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

Multilevel recording has an advantage of increasing the recording density w ithout extensively altering the current optical configuration. Four-level r ecording schemes in phase-change disks using either two or three recording layers were demonstrated. By modulating the intensity of the laser pulse, m arks were recorded in any of the recording layers, forming four recording r egions. Utilizing the large difference in refractive indices between the am orphous and crystalline states, each recording region corresponds to a diff erent reflectance. For the four-level recording disk, if each recording reg ion represents two-bit data, then the recording density of the four-level d isk can be higher by a factor of two than that of a conventional disk recor ded by pulse position modulation (PPM).