Phase-change information recording medium and optical recording method to make higher rotation rates feasible

An optical recording method for writing or overwriting of information to an information recording medium through a pulsewidth modulation PWM conversion, comprising the steps of:

• providing an information recording medium which comprises a substrate, a first protective layer formed on the substrate, a recording layer formed on the first protective layer, and a second protective layer formed on the recording layer, the first protective layer having a thickness in a range from 65 nm to 130 nm, the second protective layer having a thickness in a range from 15 nm to 45 nm, the recording layer having a thickness in a range from 15 nm to 35 nm and being composed of a phase-change alloy with the composition in terms of atomic percentage being represented by the formula
• AgaInbSbcTed where 0<a≦6, 3≦b≦15, 50≦c≦65, 20≦d≦35, and a+b+c+d=100,
• the recording medium being configured to provide capabilities of reading, writing and overwriting of information at linear rotation rates ranging from 1.2 m/s to 11.2 m/s;
• and
• supplying a data signal to a semiconductor laser via a PWM converter so that the laser is operated in accordance with a controlled power supplied by the PWM converter and the data signal is encoded in the recording layer of the recording medium by a laser beam emitted by the laser, wherein the data signal has a waveform of power-control pulses with a fixed clock duration, the controlled power supplied to the laser including at least three power levels including a write power level Pw, an erase power level Pe and a biased power level Pb, where Pw>Pe>Pb, the waveform of power-control pulses including a multi-pulse portion with a number of cycles, each cycle including a selected one of either the write power level Pw for a pulse duration ton and the biased power level Pb for a pulse duration toff, or the erase power level Pe for a pulse duration ton and the biased power level Pb for a pulse duration toff,
• wherein each of the linear rotation rates of the recording medium is set to an integer multiple of a basic linear rotation rate Vmin which ranges from 1.2 m/s to 1.4 m/s, and wherein a duty ratio of the multi-pulse portion in the data signal represents a ratio of an on-time of the semiconductor laser to a multi-pulse duration and is given by ton/(ton+toff), and the duty ratio is set to one of a plurality of ranges of 0.05 to 0.4, 0.1 to 0.6, 0.2 to 0.8, 0.25 to 1.0, and 0.3 to 1.0 for the linear rotation rates Vmin, 2Vmin, 4Vmin, 6Vmin, and 8Vmin, respectively.