Using Mechanically-Induced Long-Period Fiber Gratings for OAM Modes Generation Based on Anti-Resonant Mechanisms in Ring-Core Fibers

A flexible first-order OAM modes generation in ring-core fibers (RCFs) based on a mechanically-induced long-period fiber grating (MLPFG) is proposed. By directly fusion splicing a standard single-mode fiber (SMF) and RCFs, the anti-resonant reflection guidance mechanism (ARRGM) is introduced in the RCF, leading to the guided-core modes, so-called anti-resonant modes (AR modes), which have a Gaussian field distribution and are confined in low-refractive index central core of RCFs. A mechanical long-period fiber grating is used to couple the AR modes into the OAM modes propagated in the high-index ring core of the RCF. Results show that the generated OAM modes are the combination of two orthogonal linearly polarized (LP) modes with +/-pi/2 phase shift in the ring core. The mode conversion efficiency of OAM(+1) and OAM(-1) can reach 95% and 94%, respectively, indicating promising applications in sensing and communication of the RCFs.