Abstract
The third-order orbital angular momentum (${{\rm OAM}_{\pm 3}}$) guided mode generation is demonstrated for the first time, to the best of our knowledge, by employing an asymmetric long-period fiber grating (AS-LPFG). The proposed AS-LPFG is modeled by coupled local-mode theory, which is extended to the coupling of core modes and is fabricated by multicycle scanning ablation with increasing power in a six-mode fiber. The experiments demonstrate that one fabricated AS-LPFG can convert the ${{\rm LP}_{01}}$ mode to the third-azimuthal-order (3AO, ${{\rm LP}_{31}}$ or ${{\rm OAM}_{\pm 3}}$) guided mode with efficiency of ${\sim}{99.8}\%$. The model and the method presented, in principle, can be used to generate any other high-order modes.
© 2020 Optical Society of America
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