Excitation of high-quality orbital angular momentum vortex beams in an adiabatically helical-twisted single-mode fiber.

A novel method to control the parameters of a chiral fiber grating structure is proposed. Mode couplings are controlled in real time during the twisting fabrication process. This chiral grating structure can satisfy the phase-matching condition for generating high-quality orbital angular momentum (OAM) beams, with an order mode of conversion efficiency over 99.9%. Both theoretical analysis and experimental results of this OAM mode conversion have been investigated, with good agreement. The results demonstrate a dual-OAM beam converter with a charge of ±1 for the right- and left-handed CLPGs, respectively. The high-quality OAM beam generated in this twisted single-mode fiber process may find excellent applications in optical communications.