蓝绿激光跨介质下行链路的偏振传输特性
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.
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 three-dimensional 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.
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).
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.
1 引言
海洋环境蕴含着丰富的资源,成为国家经济和安全的重要研究领域。与声通信和射频通信等传统通信技术相比,无线光通信在水下介质中能够实现低功耗、高速率的安全传输[1-3]。尽管如此,水下无线光通信的性能仍然受到吸收散射和空气-海水随机界面干扰的影响[4-8]。针对空气-海水跨介质通信,通常使用浮标中继节点连接大气与水下,然而中继浮标通常需要提前部署且价格昂贵[9-11]。直接进行空气-海水通信受海面随机起伏波浪的影响,激光束在空气-海水界面发生随机折射,导致接收光产生能量衰减和光束漂移,从而产生退偏效应[12-14]。因此,研究随机动态海面对偏振光偏振态的影响尤为重要。
目前,研究随机海面多使用分形函数法和蒙特卡罗法,大量实验证明分形方法只适用于重力波谱,因此基于海浪谱的蒙特卡罗法模拟海面更加真实有效[15-16]。现阶段针对激光跨介质通信的研究大多考虑激光空气-海水传输的时、空域特性。Sahoo等[17]通过双向透射分布函数对光在空气-海水界面的传输进行建模,研究了不规则海面、相关气泡层和非均匀吸收散射影响下的通信信道特性。李聪等[18]在晴天、层云和卷云条件下,分析了光在空气-海水跨介质下行信道不同水下深度的光斑特征,但并未考虑不同风速下海面高度起伏的影响。徐正元团队[19]考虑了三维海面高度起伏的影响,建立并实验验证了通过动态空气-海水界面的水对空可见光通信系统,仿真和实验模拟了不同尺度波条件下信道链路增益和变化时间。通常,入射光子与介质粗糙面相互作用发生反射和折射,反射和折射光的能量与偏振状态会发生变化。Mobley[20]使用蒙特卡罗光线追踪研究了风致粗糙海面在可见光谱总的偏振反射率和透射特性,并给出了海面辐射反射率因子改进值。宿德志等[21]基于双向反射分布函数和菲涅耳反射分布函数推导了长波红外偏振度计算模型,并仿真分析了在不同风速下,红外波段不同探测角的偏振特性,在三维海面条件下分析了粗糙海面对反射光偏振态的影响。
综上所述,关于空气-海水信道的研究,较少考虑随机空气-海水界面对偏振光的影响。本文推导了光子在空气-海水界面的折射模型和Stokes矢量变化,建立了激光跨介质下行信道偏振传输模型,对偏振光在空气-海水通信系统中的应用具有参考价值。
2 基本原理
2.1 三维动态海面建模
由于风浪与水面的相互作用,真实海面呈现出无规则高低起伏。为模拟真实三维海面,近年来基于海浪谱和快速傅里叶变换的海面方法得到广泛应用。Elfouhaily海浪谱是由高频张力波和低频重力波组成的全波数谱模型,其理论统计量与Cox-Munk一致,能够很好地描述对海面光散射起主导作用的海面毛细波[22]。
本文使用Elfouhaily海浪谱及其对应的双边方向传递函数,通过谱快速傅里叶变换的蒙特卡罗法进行三维海面建模。假设仿真海面大小为
式中:
图 1. 不同风速下的三维海面。(a)风速为5 m/s;(b)风速为10 m/s;(c)风速为15 m/s;(d)风速为10 m/s时三维海面的局部放大图
Fig. 1. Three-dimensional sea surfaces under different wind speeds. (a) Wind speed is 5 m/s; (b) wind speed is 10 m/s; (c) wind speed is 15 m/s; (d) partial enlarged view of three-dimensional sea surface at wind speed of 10 m/s
由
图 2. 不同风速下海面倾斜角分布概率密度
Fig. 2. Probability density of sea surface inclination angle distribution under different wind speeds
由图中统计的风速分别为5、10、15 m/s三维海面的海面倾斜角分布概率密度可以看出,随风速变大,海面倾斜角分布逐渐分散,向大角度移动。以海面倾斜角均方斜率表征海面粗糙度,统计不同风速下相关数据如
表 1. 不同风速下海面倾斜角相关统计数据
Table 1. Statistical data related to sea surface inclination angle under different wind speeds
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2.2 风致波浪水下偏振度仿真
激光跨介质下行通信系统在传输时依次经过大气信道吸收散射、空气-海水界面折射以及海水的吸收散射。
光子在空气和海水中传输被视为经历单分散体系Mie散射,Mie散射相函数与散射Muller矩阵根据Mie理论计算得到[23-24]。光子在大气和海水信道中经过多次散射并随机改变传输方向,使光产生退偏。使用Stokes矢量和Muller矩阵描述光子在信道中发生散射和折射,得到接收光的偏振态。Stokes矢量定义为
当光到达空气-海水界面时,发生折射,光子偏振态随之改变,折射前后Stokes矢量更新计算公式[27]为
式中:
首先,根据生成的三维海面网格信息和入射光方向得到光在海面的入射点位置及其法向量。计算得到入射点倾斜海面微面元倾斜角
其次,根据球面三角余弦定理求折射旋转角
式中:
最后,根据球面三角余弦定理求得折射平面
式中,
对于空气-海水界面的折射,假定发生折射时的入射角为
式中,
3 分析与讨论
光子在空气-海水信道中传输时,发生吸收散射和随机倾斜海面的折射,在不同风速下,三维粗糙海面和海面倾斜角随机变化。使用多次大气Mie散射、三维海面折射和水下多次Mie散射描述光子的Stokes矢量变化,得到水下光子偏振度。本节通过改变海面风速和光束发散角对接收光偏振度和闪烁指数进行仿真计算。
为探究海面风速对水下偏振度的影响,设置海面风速分别为5、10、15 m/s,仿真相应风速下水下偏振度的变化情况。设置发射激光为高斯光束,光波长为532 nm,束腰半径为3 cm,发散半角为10 mrad,发射光Stokes矢量分别为
图 5. 不同风速下接收光的偏振度。(a)圆偏振光;(b)线偏振光
Fig. 5. Degrees of polarization of received light at different wind speeds. (a) Circularly polarized light; (b) linear polarized light
如
在光子的跨介质传输中,海面倾斜角大小随机,但其统计特性代表的海面粗糙度随风速的变化呈现相关规律,因此海面的随机折射致使接收端光斑位置、形状
和接收强度发生变化。如
图 6. 不同风速下随机动态海面的接收光斑。(a)风速为5 m/s;(b)风速为10 m/s
Fig. 6. Receiving spots of random dynamic sea surface under different wind speeds. (a) Wind speed is 5 m/s; (b) wind speed is 10 m/s
因此仿真风速分别为5、10、15 m/s条件下0~20 s内接收光强的变化并计算相应条件下的闪烁指数。闪烁指数采用接收坐标范围内的光强波动进行计算,定义[28]为
式中:
设置接收面坐标范围为
图 7. 不同风速下0~50 s内接收光强的波动变化
Fig. 7. Fluctuation of received light intensity within 0-50 s under different wind speeds
从
仿真研究风速为5 m/s和15 m/s时不同偏振态发射光束经过跨介质信道后的闪烁指数,设置接收面坐标范围为
图 8. 不同风速下接收光的闪烁指数。(a)风速为5 m/s;(b)风速为15 m/s
Fig. 8. Scintillation indecies of received light under different wind speeds. (a) Wind speed is 5 m/s; (b) wind speed is 15 m/s
在10 m/s风速条件下,探究光束发散角对动态空气-海水信道闪烁指数的影响。设置发射激光的Stokes矢量为
图 9. 不同光束发散角下接收光的闪烁指数
Fig. 9. Scintillation indecies of received light under different beam divergence angles
4 结论
本文建立了海面风速影响下的激光跨空气-海水界面的偏振光传输模型。通过改变海面风速、链路距离和激光束发散角,对接收端光束的偏振度和闪烁指数进行了统计。海面风速影响了三维海面的倾斜角统计分布变化,同时影响了海面粗糙度。风速越大,海面粗糙度越大,水下光子束偏振度降低;信道链路距离越长,偏振度越低。而闪烁指数则衡量了海面随机折射对光强变化程度的影响,仿真结果表明,风速越大,闪烁指数越大,光强起伏受三维海面影响更大;当发散角变小,闪烁指数逐渐增大。本文研究结果可为激光跨粗糙空气-海水界面的光通信信道模型的研究提供理论参考。
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Article Outline
张建磊, 田雨欣, 王洁, 朱云周, 张鹏伟, 杨祎, 贺锋涛. 蓝绿激光跨介质下行链路的偏振传输特性[J]. 光学学报, 2024, 44(6): 0606005. Jianlei Zhang, Yuxin Tian, Jie Wang, Yunzhou Zhu, Pengwei Zhang, Yi Yang, Fengtao He. Polarization Transmission Characteristics of Blue-Green Laser in Cross-Medium Downlink[J]. Acta Optica Sinica, 2024, 44(6): 0606005.