A graded index ring-core fiber supporting 22 OAM states

We present a graded index ring-core fiber for long-distance OAM mode transmission. Due to the graded index profile of the ring-shaped fiber core, the supported OAM modes are more pure and degenerate than those in the step-index ring-core fibers. A simple ring-core fiber design scheme is introduced. This scheme illustrates the relationship between the mode effective refractive indices and the designed ring-core structure parameters. 5 orders of OAM modes are excited in the fiber. The purity of OAM states after 1 km transmission is verified by spiral phase demodulation.