應用於可見光波段高效率偏振器之設計

Abstract

傳統的偏振器是吸收特定極化的偏振光，因此光的使用效率最高只有百分之五十。為了提高光使用效率，本論文利用極化分光器實現了一種應用於可見光的高光使用效率(light utilization efficiency, LUE)的偏振器。我們設計的極化分光器，在特定情況下，能使用百分之八十的光，最多則有百分之九十的光可利用。 本論文設計的極化分光器是使用繞射光柵(Diffraction grating)。並利用利特羅裝置(Littrow mounting)的現象來分析其特性。數值方法是採用嚴格耦合波法(Rigorous Coupled Wave Analysis, RCWA)。我們利用一些物理理論和公式在論文中調整各個參數來達到優化的目的。 傳統的極化分光器有的是利用雙折射效應或多層結構，雖然不會有過多的損耗，卻因為入射角度的允許範圍不大或是體積龐大而受限。另有一種極化分光器是利用金屬的線柵結構，這種結構雖然能夠達到很好的消光比，而且也能夠應用在整個可見光波段，卻很難避免損耗，其中又以藍光波段為甚，論文的例子中至少都有百分之十的損耗，最多則約有百分之二十的損耗。也因此，和我們的設計相比，能夠利用的光最多相差約百分之十，最少也有百分之三到四的差距。 雖然我們的設計未能涵蓋整個可見光波段，但這個問題亦能分化成紅綠藍三個波段個別解決。此外，藉由光源頻譜的高斯函數近似，適當地對波段取捨，能進一步提高允許的入射角度範圍。同時，在論文的最後，我們針對光源的出光角度，介紹了幾種能夠縮小出光角度的方法。 極化分光器的一個應用是液晶顯示器的背光模組。一旦提高了光使用率，就能進一步達到節能的效果。

This thesis designs a polarization beam splitter (PBS) to achieve high light utilization efficiency (LUE). Compared with conventional absorption polarizers of which the LUE is at most 50%, the LUE of our design is about 80% under certain conditions. And, the 90% maximum light utilization efficiency was also achievable. We use dielectric diffraction grating to design the PBS and employ the so-called Littrow mounting phenomenon to analyze its characteristics. We use the 'Rigorous Coupled Wave Analysis'(RCWA) to perform the numerical simulation. And we also describe the optimization process by using the physical theorems and equations. Some conventional PBSs employ birefringent crystals, multilayer structure, and birefringent multilayer structure. However, these attempts suffer either the relatively small acceptance angle or very bulky. The wire-grid polarizer/PBS which utilizes metal is also commercially available; nevertheless, the attenuation due to metals is inevitable. Compared with a wire-grid polarizer/PBS example in this thesis, our design would lead nearly 10% higher LUE in blue-violet band. However, there are some issues we need to concern about. First, we cannot apply whole visible band with one PBS. Second, the field of view (acceptance angle) of our PBSs is relatively too small to collect all light emitted from light sources with large divergence angle. To deal with the first issue, we may use red, green and blue light-emitting diodes (LED) for three separated band. The field of view could be improved by ignoring low-energy wavelengths in a Gaussian approximated LED spectrum. At the end of this thesis, we introduced some possible methods to reduce the divergence angle of LED light sources. An application of the PBS is backlight units (BLU) in the liquid crystal display (LCD). The usage of PBS can increase light utilization efficiency, and therefore, achieve high efficient energy utilization.