In this Letter, we demonstrate, to the best of our knowledge, a novel method to generate an orbital angular momentum (OAM) based on the principle of the modal interference in a two-mode fiber. At the interference dips, the left- or right-handed circular polarized ${\mathrm{HE}}_{11}$ modes can be ideally converted into the $\pm 1$-order OAM beam. To verify this concept, we employed the femtosecond laser micro-processing technology to write micro-waveguides in the two-mode fiber and hence realized the in-line modal interferometer. To enhance the mode conversion efficiency at the dips, we optimized the waveguide parameters both theoretically and experimentally. The interference spectrum and spiral/fork patterns confirm the OAM beam generation with an efficiency as high as 99%.

中文翻译：

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基于模态干涉原理的二模光纤轨道角动量生成

在这封信中，我们将根据我们的知识，论证一种基于双模光纤中模态干涉原理产生轨道角动量（OAM）的新颖方法。在干扰波谷处，左旋或右旋圆极化${他}_{11}$ 模式可以理想地转换为 $\pm \mathrm{1个}$阶OAM光束。为了验证这一概念，我们使用飞秒激光微处理技术在双模光纤中写入了微波波导，从而实现了在线模态干涉仪。为了提高在骤降时的模式转换效率，我们在理论上和实验上都优化了波导参数。干涉光谱和螺旋/叉形图确认了OAM光束的产生，效率高达99％。