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• A mitigating crosstalk method is proposed by simultaneously propagating a probe beam with a data beam when the multiplexed vortex beam are propagated over a atmosphere turbulence (AT) free space. • A conjugated crosstalk is achieved by the probe beam during the square-law mixture in a photoelectric detector. • The conjugated crosstalk phase can effectively synthesize the crosstalk of data beam induced by AT. • A proof of concept is built to investigate the crosstalk, bit error rate, constellation plot, stability of system, etc. for verifying the feasibility of the proposed concept in practice. To overcome the upcoming famine of channel capacity driven by explosive increases of new applications, optical orbital angular momentum (OAM) carried by a vortex beam has been considered to be a promising candidate for space-division multiplexing (SDM) in a novel physics domain. However, owing to the inescapable disturbance induced by atmosphere turbulence (AT), the performance of OAM-based free-space optical communication (FSOC) is seriously restrained, which can be ascribed to the crosstalk among different OAM modes induced by the distorted helical phase. For promoting the development of OAM-based FSOC, a synthesizing-crosstalk method is proposed for mitigating the AT by coaxially propagating an OAM-probe beam (OPB), wherein the OAM mode of the extra OPB is identical to the OAM mode of the propagated OAM-data beam (ODB). The corresponding theory analysis of the proposed method demonstrates that the conjugated distortion of the crosstalk can be generated by a hybrid of the OPB and the ODB in a photoelectric receiver/detector (PD/PR). The generated conjugate distortion of the OPB can effectively counteract the crosstalk of ODB induced by the turbulence. To verify the feasibility of the proposed scheme, an experimental setup is built. The measured results demonstrate that the bit error rate (BER) and constellation plot (CP) of the employed two OAM channels can be significantly improved under different atmosphere turbulence (AT) by utilizing the proposed scheme. A continuous 30 min-long-BER-testing results depict that the proposed concept can stably work well. The proposed concept can be also suitable for multiplexing other OAM modes as well as the employed two OAM channels. Besides, with the increase of data speed, the proposed scheme can also effectively mitigate the crosstalk and improving the system performance. Consequently, the proposed concept is valid and feasible both in theory and practice, which can be used for compensating the distorted helical phase and improving the system performance of an OAM-based FSOC in the future. [ABSTRACT FROM AUTHOR]

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 OAM-mode 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. [ABSTRACT FROM AUTHOR]