次波長光柵式偏極分光器之設計與分析

Abstract

一般常用的偏極分光器如Wollaston Prism和MacNeille Polarizer具有高分光比 (Extinction ratio)、高分光效率及寬頻的優點。但是因為其本身的特性使得這些分光器適用於操作的入射角範圍很小，而且本身的體積甚大。為了克服這些缺點，我們利用次波長光柵來設計一具有高效率和廣入射角範圍優點的平面式偏極分光器。 次波長光柵可被視為一具有高複曲折射率 (birefringence) 的單軸晶體，而且其複曲折射率可藉由光柵的填充比 (fill factor) 來改變。利用這些特性，可經由設計使得光柵對一偏極方向的入射光形成反射鏡，而對另一偏極方向的入射光形成抗反射膜。再加上多層膜的結構，我們可得到一效率高達97 % 的偏極分光器。除此之外，由於次波長光柵的單軸晶體特性在小角度內幾乎和入射角無關，因此這一偏極分光器的入射角範圍高達10度。

Traditional Polarizing Beam Splitters (PBSs), such as Wollaston Prism and MacNeille Polarizer, have advantage of high extinction ratio, broad band, and high efficiency. However, owing to the inherent nature of their asymmetric properties, these PBSs have small field of incident angle and are in huge volume. In order to overcome this shortage, subwavelength grating is used to design a planar PBS with high efficiency and wide field of incident angle. Subwavelength gratings can be treated as uniaxial media with large birefringence. Moreover, the birefringence can be controlled through the fill factor of grating. With these properties, it is possible to design the grating so that it forms a dielectric mirror under one polarization, and forms an antireflection coating under another polarization. Moreover, using the multilayer structure, we have designed a planar PBS with an efficiency of 0.97 for both polarizations. Moreover, since the uniaxial behavior of subwavelength grating is almost independent of incident angle, the field of incident angle is as large as ±10 degree.