Flexible generation/conversion/exchange of fiber-guided orbital angular momentum modes using helical gratings

By exploiting helical gratings (HGs), we propose and simulate flexible generation, conversion, and exchange of fiber-guided orbital angular momentum (OAM) modes. HGs can enable the generation of OAM modes, and the OAM conversion between two arbitrary modes guided in fibers. A specific HG can exchange the OAM states of a couple of OAM modes, i.e., OAM exchange. In addition, a Fabry-Perot (F-P) cavity cascaded with two identical reflective HGs can reflect converted OAM modes with a comb spectrum. The HG-based generation/conversion/exchange of OAM modes are dependent on helix period, orientation, and the fold number of helical fringes. The proposed method of generation, conversion, and exchange of fiber-guided OAM modes using HGs is flexible and well compatible with OAM fibers, featuring a high conversion efficiency close to 100% and a conversion bandwidth about 10 nm in transmission spectra, while less than 1 nm in reflection spectra.