Rewritable optical disk technologies

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Abstract
The two mainstream technologies for rewritable optical data storage are based on magneto-optical (MO) and phase-change (PC) media. In both cases a focused laser beam is used to raise the temperature of the medium beyond a certain critical temperature (i.e., melting and crystallization temperatures in the case of PC, and the Curie temperature in the case of MO) for writing, erasure, and overwriting of data. The readout of information from these media relies on the change of reflectivity of the medium (PC), or the effect of the medium on the state of polarization of the laser beam (MO). The performance of these data storage systems is characterized by the storage density of the media, achievable data rates during recording and readout, longevity, reliability, and cost of the finished products. These performance criteria in turn are determined by a host of physical and technological factors, among them: (1) Wavelength of the available semiconductor laser diodes; (2) Type of optics used in shaping the laser beam and confining it to sub-micron regions (i.e., conventional optics, near-field optics, flying lasers, integrated optics, etc.); (3) Availability of one- or two- dimensional arrays of lasers and the corresponding optics; (4) Miniature magnetic heads for thermally-assisted writing (inductive) and magneto-resistive readout; (5) Adaptive optics for beam-shaping and/or polarization control; (6) Availability of low-noise media that can deliver large readout signals; (7) Advanced signal processing techniques (both optical and electronic); (8) Multi-layer recording on stacked layers of media; (9) Novel encoding/decoding schemes for efficient use of available space on the media; (10) Mass-production of flat, rigid, low-noise patterned substrates (plastic or glass); (11) Multi-level recording. We discuss the present state of the art in optical data storage, describe the potentials and pitfalls of the existing technologies, and draw conclusions about the future of this field.

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