Erasable compact disk utilizing phase-change material and multipulse recording method

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Abstract
A feasibility of phase change media to an erasable compact disk (CD) was studied. We proposed a quadrilayered optical disk having a recording layer of Ge12Sb39Te49 thin film. The ternary material film was crystallized and also amorphized reversibly by laser irradiation of 0.7 microsecond(s) . This transition time was appropriate for the CD which revolves at 1.2-1.4 m/s. A multi-pulse (MP) recording method was also proposed to record EFM signals without teardrop shaped distortions. On the revolving disk, each recording mark of amorphous state was formed by irradiation of 'multi-pulses': a set of one preceding longer pulse and succeeding shorter pulses. Newly developed simple electric circuit for producing the MP waveform was examined to overwrite the single frequency signals of 196 kHz and 720 kHz, alternatively using only one laser beam. As a result, carrier to noise ratio (CNR) of more than 50 dB, secondary harmonic distortion (SHD) of -40 dB, and erasability of 40 dB were obtained. It was observed by a transmission electron microscope that the teardrop shaped distortion of the recording mark was reduced. Eight to fourteen modulation (EFM) signals of the audio CD format were also overwritten successfully and a clear eye-pattern was confirmed. The combination of the GeSbTe phase change media and the MP recording method will be applicable to the erasable compact disk.

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