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IBM Scientists Demonstrate Multilevel Optical Disks

Could Increase Optical Data Capacities 10-fold or More

May 12, 1994

Scientists at IBM's Almaden Research Center 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.

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.

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.

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 area density, such as blue lasers or high numerical-aperture lenses.

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.

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