Ma Yuehui, Zhou Kaiming, Kamarudin Lukman, Jiang Chen, Chen Siyu, Li Jiaxuan, Huang Qiushi, Sun Wei, Wang Xutao, Yang Zhiqun, Liu Yaping, Zhang Lin, Mou Chengbo, Liu Yunqi
Opt Lett. 2024 Oct 15;49(20):5949-5952. doi: 10.1364/OL.538511.
We demonstrate the realization of mode conversion using hollow cylindrical long-period fiber gratings (LPFGs) inscribed in graded-index few-mode fibers (FMFs) by a femtosecond laser. By precisely shaping the refractive index modulation into a hollow cylindrical structure, we enable efficient coupling from the fundamental mode to high-order modes (LP, LP, LP, LP, LP, LP, and LP modes). The method achieves high-efficiency and low-loss mode conversion across various mode groups, marking a first for graded-index FMFs with different grating periods. The influence of the hollow cylindrical LPFG radius on mode conversion efficiency and spectral characteristics is thoroughly investigated. Additionally, incorporating a linear polarizer allowed for distinguishing between LP modes within a mode group, enhancing the tunability of the mode converter. These mode conversion devices have significant potential for applications in mode gain equalization and mode scrambling for mode division multiplexing (MDM) systems.
我们展示了利用飞秒激光在渐变折射率少模光纤（FMF）中写入的空心圆柱长周期光纤光栅（LPFG）实现模式转换。通过将折射率调制精确地整形为空心圆柱结构，我们实现了从基模到高阶模（LP₀₁、LP₁₁、LP₂₁、LP₃₁、LP₄₁、LP₅₁和LP₆₁模）的高效耦合。该方法在各种模式组中实现了高效低损耗的模式转换，这在具有不同光栅周期的渐变折射率FMF中尚属首次。深入研究了空心圆柱LPFG半径对模式转换效率和光谱特性的影响。此外，加入线性偏振器能够区分模式组内的LP模式，提高了模式转换器的可调谐性。这些模式转换器件在模式增益均衡和模式分割复用（MDM）系统的模式加扰方面具有重要的应用潜力。
