The restricted power supply problem for equipment both on and below the sea surface has seriously affected the development of marine equipment, as well as the exploration and exploitation of marine resources. Traditional power supply methods, such as cable and resonant power supplies, come with several challenges, including limited transmission distance, complex equipment, and high costs. In contrast, laser wireless power transmission, with its advantages such as high power density and small equipment size, holds broad application prospects. However, most studies on laser transmission characteristics focus on the behavior of lasers in a single medium, and further research is needed for the successful realization of practical scenarios where airborne lasers supply power to devices on and below the sea surface. In this study, we use the Monte Carlo method to simulate and analyze the transmission characteristics of lasers in multi-medium coupling links and investigate the characteristics of lasers during cross-medium transmission. We hope that our research results and strategies will provide theoretical references for the design and implementation of cross-medium laser wireless energy transmission systems.

Firstly, this study analyzes the optical characteristics of different transmission media, including the atmospheric medium, air?sea interface, and seawater medium, and models the optical coefficients of each transmission medium using relevant theories and mathematical models. Secondly, based on the medium models established by these theories and mathematical models, the Monte Carlo method is employed to simulate the entire physical process of a large number of photons transmitting through the media, obtaining statistical results that closely approximate the actual situation.

We analyze the relationships among transmission media, optical coefficients, and important indicators of the laser energy transmission system through simulation. In the atmospheric transmission medium, as the particle radius increases, the atmospheric optical coefficient gradually increases. Moreover, there is no obvious correlation between the total extinction coefficient and the complex refractive index, but the scattering coefficient and the absorption coefficient are positively correlated with the real and imaginary parts of the complex refractive index, respectively (Fig. 2). When the laser wavelength varies within the range of 400?700 nm, the scattering coefficient of the seawater medium gradually decreases as the wavelength increases, while the extinction coefficient first decreases and then increases. In addition, the optical coefficients of seawater increase with the mass concentration of phytoplankton or non-pigment suspended particles (Fig. 3). This study provides a basis for the selection of laser wavelengths in different media. Lasers with a wavelength of 720 nm have a relatively large optical coefficient in the atmosphere and will experience greater scattering losses in the seawater medium. Blue light with a wavelength of 450 nm and green light with a wavelength of 500 nm are more suitable for use in the seawater medium (Fig. 4). We also use the Monte Carlo method to conduct a qualitative analysis of photon transmission characteristics, including photon transmission trajectories (Fig. 5) and the energy flow distribution of successfully received photons (Fig. 6). Through quantitative analysis, it is found that the normalized received power decreases as the transmission distance increases (Fig. 8). The increase in wind speed reduces photon transmittance and the degree of spot expansion without significantly affecting the laser normalized received power (Fig. 9). The normalized received power changes with the increased beam waist radius when using nonconstant asymmetry factors, but an increase in the receiving radius will increase the normalized received power to a certain extent (Fig. 10).

Based on the Monte Carlo method, we focus on the influence of relevant factors, such as suspended particles and wind speed at the air?sea interface, on laser transmission characteristics throughout the entire process—from the atmosphere to the air?sea interface and into the seawater. The research results show that when lasers are transmitted through the atmosphere, the optical coefficient increases with the size of atmospheric aerosol particles. Additionally, the scattering coefficient increases with the real part of the complex refractive index, and the absorption coefficient increases with the imaginary part of the complex refractive index. When lasers are transmitted in water, the optical coefficient increases with the mass concentration of phytoplankton and non-pigment suspended particles. The extinction coefficients of blue?green lasers are smaller, and the extinction coefficient reaches a minimum when the wavelength is 570 nm. The normalized received power decreases with increasing transmission distance, with this decrease being more pronounced in the underwater environment. The extinction coefficients of lasers in the blue?green and near-infrared bands are relatively low, which results in higher transmission efficiency. Both photon transmittance and the deflection angle of the photon transmission direction decrease as wind speed at the air?sea interface increases. These changes exhibit an opposite trend compared to that of the normalized received power, leading to a minimal impact of wind speed on the normalized received power. When the asymmetry factor is constant, the trend of normalized received power is not significant for the laser beam waist radius. However, with nonconstant asymmetry factors, the trend becomes more noticeable. The normalized received power increases substantially with the radius of the receiver. These findings offer a theoretical reference for the realization of airborne laser cross-medium downlink wireless energy transmission.