Wavelength tunable OAM mode converters based on chiral long-period gratings

We have demonstrated direct generation of high-order orbital angular momentum (OAM) modes by chiral long-period gratings (CLPGs) written in a few mode fiber (FMF) via a CO2 laser. The high-order core modes (LP11, LP21, and LP31 modes) were realized by the CLPGs through high order diffraction modes with efficiencies of more than 99.69%. The -1st, −2nd, and −3rd order OAM modes can be generated directly by the CLPGs with various grating periods. The proposed method opens a new path to implement all fiber high-order OAM mode generation. The OAM mode converters can be adopted as wavelength tunable mode converters by adjusting twist rate of the CLPGs, which could be widely used in optical communication and fiber sensing system based on FMFs.