三元次容積光碟之讀取信號與雜訊的研究

Abstract

人類的文化進展日新月異，工業技術的發展和高科技產品的普及已為人類的生活帶來巨大的影響。隨著科技的日益進步，人類對資訊的需求更加殷切，不但要求資訊的快速處理，更要求資訊的大量儲存，具備高密度和可攜性的光碟記錄媒體因而蓬勃發展。因應電腦、通訊、和多媒體等科技的發展，更大的記錄容量、高速的資訊存取、和多樣化的讀寫功能等成為光碟技術發展的指標。其中，超解析讀取 (super-resolution technology) 和三元次容積光碟 (three-dimensional volumetric optical disk) 等光學儲存技術由於具備巨大的潛力，因而受到廣泛的注意。 提升光碟記錄密度的方法可分為兩大類：欲提升光碟片的面積密度 (areal density)，可應用超解析讀取法等技術；欲提升光碟片的整體容量，除了提高其面積密度之外，可在垂直於碟面的方向上設置多於一層的記錄面，以倍數提升光碟片的容積密度 (volumetric density)，即應用三元次容積光碟技術。本論文探討應用脈衝雷射 (pulsed laser) 於可擦拭光碟、及應用多記錄層 (multi-layer) 和多記錄功能 (multi-functional) 媒體等於三元次容積光碟中等技術，以提昇光碟片的記錄容量並擴展光碟系統之應用領域。 超解析讀取法是用低於繞射極限的 (below-diffraction-limited) 有效檢出孔徑 (effective aperture) 來檢測光碟上微小信號的技術。和傳統的直流光源相較，應用脈衝雷射光可在光碟片上產生較微小、較均勻、和近似圓形的有效讀取孔徑，且其軌間串訊 (intertrack crosstalk)、孔徑抖動率 (aperture jitter) 和磁壁雜訊 (aperture wall noise) 等雜訊均較使用直流光源時為小。故使用脈衝光源，於讀取微小記號時可得到更好的效果，因此可有效提升光碟的面積密度。 使用三元次光碟的技術可提升光碟片的容積密度，但將導致層間串訊 (interlayer crosstalk) 的產生；故欲有效提升其信號雜訊比 (signal-to-noise ratio)，層間串訊的抑制是非常重要的。我們以雙記錄層光碟 (dual-layer optical disk) 為例來分析雷射光在其中的讀取過程，藉以探討層間串訊的成因及其對檢出信號的效應。讀取多層光碟時，在非目的層上產生的未聚焦光點將產生雜訊，因而減低讀出信號的強度和對比度。我們分析了三元次光碟各記錄層的反射率、檢出信號和頻譜等，並藉相關的實驗予以驗證。經由信號與雜訊的分析，可設計信號雜訊比較小的三元次光碟，有效提升光碟片的容積密度。 若進一步在多記錄層光碟上分別應用不同種類的記錄媒體，則可進一步將光碟系統擴展至多記錄功能 (multi-functional) 的領域。在本論文中，我們設計了一種稱為PC-ROM (Phase-Change/Read-Only-Memory) 的雙記錄層光碟，兩記錄層分別是一穿透率高的唯讀層和一可讀寫的相變化 (phase-change) 記錄層。此光碟具有相當於相變化光碟的記錄效果和相當於CD-ROM的讀取效果，因而具備「唯讀」與「可擦拭」的雙重記錄功能 (dual-functionality)。配合超解析和三元次容積光碟等技術，可提升光碟的記錄容量和信號強度，抑制各種雜訊的產生，使下一代光碟的記錄容量和記錄功能均能進一步發揮。

As time goes on, the culture of human being advances, and the impact of high technology on the world becomes more tremendous. The huge information created by people demands higher-speed signal processing and larger-scale data storage. Optical recording media, with merits of high density and portability, become important in human life. To fulfill the requirements of computer, communication, and multi-media applications, higher recording density, higher data rate, and multiple recording functions become the main objectives in developing advanced optical recording technologies. Among the advanced techniques, the super-resolution and three-dimensional optical storage technologies attract more attentions because of their great application potentials. The advanced optical storage technologies can be categorized into two groups: one is to increase the areal density of a single-layer optical disk by employing narrow track-pitch or super-resolution technologies; the other is to multiply the volumetric density of a three-dimensional volumetric optical disk by implementing multiple data layers along the direction perpendicular to the data surface. The thesis is devoted to increasing the areal density of an optical disk by applying pulsed irradiation on super-resolution optical disk, and to promoting the volumetric density and/or extending the application potential by employing multi-layer/multi-functional optical storage technologies. Super resolution is a readout method employing below-diffraction-limited apertures to detect sub-mm data marks. Compared with DC source, pulsed irradiation produces smaller, more regular, and more circular apertures, thus, is more suitable for readout on high-density erasable optical disks. Inducing limited intertrack crosstalk, smaller aperture jitter, and lower aperture wall noise, nanosecond-order laser pulses yield better readout performance and higher areal density. Besides inter-symbol and inter-track crosstalk, interlayer crosstalk is induced in reading a volumetric optical disk. To improve the signal-to-noise ratio of a multi-layer optical disk, the suppression of interlayer crosstalk is essential. The readout process on dual-layer optical disks was analyzed to study the source of interlayer crosstalk and the effect of interlayer crosstalk on readout characteristics. Experiments were conducted to correlate with the analytical results. Schemes were reported to suppress interlayer crosstalk to promote the volumetric density of a multi-layer optical disk. Employing multiple data layers of different recording functions on an optical disk further extends the application of optical recording to multi-functionality. A semi-transparent read-only layer and an erasable phase-change layer were adopted in a dual-functional optical disk called PC-ROM (Phase-Change/Read-Only-Memory). The PC-ROM was optimally designed to be with writing sensitivity compatible to that of PD and readout sensitivity compatible to that of CD-ROM. With advanced technologies such as super-resolution and volumetric optical recording, the storage capacity and application potential of optical disks can be further improved.