Multiple core modes conversion using helical long-period fiber gratings
Chen Jiang, Kaiming Zhou, Bing Sun, Ying Wan, Yuehui Ma, Zhiqiang Wang, Zuxing Zhang, Chengbo Mou, Yunqi Liu
Abstract
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 01 mode to high-order core modes, including LP 11 , LP 21 , LP 31 , LP 02 , LP 12 , and LP 41 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.