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为了进一步探讨大模场光纤激光的模式控制方法,需要研究大模场光纤的弯曲特性。从亥姆霍兹方程出发,通过对比直光纤和弯曲光纤的模场分布,确定大模场光纤直弯过渡中的模场演变方式。应用模场的叠加和激发理论,推导得到光纤直弯过渡中的模场耦合效率公式。应用耦合公式和有限元方法,计算了阶跃折射率分布的大模场光纤在直弯过渡中的模场耦合效率。结果表明:在光纤的直弯过渡中,存在模式的自耦合和互耦合过程,耦合程度随着弯曲程度和模场阶次的变化而变化,其中基模的耦合效率始终最大,高阶模的耦合损耗大于低阶模。光纤的直弯过渡以总能量的损耗为代价换取对高阶模的有效抑制。
In order to further explore the mode control method of the large mode field fiber laser, it is necessary to study the bending characteristics of the large mode field fiber. Based on the Helmholtz equation, the mode field distribution in the straight-bend transition of the large mode field fiber is determined by comparing the mode field distribution of the straight fiber and the bent fiber. By using the superposition and excitation theory of mode field, the mode-field coupling efficiency formula in the straight bend transition of fiber is deduced. The mode field coupling efficiency of large mode field fiber with step index distribution in the straight bend transition is calculated by using the coupling formula and the finite element method. The results show that in the straight bend transition of optical fiber, there exists the mode of self-coupling and mutual coupling. The coupling degree changes with the degree of bending and the order of mode field. The coupling efficiency of the fundamental mode is always the maximum and the coupling loss Greater than the lower order mode. The straight bend of the optical fiber is exchanged for the effective suppression of higher order modes at the expense of total energy loss.