All-fiber multiplexing of vortex beams carrying orbital angular momentum (OAM) through a two-mode-selective coupler and a ring-core-fiber-polarization controller

Abstract

To improve channel capacity, a novel concept of orbital angular momentum (OAM) multiplexing by a two-mode-selective coupler and a ring-core-fiber- polarization controller (RCFPC) is proposed. A ring-core fiber (RCF) with larger mode-separation degrees between adjacent vector modes is also designed and fabricated, which can support the first-and second-order vector/OAM beams propagations. The TMSC comprised of two SMFs and the RCF is manufactured and used to convert HE11 in two single-mode fibers (SMFs) into the HE21/HE31 mode in the RCF. The RCFPC is used to adjust the phase differences between the even and odd modes of HE21/HE31 for generating and multiplexing OAM modes (topological charge = 1 and 2) with left/right-circle polarization in an all-fiber system. To verify the feasibility of the proposed concept, a new experimental platform is also designed and established. The measured results are consistent with the simulation results overall, and then also demonstrate that the proposed concept is correct in theory and practice. In addition, bit error rate and constellation graph are also investigated to evaluate the influences of some parameters and multiplexing ways on the system performances. The results also demonstrate that the system performances decline with the increases of RCF length and wavelength. In addition, compared to the case of only one OAM mode propagation, the system performances significantly decrease when OAM modes are multiplexed. The measured powers of OAM modes in the receiver are also employed to analyze the crosstalk distribution among different OAM modes. The suffered crosstalk of an OAM mode with less topological charge is less than another OAM mode with a larger topological charge. The crosstalk between two OAM beams with identical polarization is larger than the crosstalk between two OAM beams with different polarization.