Design and optimization of GI-PCF supporting the orbital angular momentum modes based on negative curvature structure.

Based on the negative curvature structure, we design a graded-index photonic crystal fiber (GI-PCF) supporting the orbital angular momentum (OAM) mode transmission and discuss its optimization strategy. The core of the designed GI-PCF is sandwiched by three-layer inner air-hole arrays with gradually decreasing air-hole radii and a single outer air-hole array, where the inner side of the annular core forms a graded refractive index distribution. All these structures are clad with negative-curvature tubes. By optimizing characteristic structural parameters, including the air-filling fraction of the outer array, the air-hole radii of the inner arrays, and the thickness of the tubes, the GI-PCF can support 42 OAM modes and most of them have a purity greater than 85%. Compared with conventional structures, the present design of GI-PCF has better properties on an overall level, which can stably transmit multiple OAM modes with high mode purity. These results inject new interest in the flexible design of PCF and have potential applications in various fields, including but not limited to the mode division multiplexing system and terabit data transmission.