120 km low-loss propagating OAM beams enabled by OAM-mode group multiplexing and a ring-core fiber

Due to the mutual orthogonality among different orbital angular momentums (OAMs), vortex beams (VB) carrying OAMs have been widely employed in the form of encoding/decoding or mode-/space-division multiplexing (M/SDM) for further promoting the channel capacity and spectrum effectiveness in a free-space optical communication (FSOC) and an optical fiber communication (OFC), respectively. For improving the propagation distance and loss in the OFC, a propagation concept of VB is proposed by multiplexing OAMmode groups (OMGs) over a homemade ring-core fiber (RCF). The step-index RCF is proposed and fabricated in our Lab for supporting OMGs propagation and multiplexing with lower loss, which can support 6 OMGs. A proof-of-concept experimental setup is also proposed and performed for verifying the feasibility and validity of the proposed concept, where 16-quadrature amplitude modulation (16-QAM) signal is employed. The detected intensity profiles and interference graphs demonstrate that 5 OMGs can be successfully propagated over the RCF and received at the receiver. For evaluating the performance of the proposed scheme, bit-error rates (BER) and constellation graph (CG) are utilized. The measured BERs and CGs show that the system performance degrades as the order of OMGs or data rate increases and the influence of carrier wavelength on the performance of system is relatively weak in C band. In addition, the measured crosstalk inter-OMGs is much lower than the crosstalk intra-OMGs. The aforementioned results demonstrate that the proposed approach is correct and reliable and can enhance propagation distance and reduce the loss for a long-haul OFC.