飛行光學讀寫頭之近場記錄

Abstract

目前市面上的光儲存媒體，皆以基板入射型（substrate - incidence）的方式進行讀寫，這種記錄方式受限於基板的厚度，無法使用高數值孔徑（numerical aperture, NA）的物鏡，記錄密度因而受限。 表面入射型（surface - incidence）光碟是利用讀寫頭直接在表面進行讀寫，不需穿過基板，克服了傳統記錄方式的限制，便可使用高數值孔徑的物鏡。而且，因飛行光學讀寫頭（flying optical head）採用固態浸沒透鏡（solid immersion lens, SIL）的設計，物鏡有有效NA值大於1，可使聚焦光點更小。但讀寫頭必須低飛於碟片表面，才能使光點縮小的效應耦合在記錄層（recording layer ）上，當飛行高度（flying height）小於波長時，會有近場（near - field）效應發生，一般遠場理論（far - field theorem）已不適用，須以近場光學的理論重新為近場記錄（near - field recording）的方式建立模型。 本論文以飛行光學讀寫頭在表面八射型相變化碟片的應用為理論架構，建立此系統的近場模型，模擬計算各系統參數的改變。對雷射功率及有效光點大小（spot size）的影響，並估算近場記錄可能達到的最小光點。模擬結果發現：欲達到近場記錄的效果，飛行高度需小於0.4λ，介電層厚度不可大於180nm。當飛行高度為0.2λ，介電層厚度在100nm左右時，此時的spot size約為0.15um，一個4.7吋的碟片容量約可達40GB / side。

Most of the commercial optical storage media today use substrate - incidence recording technique which has restrictions in using high numerical aperture objective lens to achieve higher recording density. The newly contrived technique of surface - incidence recording, where the read / write beam does not have to penetrate through the substrate, can utilize objective lens of high numerical aperture for ultra - high density optical recording. The flying optical head further reduce the size of diffration - limited spot by using solid immersion lens (SIL) ,the effective NA of which can be designed to be larger than unity. Because the optical head in this method must fly near the disk surface within the range of wavelength to couple the optical energy onto the recording layer, the nearfield effect emerges. Therefore, the process of near - field recording was modeled by near - field optics because far - field theorem is no longer applicable in this case. This thesis is devoted to establish a near - field model for the flying optical head recording system. We simulate the dependence of laser power and effective spot size on various parameters of the system. Consequently, in order to achieve the effect of nearfield recording, the flying height should be less than 0.4λ and the dielectric - layer thickness be smaller than 180 nm. The storage density as high as 40 GB / side is achieved with flying height of 0.2λ, dielectric - layer thickness of 100 nm, and effective spot size of 0.15 μm.