phase change - a rewritable optical disk technology developed by Panasonic (Matsushita) Phase change was used in a line of optical drives in the early 1990s. It was later used in Panasonic's PD drive and is also the technology used for writing CD-RWs and DVD-RAMs. Phase change is a pure optical technology and does not rely on any magnetic influence as does magneto-optic and its progeny. A short, high-intensity laser pulse turns a bit in the recording layer from its natural crystalline state (reflective) to an amorphous one (dull), which does not reflect light as well. A medium-intensity pulse restores the crystalline structure, and a low-intensity pulse reads the bit.