蓝绿激光跨介质下行链路的偏振传输特性

Polarization Transmission Characteristics of Blue-Green Laser in Cross-Medium Downlink

为了研究粗糙动态海平面对折射偏振光的影响,利用Elfouhaily海浪谱和快速傅里叶变换生成随机动态海面,建立了激光跨粗糙空气-海水界面的偏振光传输模型。研究了不同风速、不同距离、不同光发散角下激光穿过空气-海水信道后的偏振特性和闪烁指数。仿真结果表明,风速越大,接收偏振度越小,相同条件下,圆偏振光偏振度远大于线偏振光;信道距离越长,偏振度越低;风速和链路距离变大时,闪烁指数也变大;当光束发散角减小时,闪烁指数变大;反之闪烁指数减小。验证了所提模型的可行性。本文研究为空气-海水光通信提供了理论依据。

Objective Cross-medium atmospheric seawater communication is influenced by the absorption,scattering,and random wave refraction of the sea surface,which results in intensity attenuation,beam drift,and depolarization effects on the received light.Currently,the research on atmospheric seawater cross-medium channels mainly focuses on sea surface reflection and non-polarization.We aim to study the transmission of polarized light,derive the refractive model,and analyze the Stokes vector variation of photons at the atmospheric seawater interface.Furthermore,a complete polarization transmission model for laser cross-medium downlink channels is built to provide valuable references for the implementation of polarized light in atmospheric seawater communication systems.Methods To investigate the influence of rough dynamic sea levels on refracted polarized light,we first utilize the Elfouhaily wave spectrum and its corresponding bilateral directional transfer function to generate a dynamic threedimensional sea surface via fast Fourier transform simulation.We then build a polarized light transmission model of laser crossing a rough air-seawater interface using the Monte Carlo method,which incorporates the photon scattering in atmospheric channels,refraction at the atmospheric seawater interface,and scattering in underwater channels.Finally,the received intensity and polarization are obtained.By adopting this model,the polarization characteristics and scintillation indices of laser propagation through atmospheric seawater channels are analyzed under different wind speeds,distances,and light divergence angles.Results and Discussions The establishment of a dynamic three-dimensional sea surface and its related statistical data indicates that the increasing wind speed leads to gradually roughening sea surface and rising fluctuation range of sea surface tilt angle.When photons pass through the sea surface,the sea surface tilt angle during refraction is also more random.In exploring the effect of different sea surface wind speeds on underwater polarization,the polarization of received light decreases with the rising wind speed,which is positively correlated with the sea surface roughness at different wind speeds.As the wind speed increases,photons refract and diverge outward,resulting in an increase in scattering times and a decrease in polarization(Fig.5).Additionally,the received light scintillation indices under different wind speeds and beam divergence angles are analyzed.As the wind speed increases,the scintillation index also rises,and the influence of the sea on the light intensity fluctuation becomes greater(Fig.8).By changing the divergence angle of the beam,if the divergence angle gets smaller,the received photons will be refracted by a smaller area of the sea surface,thus bringing beam drifting and an increase in the scintillation index.Conversely,if the divergence angle increases,the scintillation index decreases(Fig.9).Conclusions We build a dynamic three-dimensional sea surface model influenced by wind speed and a polarization transmission model for laser light across the air-seawater interface.By varying the wind speed,link distance,and laser beam divergence angle,we statistically analyze the polarization degree and scintillation index of the received beam.The results show that wind speed influences the statistical distribution of three-dimensional sea surface tilt angles and roughness.As wind speed increases,sea surface roughness rises,resulting in a decrease in the polarization degree of underwater photons.Meanwhile,the longer link distance leads to a lower polarization degree.The scintillation index measures the degree of light intensity fluctuations caused by random refraction at the sea surface.The simulation results indicate that as the wind speed increases,the scintillation index grows and the light intensity fluctuation is more strongly influenced by the three-dimensional sea surface.When the divergence angle decreases,the received light intensity is more strongly affected by the dynamic sea surface,increasing the scintillation index of the received light.Our research findings can provide theoretical references for the laser communication channel model across the rough air-seawater interface.