We have demonstrated direct generation of high-order orbital angular momentum (OAM) modes by chiral long-period gratings (CLPGs) written in a few mode fiber (FMF) via a CO2 laser. The high-order core modes (LP11, LP21, and LP31 modes) were realized by the CLPGs through high order diffraction modes with efficiencies of more than 99.69%. The -1st, −2nd, and −3rd order OAM modes can be generated directly by the CLPGs with various grating periods. The proposed method opens a new path to implement all fiber high-order OAM mode generation. The OAM mode converters can be adopted as wavelength tunable mode converters by adjusting twist rate of the CLPGs, which could be widely used in optical communication and fiber sensing system based on FMFs.

中文翻译：

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基于手征长周期光栅的波长可调OAM模式转换器

我们已经证明了通过 CO2 激光器在少数模式光纤 (FMF) 中写入的手征长周期光栅 (CLPG) 直接生成高阶轨道角动量 (OAM) 模式。高阶核心模式（LP11、LP21 和 LP31 模式）由 CLPG 通过高阶衍射模式实现，效率超过 99.69%。-1 阶、-2 阶和 -3 阶 OAM 模式可以由具有不同光栅周期的 CLPG 直接生成。所提出的方法开辟了一条实现全光纤高阶OAM模式生成的新途径。OAM模式转换器可以通过调整CLPG的扭曲率作为波长可调模式转换器，可广泛应用于基于FMF的光通信和光纤传感系统。