将两种分别具有正负双折射特性的缺陷结构相结合,在一定波长范围内,设计了一种拍长对波长变化不敏感的多孔双折射光纤。设计方案从均匀分布圆形微孔的光纤端面出发,将靠近中心的一对微孔的半径增大,同时将分布于包层中的其余微孔变为椭圆形状,在不同的波长上分别产生符号相反的模式双折射进行调节补偿。采用有限差分波束传播法计算分析了不同结构参数对多孔光纤双折射的影响,结果表明,调控这些结构参数能在特定的波长窗口获得稳定的拍长,且拍长值也可以根据实际需求设计调整。通过优化结构参数,在1310 nm波长窗口得到了带宽大于180 nm的平坦拍长曲线。

A holey fiber with beat length independent of wavelength is designed by combination of two defect structures respectively with positive and negative birefringence. The radius of a pair of air holes adjacent to the core is enlarged and the other circular air holes in the cladding are substituted with elliptical holes at the same time. Birefringence with different signs generated by these two changes may interact with each other so that the beat length is less sensitive with wavelength. Finite-difference beam propagation method (FD-BPM) is used to calculate and analyze the influence of different structural parameters on birefringence of holey fiber. Stable beat length can be achieved at some wavelength window by adjusting the structural parameters and the value of the beat length can also be tuned according to practical requirement. Uniform beat length with more than 180 nm bandwidth at 1310 nm wavelength window is obtained by optimizing structural parameters of holey fiber.