光波導與微光學元件在高密度光資訊儲存的應用

Abstract

為了因應電腦，多媒體與網路等科技的發展，更大的記錄容量與高速的資訊存取成為光資訊儲存技術發展的指標。對於光碟機系統而言，發展一高解析(high-spatial-resolution)，高光學效率(high-optical-efficiency)的微型光學讀寫頭將成為必要的趨勢。 在本論文的研究中，利用固態浸沒透鏡(SIL: Solid Immersion Lens)此種高光學效率的近場記錄方式來進一步提升光碟片的記錄密度。在此種高數值孔徑(NA>1)的近場系統，固態浸沒透鏡與碟片間距嚴重的影響了聚焦光點與讀出訊號的品質。因為固態浸沒透鏡與碟片的空氣間距是處於次波長範圍，一般的光學遠場理論(far-field theory)無法適用於描述光波在此空間中的行為，因此，我們建立了一根據光學平面波展開(plane wave expansion)為基礎的近場模型。由我們的計算結果得知，當空氣層由0 nm增加至300 nm時，由於光波經過空氣間層時所造成的振幅與相位的切趾效應(稱為gap-induced aberration)，聚焦在記錄層的光點大小將擴大90 %，聚焦光點中心能量將減少85 %。除此之外，高數值孔徑所造成的偏極化效應將會使得讀出信號對於空氣間層有很高的敏感度。對於一道x線偏振的入射光讀取相變化碟片而言，反射訊號在x與y線偏振態對於空氣層將會有不同的函數，若我們利用偏極化元件將y線偏振態做濾波，信號對比度將會提升1.0至1.35倍。 在此論文的第二部份，我們利用248 nm KrF準分子雷射製作固態浸沒透鏡。經由灰階光罩的設計，我們可以在塑膠基板上製作出一連續的折射球面，對於投影系統的非線性效應做適度的補償之後，球面將可以控制在5%的誤差範圍內。利用knife-edge scanning量測所得到的結果，此種方式所製作的固態浸沒透鏡將可以提升系統數值孔徑約1.5倍。 光纖具有輕小，易彎曲等優點，於是在此論文的第三部份，我們在光纖端面製作微透鏡作為聚焦元件，用以替代傳統厚重的物鏡，使得讀寫頭達到高速的存取速度。藉由電弧放電加工技術，光纖微透鏡可以達到約1 mm的聚焦光點與145 mm的工作距離。除此之外，為了更進一步縮小聚焦光點與增強光纖元件機械結構，我們設計一個積體微光學模組，此模組利用半導體製程整合了固態浸沒透鏡，微孔徑技術與光纖光波導，初步得到一輸出效率為10-1，小於繞射極限(diffraction limited)的光源。此微小模組(3mm*3mm*500mm)可以利用微致動器(actuator)做光點的位置控制，作為將來近場飛行讀寫頭的雛形架構。

Optical data storage industries are continually growing with rapid progress of computer, multimedia, and network markets. In this trend, technologies capable of recording more information thus become increasingly demanded. Since optical heads are a key component of the recording system, developing a high-spatial-resolution, high-optical-efficiency and small-sized optical head is thus essential to enable the system more competitive in price and performance. For increasing the recording density, a near-field optical approach using a solid immersion lens (SIL) was developed. The effective numerical aperture (NAEFF) of the system can be achieved above theoretical upper limit of 1.0 in air. The air spacing between the SIL and the recording medium is an important factor that influences the focused spot quality. Compared with gap width h = 0 nm, the spot width is increased by 90 % at h = 300 nm. The primary factor causing increased spot size at larger air spacing is due to the gap-induced aberration, which is introduced by the phase and amplitude apodization when the incident light passes through the air gap. On the other hand, the peak intensity of the focused beam is decreased by 85 % when the gap width h changes from 0 to 300 nm. The reflection coefficients of the thin-film structure are both angle and polarization dependent, the contrast of readout signal is a different function of air gap width for x and y polarizations. For an x-polarized incident beam, the simulation shows that a method by filtering y-polarized light results in a factor 1.0 to 1.35 higher than non-polarized detection for signal contrast in phase-change media. We then demonstrated a microfabrication of SIL array by using a 248 nm excimer laser micromachining with a gray-tone mask photolithography. With pre-correlation to the nonlinear exposure process, a 30-mm-radius hemispherical SIL array was achieved with a deviation of less than 5%. The fabricated SIL array was used with a 0.54 NA objective to achieve 0.87 effective NA measured by the knife-edge scanning. In order to make the optical head smaller and lighter for fast access, we designed and fabricated a hyperbolic-shaped microlens on a single-mode fiber (SMF) to achieve the focused spot 1/e2 = 0.82 mm (x-direction) and 0.89 mm (y-direction) at 145 mm working distance by the discharged arc method. To overcome the drawbacks of low NA and fragility of the fiberlens-type optical head, a well-controlled mechanical structure with a fiberlens, a SIL and a submicron aperture was proposed as a heat source for near-field recording. Through this structure, a below-diffraction-limited submicron aperture (600 nm) within the diffraction-limited fiberlens illumination was used to realize a super small spot size with 10-1 throughput efficiency. This small-size (3mm*3mm*500mm) highly integrated module can be driven by a radial actuator for beam steering, which can potentially function as a flying head in next-generation optical storage systems.