A novel design of highly birefringent photonic quasi-crystal fiber based on ZrF4-BaF2-LaF3-AlF3-NaF glass with twin grapefruit air holes near the core and twofold symmetry is proposed. The basic unit is composed of one square and its neighboring regular triangle. Using the finite element method, the birefringence and confinement loss are investigated simultaneously by changing the pitch of air holes and sizes of air holes. Numerical results show that the fiber maintains single mode operation in a wide wavelength range from 1.8 µm to 2.2 µm, and the birefringence is on the order of 10-2, two orders of magnitude larger than that of the conventional polarization-maintaining fibers, which is largest (around 2 µm) ever reported to our knowledge and the same order of magnitude as that obtained by fiber using elliptic air holes But this designed fiber is easy to fabricate compared with the fibers using elliptic air holes.%提出了一种具有二重对称性的ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN)氟化物玻璃基的光子准晶光纤。光纤结构是以ZBLAN氟化物玻璃为背景材料,外包层由圆形空气孔以正方形与正三角形分布组成的基本单元构成,在芯区两侧引入两个对称的葡萄柚空气孔,以增加光纤的双折射。应用全矢量有限元法,研究了光纤的双折射和限制损耗特性与结构参数的关系。通过优化光纤结构参数,在1800-2200 nm的波长范围内获得了具有单模传输特性的高双折射光纤,其模式双折射高达10-2,比普通保偏光纤高出两个数量级,与目前报道的采用椭圆空气孔微结构光纤获得的高双折射具有同样的量级。但与具有椭圆空气孔微结构光纤相比,提出的光纤结构更易于制作。研究结果为开辟2µm波段光器件的研究做出了有益的探索。
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