An all-fiber orbital angular momentum (OAM) generator enabling direct turning of the fundamental mode (${{\rm HE}_{11}}$) to the second OAM modes (${ l} = {\pm 2}$) with an efficiency of $\sim90\% $ has been proposed and experimentally demonstrated, which is realized based on utilization of a second-order helical fiber grating written in a few-mode fiber. This is the first time, to the best of our knowledge, that an all-fiber second-order OAM has been achieved with using only one component, i.e., the helical long-period fiber grating. The proposed method opens a new way to efficiently generate an all-fiber higher-order OAM using a conventional multimode fiber.

中文翻译：

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基于单螺旋螺旋光纤光栅的全光纤二阶轨道角动量发生器

全光纤轨道角动量（OAM）生成器，可将基本模式（$ {{\ rm HE} _ {11}} $）直接转换为第二个OAM模式（$ {l} = {\ pm 2} $已经提出并通过实验证明了效率为$ \ sim90 \％$），这是基于利用写在少数模光纤中的二阶螺旋光纤光栅实现的。据我们所知，这是第一次仅使用一个组件即螺旋长周期光纤光栅即可实现全光纤二阶OAM。所提出的方法开辟了一种使用常规多模光纤有效生成全光纤高阶OAM的新方法。