设计了几种特殊结构的带隙光子晶体光纤,采用平面波展开法和频域有限差分方法研究了基于不同结构光子晶体光纤的带、模式、色散和色散斜率。由平面波展开法和频域有限差分方法求解麦克斯韦方程组导出本征值方程，可以得到带隙光子晶体光纤中可能存在的不同模式的传播常数、电场分布、本征频率和磁场分布。分别给出了第二圈填充率取三种不同值时带、模式、色散和色散斜率的变化。结果表明，当第二圈填充率改变时其特性有明显的变化。随着填充率从0.75减小到0.45，其二阶模分裂成两个类LP01模，可降低损耗。计算结果对单模和低损耗的光子晶体光纤的设计有参考价值。

Several special structures of the photonic bandgap fiber are designed. By using the plane wave expansion method and the frequency domain finite difference method, the gap, mode dispersion and dispersion slope for different structures of photonic crystal fiber are investigated. The eigenvalue equation derived from Maxwells equations are solved by the plane wave expansion method and FDFD method. And then the propagation， electric field distribution, eigenvalue frequency and magnetic field distribution of different modes existing in band-gap photonic crystal fiber are calculated. The gap, mode, dispersion and dispersion slope of photonic crystal fiber are calculated when the fill rate in the second ring changes. The simulated results indicate that the characteristics of PCF change markedly with the fill rate in the second ring. With the fill rate changing from 0.75 to 0.45, the second-order mode divides into two quasi-LP01, which can decrease the loss of fiber. The calculated results make reference to the design and application of single-mode and the low-loss photonic crystal fibers.