The third-order orbital angular momentum (${{\rm OAM}_{\pm 3}}$) guided mode generation is demonstrated for the first time, to the best of our knowledge, by employing an asymmetric long-period fiber grating (AS-LPFG). The proposed AS-LPFG is modeled by coupled local-mode theory, which is extended to the coupling of core modes and is fabricated by multicycle scanning ablation with increasing power in a six-mode fiber. The experiments demonstrate that one fabricated AS-LPFG can convert the ${{\rm LP}_{01}}$ mode to the third-azimuthal-order (3AO, ${{\rm LP}_{31}}$ or ${{\rm OAM}_{\pm 3}}$) guided mode with efficiency of ${\sim}{99.8}\%$. The model and the method presented, in principle, can be used to generate any other high-order modes.

中文翻译：

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采用不对称长周期光纤光栅的全光纤三阶轨道角动量模式生成。

据我们所知，通过使用非对称长周期光纤光栅，首次演示了三阶轨道角动量（$ {{\ rm OAM} _ {{pm 3}} $） （AS-LPFG）。所提出的AS-LPFG是通过耦合局部模式理论建模的，该模型已扩展到纤芯模式的耦合，并通过在六模光纤中通过增加功率的多周期扫描烧蚀来制造。实验表明，一个预制的AS-LPFG可以将$ {{\ rm LP} _ {01}} $模式转换为第三方位阶（3AO，$ {{\ rm LP} _ {31}} $或$ {{\ rm OAM} _ {\ pm 3}} $）引导模式，效率为$ {\ sim} {99.8} \％$。原则上，提出的模型和方法可用于生成任何其他高阶模式。