Abstract

We demonstrate high-order mode conversion in a few-mode fiber (FMF) via ${{\rm CO} _2}$ laser inscribed long-period fiber gratings (LPFGs) at both the 1.55 µm and 2 µm wavebands. At the 1.55 µm waveband, five high-order core modes (the ${{\rm LP}_{11}}$, ${{\rm LP}_{21}}$, ${{\rm LP}_{02}}$, ${{\rm LP}_{31}}$, and ${{\rm LP}_{12}}$ modes) can be coupled from the ${{\rm LP}_{01}}$ mode with high efficiency by the FMF-LPFGs. The orbital angular momentum beams with different topological charges $({\pm 1, \pm 2, \pm 3})$ are experimentally generated by adjusting the polarization controllers. At the 2 µm waveband, three high-order modes (the ${{\rm LP}_{11}}$, ${{\rm LP}_{21}}$, and ${{\rm LP}_{02}}$ mode) can be coupled by the FMF-LPFGs with different grating periods. The proposed LPFG-based mode converters could have a potential prospects in mode-division multiplexing and multiwindow broadband optical communication applications.

© 2020 Optical Society of America

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