at IBM's Almaden Research Center here have demonstrated new multilevel optical disks that are capable of huge gains in optical-disk data storage capacities. The disks are made by stacking two or more recording surfaces on top of each other. Moving the optical disk drive's focusing lens up and down selects the surface on which data is read or written. The impact of multilevel disks is expected to be huge. At today's optical data storage densities, a 10-layer disk would store some 6.5 billion bytes (or gigabytes) of information -- equivalent to more than a million pages of printed text. This would permit feature-length movies to be stored as high-resolution digital video on a single compact disk (about 4.7 inches/120 millimeters in diameter). It would also permit dramatic increases in the capacities of the multi-disk optical libraries, or "jukeboxes," used by industry to store vast amounts of data (trillions of bytes) with about 10-second access times. Multimedia and video-game authors would also be able to use the extra capacity to add more high-quality images, sound and video to their products. "With the advent of the information superhighway, many people will be also asking for access to huge amounts of data," says Edward M. Engler, program director of IBM's Optical Storage Laboratory, a joint program between the company's Research and Storage Systems divisions. "High-capacity, multilevel optical disks could provide a very cost-effective means of storing this data at both the sending and receiving ends." To date, capacity increases in optical data storage have been largely due to increases in the density of information that could be written on a single disk surface. "By using multilevel disks, we are now extending optical data recording into the third dimension," says Hal J. Rosen, manager of Novel Recording Studies at Almaden. IBM's approach to multilevel disks is to glue individual layers together into a stack with spacers providing a gap between the disks. Data is contained on any disk surface inside the stack. Rosen says disks containing four or more surfaces would probably be made of materials thinner than that used in today's optical disks to keep the stack from becoming too thick. The IBM scientists have demonstrated in their laboratory that data can be both read on 2-, 4- and 6-layer read-only disks and also written and read on 2- and 4-layer write-once disks with essentially product-level signal-to-noise quality. "This is not the end," Rosen adds. "I see no technical reason why this approach would not also be extendable to many more surfaces." By adjusting the position of the optical disk drive's lens, the unit's laser light can be focused on any of the data surfaces. Movable lenses already exist in today's optical disk drives to maintain focus, even on warped disks. In fact, the Almaden researchers essentially doubled the capacities of several present-day disk drives by making relatively minor modifications that enabled them to play audio and video tracks (on a CD-ROM drive) and to read data (on an IBM high-performance optical disk drive) on two-layer, read-only disks. "While the implications for the future are truly staggering, I'm very excited that the two-level disks require only minor adjustments to existing devices," says Robert Scranton, director of Storage Systems and Technology at Almaden and also director of Advanced Technology for IBM's Storage System Division, which develops, manufactures and markets data storage products. IBM's multilevel approach is also fully "backward compatible" -- that is, today's single-layer optical disks could still be usable in any future drives designed for multilevel disks. (Multilevel disks cannot be used in today's drives, however.) Multilevel disks could also accommodate future advances in areal density, such as blue lasers or high numerical-aperture lenses. "I can imagine that future multilevel disks that include these other capacity increases could store 30 gigabytes of data or more," adds Engler. "A single disk could then hold the scanned images of the entire contents of several thousand 200-page books. You could carry around a small library in the palm of your hand." In their tests, the IBM scientists also demonstrated that interference from adjacent disk surfaces is minimal, due to the sharp focusing mechanism in today's optical drives. Light focused on any layer is 10,000 to 100,000 times more intense than that hitting any adjacent layer, the scientists found. Each disk layer must be partially transparent in order to allow the optical drive's laser beam to penetrate to all the layers in the stack. Each surface must also have sufficient reflectivity to direct enough light back to the disk-drive's detectors so the data can be read accurately. Ultimately, the maximum number of surfaces in a disk stack is limited by the power of the laser, the transparency of the layers and the cost of making multilevel disks compared with their single-surface competitors. The maximum number of layers in a writable disk would typically be less than in a CD-ROM because the writing process also requires that the disk materials absorb some of the laser light, thus reducing the transparency of each layer. Although a few basic studies relating to multilevel optical storage have been conducted over the past decade or so, IBM's Almaden team is the first to present comprehensive test results of a potentially practical implementation. The Almaden group -- Rosen and colleagues Kurt A. Rubin, Wayne I. Imaino, Wade C. Tang, Timothy S. Strand and Margaret E. Best -- will present two technical reports of the multilevel disk technology on May 18 at the Optical Data Storage conference in Dana Point, California.