Optical data storage

The distribution of audio information in disk format—sound stored as modulations in a flat surface—has had a long and successful track record. At first, analog signals were imprinted on shellac and then later vinyl platters. But recently, sound has been almost exclusively distributed as and replayed from digitally encoded data, molded into the surface of compact polymer disks. This migration was driven by consumer demand for higher fidelity, i.e., higher bandwidth in recording, replication and playback, which in turn required dramatic increases in the amount of stored information. At first the diameter of the disk was increased, but this soon reached its practical limit. With a mechanical stylus, increasing the areal storage density increased the susceptibility to wear and tear, which forced a transition from mechanical detection to a non–contact optical scheme. Optical detection retrieves the stored data by sensing changes in the intensity or polarization of a reflected laser beam. In the form of the read-only compact disk, this data storage medium became the dominant vehicle for music distribution (CD) and later for computer software (CDROM) [1,2]. With the ferocious appetite of consumers for ever more information at ever higher data rates, the CD–ROM is currently undergoing a metamorphosis into the Digital Versatile Disk (DVD) [3]. The DVD standard offers higher areal density per layer, and as many as four layers of prerecorded information, offering sufficient readout bandwidth and capacity for distribution of several hours worth of high quality compressed video. Table I compares the pertinent characteristics of the CD and DVD formats, highlighting the significant progress that has been made. Since its introduction into the market, DVD devices have reportedly experienced the fastest growth of quarterly sales and market penetration of any consumer electronics technology, as of the last quarter of 2001. In addition to this distribution of prerecorded content on removable and interchangeable media, the desire of consumers to record information by and for themselves has been an important economic and technical driver. The wax coated drum of Edison, with analog information engraved with a vibrating needle, has given way to the CD ‘burner,’ with carefully modulated and interleaved digital data imprinted with a flashing laser beam. Recordable optical disks rely