In this study, an ultra-broadband orbital angular momentum (OAM) mode generator based on a phase-modulated helical long period fiber grating (HLPG) working at highradial order cladding mode is firstly proposed and numerically demonstrated. The proposed phase-only modulation method can considerably facilitate the HLPG’s fabrication by eliminating the grating’s apodization, which is extremely difficult to realize in the screw-type grating. Meanwhile, the proposed method provides more flexibilities in the design of HLPGs. As a typical example, a first-order OAM (OAM ₋₁ ) mode generator with a bandwidth of ~63 nm, a conversion efficiency of ~100%, and purity of ~ 91% has been numerically demonstrated. Moreover, an OAM ₋₁ mode generator with a bandwidth of ~469 nm, a conversion efficiency of ~100%, and purity of ~ 91% has been successfully obtained, covering all bands of fiber communication, i.e., o riginal- (O-), extended- (E-), short- (S-), conventional- (C-), long- (L-), and ultralong- (U-) wavelength bands, which, based on our knowledge, are the best results reported to date among those of the HLPG-based OAM mode generators. The proposed component may find potential applications in the OAM mode division multiplexing (MDM) system combined with the wavelength division multiplexing technique.

中文翻译：

---

基于高径向阶包层模相位调制螺旋光栅的超宽带OAM模式发生器


在这项研究中，首次提出并数值演示了一种基于高径向阶包层模式的相位调制螺旋长周期光纤光栅（HLPG）的超宽带轨道角动量（OAM）模式发生器。所提出的纯相位调制方法可以通过消除光栅的变迹来极大地促进 HLPG 的制造，这在螺旋型光栅中是极其难以实现的。同时，所提出的方法为 HLPG 的设计提供了更多的灵活性。作为一个典型的例子，一阶 OAM (OAM _-1 ) 模式发生器的带宽约为 63 nm，转换效率约为 100%，纯度约为 91%。此外，还成功获得了带宽~469 nm、转换效率~100%、纯度~91%的OAM _-1模式发生器，覆盖了光纤通信的所有频段，即O原始-（O- ）、扩展（E-）、短（S-）、常规（C-）、长（L-）和超长（U-）波长带，据我们所知，这是最好的迄今为止，基于 HLPG 的 OAM 模式发生器的结果已报告。所提出的组件可以在与波分复用技术相结合的OAM模分复用（MDM）系统中找到潜在的应用。