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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。
The birefringence of double-core fiber with high numerical aperture is analyzed in detail. Firstly, the geometric birefringence of the double-core fiber is analyzed by using the hyperstrouble orthogonal function method and the coupled-mode theory. The geometric birefringence calculated by the two methods is compared and analyzed. The numerical results show that the birefringence of the two-core fiber is still small, only up to 10-5 orders of magnitude when the two cores are very close. The birefringence of the double-elliptical core fiber is calculated by using the supergrating orthogonal function method. The geometrical birefringence can be 10-4 order of magnitude by changing the structural parameters. The high numerical aperture needs a high germanium content, and theoretically analyzes the stress birefringence of the dual-core fiber with high numerical aperture. The stress birefringence approaches 10-4 order of magnitude. The double-core erbium-doped fiber with good polarization maintaining property was designed and fabricated. The geometric parameters and refractive index distribution of the double-core fiber were tested and analyzed. The birefringence coefficient of the double-core fiber reached 8.4 × 10-5. Dual-core erbium-doped fiber can be used as a polarization maintaining erbium-doped fiber for producing a fiber laser with a stable single polarization state output.