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。

The large mode area in conventional fiber is achieved generally by design fiber structure, which leads to limited mode area about several hundreds square microns. Gain guiding plus index guiding is a new method to realize single mode fiber with large mode area. By analyzing the effect of gain coefficient on refractive index and the complex-valued normalized frequency, the relationship between index-difference, gain-coefficient and cutoff conditions of different modes is obtained. Finally taking a Nd doped phosphate fiber with a 100 μm diameter core for an example, where the refractive index of 10% (atomic fraction) Nd-doped core is 1.5689 and 1.5734 in the cladding, through simulation computation, the mode-diameter is lager than 90 μm at 1.064 μm. To achieve large-mode-area single-mode fiber, it is a good method of applying gain guiding and index guiding for conventional fiber.