All-fiber third-order orbital angular momentum mode generation employing an asymmetric long-period fiber grating

The third-order orbital angular momentum ( O A M ± 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 L P 01 mode to the third-azimuthal-order (3AO, L P 31 or O A M ± 3 ) guided mode with efficiency of ∼ 99.8 % . The model and the method presented, in principle, can be used to generate any other high-order modes.