Compared with traditional acoustic communication technologies, underwater vertical wireless optical communication (UVWOC) offers several advantages, including high bandwidth, low latency, compact device size, and energy efficiency. These qualities make it highly promising for applications in high-speed data transmission, multimedia content distribution, and real-time marine communication. However, the performance of UVWOC is significantly affected by the combined effects of absorption, scattering, and turbulence in seawater, all of which vary considerably with depth. Existing simulation methods face critical limitations: Monte?Carlo (MC) techniques are commonly used for modeling absorption and scattering effects, while phase screen approaches are typically employed for turbulence simulation. However, optical turbulence fundamentally arises from random variations in the refractive index along the light propagation path, driven by depth-dependent fluctuations in temperature and salinity. These environmental parameters exhibit strong stratification in ocean environments, leading to complex vertical heterogeneity that cannot be adequately captured by conventional decoupled modeling approaches. In this study, we develop an integrated photon transport model that captures the continuous interplay between particulate interactions and refractive turbulence in stratified marine environments. By unifying these physical processes within a single MC framework, we enable accurate simulation of optical signal degradation across the entire water column. The model incorporates empirical data from oceanic sensors to ensure a realistic representation of vertical stratification effects.

The MC simulation framework developed in this study employs a multi-layer photon transport model to characterize light propagation in stratified underwater optical channels. Photon packets are initialized with spatial and angular distributions that match practical laser diode outputs, which feature beam divergence angles ranging from 0.1 to 50 mrad. During propagation, each photon packet undergoes energy attenuation and trajectory deviation due to combined absorption, scattering, and turbulence effects. The model implements wavelength-dependent absorption coefficients derived from empirical seawater databases, with scattering effects calculated using the Henyey?Greenstein phase function. Turbulence is simulated using a refractive cell approach, which vertically discretizes the water column into 0.1?1 m thick layers. Each layer contains spherical turbulence elements, with refractive index fluctuations determined by local temperature and salinity gradients. The receiver module incorporates a 0.2 m aperture diameter and a 120° field-of-view constraint. Photon tracking continues until either successful detection within the receiver criteria, energy falling below the detection threshold, or divergence beyond the effective propagation range. Model validation employs three complementary approaches: first, confirming that simulated light intensity distributions under pure turbulence conditions conform to lognormal statistics; second, implementing controlled verification by comparing pure turbulence channels against composite channels with scattering artificially disabled (scattering coefficient set to 0); third, comparing with field measurements from South China Sea waters.

The simulation results demonstrate three key characteristics of underwater vertical optical channels through a comprehensive parametric analysis. Under pure turbulence conditions, scintillation index analysis reveals that the link distance contributes approximately 60% to the overall turbulence intensity, followed by refractive index variations (~30%) and layer spacing (~10%) (Fig. 6). The research defines threshold criteria for turbulence regimes of weak, moderate, and strong (Fig. 7). Path loss measurements show that absorption and scattering dominate signal attenuation, with coastal waters exhibiting a 10 dB higher loss than that of clear oceanic waters, while turbulence introduces an additional 1 dB penalty due to beam wander and distortion (Fig. 10). Comparative analysis between pure turbulence and composite channels reveals significant nonlinear interactions between scattering and turbulence effects. In turbid coastal waters (scattering coefficient >1.5 m^-1), the presence of multiple scattering amplifies turbulence-induced signal fluctuations by 35%?40% compared to clear ocean conditions, as quantified by the enhanced scintillation index values. The vertical stratification effects are particularly pronounced in thermocline regions (100?700 m depth), where temperature and salinity gradients cause scintillation indices to fluctuate between 0.8?1.3, compared to the more stable mixed layer (0?100 m, σ_SI=0.04?0.08) and deep-water regions (>700 m, σ_SI=0.05?0.1) (Fig. 12). The model’s accuracy is confirmed through excellent agreement (R²>0.9) with lognormal distributions in turbulence-only scenarios and successful reproduction of field measurement data from South China Sea campaigns, particularly in predicting the nonlinear relationship between water depth and signal degradation.

We develop a MC-based simulation framework for underwater vertical wireless optical communication (UVWOC) that systematically integrates absorption, scattering, and turbulence effects in stratified marine environments. The model demonstrates high fidelity in characterizing channel behavior, with validation results confirming its accuracy in predicting both turbulence-induced signal fluctuations (scintillation index) and beam wander effects. Key findings reveal that link distance (L) dominates turbulence intensity, which contributes approximately 60% to the observed scintillation index (σ_SI), while refractive index variation (Δn) and turbulent layer spacing (Δz) account for 30% and 10%, respectively. The research defines threshold criteria for different turbulence regimes: weak turbulence (σ_SI<0.15) occurs when refractive index variation Δn<1.8×10^-4 and turbulent layer spacing Δz>0.50 m, primarily found in optically stable surface mixed layers; moderate turbulence (0.15≤σ_SI≤1) emerges at Δn=1.8×10^-4?2.6×10^-4 with Δz=0.25?0.50 m, typically observed in thermocline transition zones; while strong turbulence (σ_SI>1) dominates when Δn>2.6×10^-4 and Δz<0.25 m. In composite channel simulations, absorption and scattering are identified as the primary drivers of power attenuation, with coastal waters exhibiting 10 dB higher path loss compared to clear oceanic conditions. The integration of real-world Argo float temperature-salinity profiles confirms the model’s applicability across distinct oceanic layers—mixed layer (0?100 m), thermocline (100?700 m), and deep water (>700 m)—where turbulence characteristics vary significantly with depth. This framework offers a robust tool for optimizing UVWOC systems in challenging scenarios such as deep-sea exploration and cross-layer communication. Future enhancements will incorporate machine learning for real-time turbulence prediction and expand experimental validation through controlled underwater trials, which further improves the model’s predictive reliability in dynamic marine environments.