In order to realize large mode area and single-mode fibers,a combining method of gain-guiding and indexantiguiding technique was adopted.Based on this method,the effect of the reflectivity of the resonator mirror on gain-guided and index-antiguided fiber lasers was analyzed in detail.Numerical simulation results show that the mode field distribution is varied with gain coefficient and negative index step,so the large mode area fiber with single mode operation can be realized by choosing optimal parameters,which is highly instructive for the design of large mode area fibers.

A simplified scheme of bend-induced mode distortion is introduced into bent holey fibres, the distorted mode distribution and mode effective area reduction are investigated using the finite difference method. Numerical results show that the modes of bent holey fibres with small bend radius shift away from the core and are deformed greatly, and the mode areas drop significantly as the bend radius decreases, which severely affects the fibre laser performance. The propagation characteristics of bent holey fibres at given wavelength are determined by fill factor and normalized bend radius. Finally, the transition normalized bend radius that represents the location of the mode area beginning to fall off is obtained.

In this paper, the characteristics of bent large-mode-area photonic crystal fibers are investigated comprehensively by using a finite difference mode solver with perfectly matched layers. Numerical results show that in bent large-mode-area photonic crystal fibers, the mode-field distributions are deformed and the effective mode area and the width of the guided region are reduced. The propagation character of bent photonic crystal fibers is determined by normalized propagation number and normalized bend radius. Meanwhile, the bending loss oscillation is observed and the position of the first loss peak is obtained in bent large-mode-area photonic crystal fibers.

传统的大模场光纤是通过设计光纤结构来获得大模场面积的,可以实现的模场面积只能达到几百平方微米.增益导引和折射率导引相结合是实现大模场单模光纤的一种新方法.通过分析增益因子对折射率以及归一化频率的影响,得到了光纤中各阶模式截止条件与纤芯包层折射率差和增益因子的关系.最后以包层折射率为1.5734,纤芯折射率为1.5689,纤芯半径为50 μm,10%(原子数分数)重掺杂钕离子的磷酸盐光纤作为模拟计算对象,当波长为1.064 μm时,得到其模场直径大于90 μm.对于普通光纤,增益导引和负折射率导引相结合的方法对实现大模场单模传输很有前景.