Underwater wireless optical communication (UWOC) offers high bandwidth and low latency for underwater data transmission, but its practical implementation is severely challenged by particle scattering and turbulence, which can cause beam distortion, signal fading, and increased bit error rates (BERs). In this work, we experimentally investigate the transmission performance of Gaussian beams (GBs) and vortex beams carrying orbital angular momentum (OAM) in UWOC systems under combined scattering and turbulence. A 450 nm semiconductor laser with 50 Mbit/s single-channel NRZ-OOK modulation is employed, and comparative experiments are conducted under different water attenuation coefficients and three turbulence conditions. The results show that the BER of both beam types increases with increasing attenuation and turbulence strength, while vortex beams consistently exhibit lower BERs and superior tolerance to the combined interference. A Monte Carlo photon-statistical model incorporating Mie scattering theory is further developed to reproduce the beam transmission characteristics and quantitatively elucidate the relationships among water scattering, turbulence, received photon number, and system BER.
To increase the transmission capacity while maintaining low system complexity, we further propose and experimentally demonstrate a dual-channel coaxial concentric-ring space-division multiplexing (SDM) system using vortex beams with topological charges of (l=±8). By employing different beam expansion ratios, the two vortex beams form spatially separated inner and outer concentric-ring channels at the receiver, enabling passive channel demultiplexing with a perforated mirror without complex phase demodulation or algorithmic compensation. The system achieves a total transmission rate of 100 Mbit/s and successfully transmits underwater LiDAR 3D point-cloud data. Even under severe scattering (18.94 m^-1) and strong thermal turbulence, a BER of (1.5×10^-3) is maintained. Crosstalk measurements further reveal that scattering and turbulence enhance inter-channel crosstalk and produce an asymmetric crosstalk characteristic, with stronger coupling from the inner-ring channel to the outer-ring channel due to radial beam broadening and outward energy migration.
These results demonstrate the potential of vortex beams for robust UWOC in complex underwater environments and validate the feasibility of low-complexity OAM-based spatial multiplexing for practical underwater sensing-data transmission. The findings provide experimental and theoretical insights into the performance limits and system design of SDM-UWOC links operating in turbid and turbulent waters.