多階記錄可擦拭型相變化光碟的設計與製作

Abstract

可擦拭型相變化碟片之記錄密度主要取決於記錄記號的大小。受限於光學的繞射極限，當記號大小小於雷射的最小聚焦光點0.6(λ/NA) (λ為雷射光波長，NA為物鏡的數值孔徑)，讀出訊號將會隨著記號縮小而變差，使得記號無法無限制的縮小，因此光碟記錄密度將受限於現有光學系統。 在本論文中，提出了新型的多階記錄可擦拭型相變化碟片，利用多階記錄的方式，可增加單一記號大小所對應的位元數，使在不縮小記錄記號大小的情形下，提昇碟片的記錄密度。多階記錄碟片具有雙層以上的記錄層，且皆位在同一聚焦深度之內，當調變寫入雷射脈衝強度，可選擇將非結晶態記號記錄在不同的記錄層上而形成多種記錄區域。利用相變化材料在結晶態（Crystalline state）和非結晶態（Amorphous state）兩種不同狀態下具有不同折射係數（Refractive index）的特性，不同的記錄區域會產生不同的反射率值，而形成多階讀出訊號。例如對於四階記錄碟片而言，可形成四種記錄區域，並且產生四階讀出訊號，每個記號大小可對應於兩個位元，使得對應於單一位元的等效記號大小有效縮小，能在不改變現有光碟機系統下，達到增加記錄密度的目的。 在實驗中，經由光學及熱學模擬所設計出的多階記錄碟片可實際產生出多階讀出訊號。由實驗結果發現，當雷射波長為660nm，物鏡的數值孔徑為0.6，且四階記錄碟片其對應於單一位元的等效記錄記號大小為0.3μm時，所得之CNR（Carrier to Noise Ratio）值可達到36dB以上，遠高於傳統DVD-RAM碟片的25dB，證明了利用多階記錄的方式，可有效的縮小可解析之等效記號大小，使碟片之記錄密度及容量得以大幅提昇。

Recording density of the erasable phase change optical disk is determined by the recorded mark size. When the mark size is smaller than the focus laser spot size which is limited by the optical diffraction limit, the readout signal is decreased simultaneously. Therefore, the smallest recorded mark size is limited by the optical diffraction limit which is about 0.6（λ/NA）, where λ and NA are the laser wavelength and the numerical aperture of the objective lens, respectively. In this thesis, ”Multi-level recording method” was proposed to increase recording density of optical disks without decreasing the recorded mark size. A new erasable multi-level recording phase change disk is composed of multi recording layers which are in the same depth of focus of laser beam. By modulating the intensity of the laser pulse, marks can be chosen to record in any of recording layers to form multi recording regions. Utilizing the property of the phase change media whose amorphous and crystalline states differ largely in refractive indices, each recording region corresponds to different reflectance. For the four-level recording disk, four recording regions corresponding to four distinct reflectances can be produced, and each recording region represents two-bit data. Thus the effective mark length for one bit becomes only one-half the actual mark length. As a result, the recording density can be increased without altering the current optic configuration of readout system. The multi-level readout signals of optical and thermal designed multi-level disks were examined experimentally. At the laser wavelength of 660nm and NA of 0.6, when the recorded effective mark size for one bit was about 0.3μm, the CNR（Carrier to Noise Ratio）of the four-level disk was above 36dB, much higher than that of the conventional disk of less than 25dB. Therefore, the resolvable smallest effective mark size was decreased, consequently disk capacity is greatly increased.