Multilevel recording in erasable phase-change media by light intensity modulation

Multilevel recording has an advantage of increasing the recording density without extensively altering the current optical configuration. Four-level recording schemes in phase-change disks using either two or three recording layers were demonstrated. By modulating the intensity of the laser pulse, marks were recorded in any of the recording layers, forming four recording regions. Utilizing the large difference in refractive indices between the amorphous and crystalline states, each recording region corresponds to a different reflectance. For the four-level recording disk, if each recording region represents two-bit data, then the recording density of the four-level disk can be higher by a factor of two than that of a conventional disk recorded by pulse position modulation (PPM).