phase change disc

A rewritable optical disc technology developed by Panasonic (Matsushita). It was first used in the mid-1990s in Panasonic's PD drives and subsequently in all major optical drives such as CD-RW, DVD-RW, DVD+RW, DVD-RAM and UDO.

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, a short, high-intensity laser pulse turns a bit in the recording layer from its natural crystalline state (highly structured) to an amorphous state (unstructured). The crystalline bit is highly reflective, but the amorphous bit is dull and does not reflect light well. A medium-intensity pulse restores the crystalline structure, and a low-intensity pulse reads the bit.

In write-once phase change drives, such as UDO, the media starts out in an amorphous state, 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 Recording

In a rewritable DVD, different intensities of the laser change the bits in the phase change recording layer from a crystalline state to an amorphous state and vice versa.

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, www.plasmon.com)