具体分析了具有高数值孔径的双芯光纤的双折射特性。首先利用超格子正交函数法和耦合模理论分析了双芯光纤的几何双折射，并将两种方法计算的几何双折射进行了比较分析。数值计算结果表明双芯光纤在两纤芯非常接近的情况下，几何双折射仍较小，只能到10-5量级。利用超格子正交函数法计算了双椭圆芯光纤的双折射，改变结构参量可使几何双折射达到10-4量级。高的数值孔径需要高的掺锗量，理论上分析了高数值孔径时双芯光纤功率集中区域的应力双折射，应力双折射接近10-4量级。设计制作出了具有良好保偏性能的双芯掺铒光纤，测试、分析了它的几何参量和折射率分布；双芯光纤双折射系数达到了8.4×10-5。双芯掺铒光纤可以作为保偏掺铒光纤，应用到制作具有稳定的单一偏振态输出的光纤激光器。

We explore the birefringence characterization of twin??core fiber (TCF) with high numerical apertures. Firstly the super cell lattice orthogonal function method and mode couple theory are used to analyze the geometrical birefringence of the TCF, then we make comparison of the results that calculated from the two different methods. The results show that the geometrical birefringence of TCF is not enough high even when the cores are touching; the maximum geometrical birefringence is only at 10-5. The maximum geometrical birefringence of twin elliptical core fiber can reach 10-4, which is calculated from super cell lattice orthogonal function method. High concentration of germanium is needed to keep high NA. We calculate the stress birefringence of the TCF with high NA in the area of assembling of light power, which approximates to 10-4. The twin??core erbium doped fiber (TCEDF) with well polarization maintaining performance is designed and fabricated, the geometrical parameters and refractive index distribution of the twin??core fiber are measured and analyzed, and the birefringence coefficient of twin-core fiber reaches 8.4×10-5. TCEDF with high NA can be used as polarization maintaining EDF that is a key component in optical fiber lasers with single polarization output.