海水湍流对涡旋局域光波通信的影响及其抑制的研究

The key problems to realize optical communication of high speed, big data and secrecy in the underwater of long distance are stemming the signal loss caused by turbulence, scattering and absorption of seawater and remitting the communication link pointing error caused by the wander of beam center which is introduced by turbulence. Vortex localized waves are the spatiotemporal light waves with no diffraction, no dispersion and carrying orbital angular momentum (OAM). This project is to establish the models of the evolution of the OAM mode of vortex localized waves and the information capacity of the communication channel with point error. To do so, we first set up a turbulent power spectrum, which can describe the turbulent fluctuations of seawater containing high wavenumber region, and the wave structure function of the vortex localized waves in turbulent seawater. In the process of research, by taking the advantage of the characteristics of the low vacuum diffraction, the low light scattering，the low dispersion and the low turbulent scintillation of the vortex localized waves, we explore the origin　of controlling the distraction of seawater turbulence by the vortex localized waves. The research is carried out from three aspects. The three aspects are the evolution trajectory of the signal OAM mode induced by turbulence, the point error of the communication link caused by the wander of beam center and the influence of the bandwidth of OAM orthogonal expansion basis on the transmission of the OAM mode. The results of this project will contribute to deepen the understanding of the mechanism of the beam structure adjusting and controlling the turbulence interference of the seawater. The results also will provide a universal model for the investigations of underwater optical communication. The conclusions of this project will contribute to understand the crosstalk mechanism between the OAM modes of the vortex beam induced by seawater turbulence. As well as, the conclusions will enrich and penetrate into the theory of artificial controlling for the obstruction of optical signal transmission originated from seawater turbulence.

遏制海水的湍流、散射和吸收产生的信号损失，缓解束心抖动导致的光通信链路的指向误差，是实现远距离水下大数据高速保密光通信必须解决的关键问题。涡旋局域波是一种具有无衍射无色散且携带轨道角动量（OAM）的空时光束。本项目以建立描述包含高波数区海水湍流起伏规律的湍流功率谱和涡旋局域光波的波结构函数为研究基础，借助涡旋局域光波的低真空衍射、光散射、色散和湍流闪烁等优点，从湍流诱发信号OAM模演变、束心抖动产生通信链路指向误差和OAM正交展开基的带宽对OAM模传输的影响等三方面探究涡旋局域光波调控海水湍流干扰的根源，建立链路指向误差信道中传输涡旋局域光波所携带OAM模的演变模型和信道信息容量模型。本项目的研究结果有助于加深了解光束结构调控海水湍流干扰的原理，为水下光通信研究提供普适的海水湍流功率谱模型。研究结论有助于理解海水湍流诱发OAM模间串音的机制，丰富和深化人为控制湍流海水干扰光信号传输的理论。

海洋覆盖了地球三分之二的表面，海洋中拥有丰富的渔业资源和蕴藏着大量矿产资源。所以，开发发展海洋事业，加快遥控潜水器和潜艇的研制、加大海洋资源的分析、海床生态演变的调查、海底天然气和石油的勘探、海洋污染的监测等是强国固本特别是成为海洋强国的重要环节。长距离、大数据和高速率的水下无线保密光通信网络是实现此环节高速发展的重要基础之一。但是，海水的湍流与吸收限制了水下光通信网络节点间的长度，所以，遏制海水通道的湍流与光吸收、湍流产生光束抖动与光束扩展和收发系统瞄准误差产生的链路指向误差等造成的通信信号损失是实现水下长距离、大数据和高速率保密通信必须要解决的关键课题。本项目通过建立新海水湍流功率谱解决了传统湍流功率谱在高低波数端点区的理论局限性和在极低空间频率区存在违反实验事实的理论奇点问题，突破了经典海水湍流功率谱在建立中到强湍流起伏区修正海洋Rytov理论时遇到的数学障碍，开辟了采用脉冲涡旋局域波扼制吸收海水湍流通道损耗的研究方向；通过提出高斯准直局域波的相位结构函数、空时正交涡旋基、带限（束径）有限正交涡旋基、OAM载波指向误差链路的信号-噪声-串音比和揭示涡旋光束的结构参数、束心抖动和光束扩展、脉冲展宽、通信链路的指向误差、海水湍流、信号调制方法和随机涡旋场的OAM正交展开方法影响OAM信号模传输的机制，建立了水下局域载波光通信链路的OAM检测概率和信道信息容量的理论模型；通过在定位局域波传输模型中引入湍流统计理论，提出随机定位局域波的概念和建立了相应的透射率理论模型。项目研究发现涡旋局波的类型、结构参数和信号调制方式的选择能部分克服海水通道中的湍流和吸收的自然障碍，达到延长信号在水下通道中传输距离的目的。项目的研究方法和结果为水下光通信系统研究与设计提供理论基础。

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