Micro-optical Subsystems for Enhancing the Function of a Surface Micro-machined Pickup
H P David Shieh
|關鍵字:||面型微加工;光學讀寫頭;多光束光柵;極化分光器;反射系統;surface micromaching;optical pickup;multibeam grating;polarization beam splitter;reflective system|
式化、輕薄短小化、省電、低功率輸出的方向前進。利用微機電製程技術來設計與製作相容於現今光學讀寫系統的光學元件具有體積小、重量輕及適合於批次製造的優點。而自由空間型將所有元件同時製作在一晶片上，具有在設計光罩時即完成光學元件間的對準工作的優點。然而自由空間型仍面臨許多問題，諸如多晶矽製作的光學元件僅可應用於使用近紅外光(780 nm)的CD碟片，無法應用於使用650 nm的DVD碟片及405 nm的HD-DVD碟片，而單光束讀寫碟片及分光鏡效率偏低更嚴重限制其功能。為了改善這些問題，本論文提出了以在可見光波段具有高穿透性的低應力氮化矽取代原先的多晶矽，並結合光柵與微型致動器設計出一可切換式光柵，其可發展成一可切換單/多光束的微型光學讀寫頭。此外，利用多層光學膜的原理，我們製作出可應用於微型光學讀寫頭的高效率極化分光器，以讓自由空間型的微型光學讀寫頭被更廣泛且成功地應用在光儲存領域。為進一步發展出CD/DVD/HD-DVD光碟相容的微型讀寫頭，本論文提出一種二維的微型反射系統，此系統的概念可用於驗證相容型的微型讀寫頭的可行性。
In the era of multimedia, internet and mobile communication, features of portable , thin, light weight and power saving are required for consuming electronic products. Compared with conventional optical pickup systems, micromachinical optical pickup systems are smaller, lighter and can be made by a batch-fabrication technology. Among the proposed micro optical pickups, the free-space polysilicon optical bench has been realized on a single chip, on which all the optical elements can be precisely prealigned during the mask design stage. However, this device used thin-polysilicon films as optical patterns and suffered from absorption of visible light by polysilicon. Another challenge in this device was that only a single beam was used by the micro-optical pickup to read and write a disc. Moreover, the beam splitter in the device was of low efficiency. These seriously limit its performance. In this thesis, to overcome the above issues, a transparent material for visible light was derived from low stress silicon nitride and improved the light efficiency. Besides, two novel components, a switchable grating and a high efficiency polarized beam splitter, were designed to further enhance the performance of the free-space optical pickup. Conventionally, the switching function can be realized by using a voltage-controlled liquid crystal grating or a magnet-actuated glass grating. The two approaches still suffer from the difficult assembly of optical components of large size and high cost. Therefore, we propose “a micro switchable grating,” which is composed of a stress-induced curved micro actuator and a phase-type grating on the tip of the actuator. In an optical pickup, a polarized beam splitter (PBS) is used with a quarter-wave plate to improve the transmittance of the forward polarized light and the reflectance of the backward polarized light. Conventionally, the PBS is realized using a birefringence crystal or a films-coated glass. The former is large in size, while the latter is of high cost. Therefore, a micro PBS consisting of a novel stack of two silicon nitride layers separated by an air gap was developed in our research to possess the same function. Besides, a planar micro-reflective system composed of a parabolic mirror, a prism, and a thermal actuator was demonstrated to verify the feasibility of a CD/DVD/HD-DVD compatible micro-pickup. A mechanical scan angle of 38 degrees was demonstrated by applying the developed planar reflective system, which displayed more than three times the scan angle compared to that of a conventional rotary mirror. The optical simulation about the deflected beam deviating from the optical axis of the planar reflective system was also presented, and good agreement between theory and experiment is achieved. This dissertation has successfully demonstrated the great potentials of the micro switchable grating, the micro PBS, and the micro-reflective system. These sub-systems can be achieved using the surface micromachining, potentially also at very low cost. The size of them is also dramatically reduced. Further, the fabrication of these subsystems is compatible with other micro diffractive elements. This makes them feasible to build a multi-function micro-optical pickup for short wavelength optical storage applications in the near future.
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