Opt Lett. 2023 Jun 1;48(11):2965-2968. doi: 10.1364/OL.488836.
We propose and demonstrate the fabrication of an all-fiber mode converter enabling simultaneous generation of multiple high-order core modes, which is realized by inscribing a helical long-period grating (HLPG) in a few-mode fiber (FMF) using a femtosecond laser. Helical refractive index modulation is introduced by continuously irradiating the core region with a highly focused femtosecond laser, while the fiber moves in a spiral path through a three-dimensional translation stage. Mode conversion from the LP mode to high-order core modes, including LP, LP, LP, LP, LP, and LP modes, is achieved by controlling the inscription pitch of the grating. Moreover, first-, second-, third-, and fourth-order orbital angular momentum (OAM) modes can be directly generated using the HLPGs, and multiple OAM modes of different topological charges can be simultaneously excited using a single high diffraction order HLPG. This approach offers a new option for implementing with high-integration high-order mode converters or OAM mode generators.
我们提出并演示了一种全光纤模式转换器的制作方法，该方法能够同时产生多个高次芯模，这是通过使用飞秒激光在多模光纤（FMF）中刻写螺旋长周期光栅（HLPG）来实现的。通过用高聚焦飞秒激光连续辐照纤芯区域，同时通过三维平移台使光纤沿螺旋路径移动，引入螺旋折射率调制。通过控制光栅的刻写周期，可以实现从 LP 模式到高次芯模（包括 LP、LP、LP、LP、LP 和 LP 模式）的模式转换。此外，还可以使用 HLPG 直接产生一阶、二阶、三阶和四阶轨道角动量（OAM）模式，并且可以使用单个高衍射阶 HLPG 同时激发具有不同拓扑电荷的多个 OAM 模式。这种方法为实现高集成度的高次模式转换器或 OAM 模式发生器提供了一种新的选择。
