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

The third-order orbital angular momentum (OAM_±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 LP₀₁ mode to the third-azimuthal-order (3AO, LP₃₁ or OAM_±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.