We propose and demonstrate the simulation and fabrication of an all-fiber orbital angular momentum (OAM) mode converter capable of generating first- to fourth-order modes simultaneously, which is realized by inscribing a cascaded preset-twist long-period fiber grating (CPT-LPFG) in a six-mode fiber utilizing a CO2 laser. A new segmented Runge–Kutta method is proposed to simulate the preset-twist long-period fiber gratings. By calculating the twist angle and relative coupling coefficient for each pitch and then solving the coupled mode equations utilizing the Runge–Kutta algorithm. The simulation illustrates that the preset-twist method significantly improves the coupling coefficient of higher-order modes, thereby reducing coupling difficulty. In the experiment, by twisting the fiber at an angle of 1080° and fabricating cascaded gratings with periods of 745 μm, 310 μm, 204 μm, and 146 μm, it is feasible to generate first- to fourth-order OAM modes simultaneously, at wavelengths of 1635 nm, 1548 nm, 1460 nm, and 1334 nm, respectively. The insertion loss is less than 1 dB, and the mode purity is over 90 %. To the best of our knowledge, this is the first time that first- to fourth-order OAM modes are simultaneously generated utilizing a single long-period fiber grating.

中文翻译：

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基于级联预扭长周期光纤光栅同时生成一阶至四阶OAM模式

我们提出并演示了一种能够同时产生一阶到四阶模式的全光纤轨道角动量（OAM）模式转换器的模拟和制造，这是通过刻入级联预扭转长周期光纤光栅（CPT）来实现的。 -LPFG）在利用 CO 的六模光纤中2激光。提出了一种新的分段龙格-库塔方法来模拟预扭长周期光纤光栅。通过计算每个节距的扭转角和相对耦合系数，然后利用龙格-库塔算法求解耦合模式方程。仿真结果表明，预置扭曲方法显着提高了高阶模态的耦合系数，从而降低了耦合难度。实验中，通过将光纤扭转1080°，制作周期为745μm、310μm、204μm、146μm的级联光栅，可以同时产生一阶到四阶OAM模式，波长分别为1635 nm、1548 nm、1460 nm和1334 nm。插入损耗小于1 dB，模式纯度超过90%。据我们所知，这是第一次利用单个长周期光纤光栅同时生成一阶到四阶 OAM 模式。