In recent years, the growing demand for ocean exploration and exploitation has led to an increasing need for underwater high-rate, high-capacity, and low-latency communications. Orbital angular momentum (OAM), as a new multiplexing dimension, can provide additional multiplexing degrees of freedom that are structurally independent of amplitude, polarization, phase, and subcarriers. This is expected to substantially improve spectral efficiency and communication capacity, which makes it a recent hotspot in research for underwater wireless optical (UWO) communications. However, when the OAM beam propagates in an oceanic random turbulence channel, the seawater medium causes both absorption and scattering of the transmitted beam. Additionally, seawater turbulence, influenced by salinity and temperature fluctuations, disrupts the phase profile of the helical wavefront, which greatly affects the performance of the UWO-OAM communication system. In practical applications, the beam must carry information from the deep ocean to the shallow ocean, encountering vertical or slant optical links with seawater parameters that vary with water depth. Furthermore, there has been no research on the performance of the UWO-OAM communication system based on real ocean data. Therefore, it is of great significance to construct a more generalized ocean-inclined optical link.

Based on the power spectrum inversion method, a random phase screen of ocean turbulence related to seawater depth is generated and compensated for. The propagation channel model for vortex beams in an oceanic turbulent slant optical link is established using the multi-phase screen approach. Numerical simulations and analyses are conducted to study the effects of the scintillation index and detection probability of Laguerre-Gaussian (LG) vortex beams transmitted through slant oceanic turbulence channels across varying transmission distances, seawater turbulence parameters, average temperature and salinity, and link tilt angles. Finally, the performance of the OAM modulation communication system for turbulence slant channels is assessed numerically using real data from Argo, a global real-time ocean observing network. The results underscore the effectiveness of the proposed channel model for underwater vortex beam turbulence slant optical links.

In our study, we present two-dimensional and three-dimensional plots depicting random phase screens of ocean turbulence at different seawater depths. These plots illustrate how the intensity and phase of LG beams vary across various transmission distances and modes. Additionally, through numerical simulations, we analyze the scintillation index and the probability of detecting LG beams. This analysis takes into account factors such as transmission distance, seawater turbulence, average temperature, salinity, and the tilt angle of the link. Utilizing data from the Argo network, we investigate how the depth of the transmitter and the slant angle of the link influence the bit error rate (BER) of the UWO-OAM communication system at specific nodes.

We propose a channel model for turbulent slant link communication using underwater vortex beams, which correlates the distribution of seawater temperature and salinity across different depths with the optical turbulence in the ocean. Based on this underwater slant optical channel model, the transmission process of LG beams is simulated in a generalized ocean turbulence environment. The results indicate that an increase in the scintillation index of the LG beam and a decrease in the probability of detection are influenced by an increase in the rate of dissipation of mean-squared temperature of turbulence, a decrease in the rate of dissipation of kinetic energy per unit mass of fluid, an increase in seawater temperature or salinity, or an increase in transmission distance. Real data from the Argo, a global real-time ocean observing network, is used to numerically simulate the effects of transmitter depth and link slant angle on the BER of the UWO-OAM communication system at specified nodes. This research holds substantial practical significance for enhancing the understanding and optimizing the performance of actual underwater communication systems.