Definition of:phase change disc
phase change disc

A rewritable optical disc. First used in drives by Panasonic in the late 1980s, the phase change technology was subsequently employed in all major optical drives, including CD-RW, DVD-RW, DVD+RW, DVD-RAM, UDO and Blu-ray. The phase change disc technology was invented by Stanford Ovshinsky, who founded Energy Conversion Devices (ECD) in 1960.

Pure Optical
Phase change discs do not rely on magnetic influence as do hard disks and magneto-optic (MO) discs. In rewritable phase change media, the heat from a short, high-intensity laser pulse turns a bit in the recording layer into an unstructured "amorphous" state, and a medium-intensity pulse sets it to a highly structured "crystalline" state. The two states (0 and 1) cause a change in reflectivity, which is sensed when reading the disc.

In "write-once" phase change drives, such as UDO, the media starts out amorphous, and bits are written by turning them crystalline. Once changed, the makeup of the media prevents the bit from being turned back to amorphous. See UDO, optical disc, PD disc and phase change memory.

Phase Change Writing and Reading
Writing a bit is accomplished by pulsing different intensities of a laser into the phase change layer, which changes the bit between crystalline and amorphous states. The bit is read by sensing the reflected light from a low-intensity laser pulse. The crystalline bit reflects the laser beam, while the amorphous bit absorbs the light.

Phase Change Bits
This magnified UDO disk from Plasmon shows non-reflective amorphous (grey) and reflective crystalline (light/dark) bits. The crystalline area is not a single crystal, but a multi-crystalline region. Due to different crystal alignments and interaction of the beam used to image the material, the reflective bits appear variously light and dark. The different sizes of bits are due to UDO's encoding method. (Image courtesy of Plasmon,