【作者】 梁琨；

【导师】 王宏远； 马泳；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2008， 博士

【摘要】 当前在大气测温技术领域,激光雷达大气测温具有实时性好、探测灵敏度高和时空分辨率好等优点,并且可连续地进行高空间分辨率大气温度垂直剖面遥感探测,因而成为一个新的研究热点。目前针对中高层大气温度探测的共振散射激光雷达和瑞利激光雷达已经有应用报道。由于低空大气中存在大量的气溶胶会产生很强烈的米氏散射信号干扰,严重影响了瑞利激光雷达的测量,因此低空大气温度探测存在测量精度非常不足的缺陷。通过测量大气瑞利-布里渊散射谱的半高线宽实现对低空大气测温的方法虽然可以在频域上减小米氏散射的干扰、提高测量精度,但是由于散射谱的半高线宽与温度在理论上仅为近似关系,所以其测得的精度也不太理想,大约为5K左右。与此同时,具有高精度测温能力的拉曼激光雷达可以用于探测0-11km低空大气的温度,其精度小于2K,但是拉曼散射信号的强度相对于米氏散射和瑞利散射要小3～4个数量级,因此为实现准确的测量需要较大的激光能量、接收望远镜系统和高精度高效率的分光器,导致该方法检测费用昂贵、成本较高,使用范围受到极大限制。针对上述方法的不足,文章提出了一种新型的、通过测量大气布里渊频移实现对低空大气温度进行实时探测的方法。由于大气中的布里渊频移与大气温度成一一对应的关系,因此只要准确的测量出大气布里渊频移量即可反映出温度大小。该方法是一种频域测量法,可以在频域上有效的滤除气溶胶产生的米氏散射干扰,同时大气布里渊频移严格正比于大气温度的平方根,因而具有更高的精度。另外,该方法只需用普通瑞利激光雷达即可实现温度的测量,亦大大降低了测量成本。文章首先以大气光学特性为理论背景,分析了大气的主要成分及其光学特性和大气中瑞利布里渊散射的基本理论。由于布里渊频移量的大小对激光雷达收发系统参数的确定非常重要,文章在大气光学特性理论基础上以激光信号在传输介质中布里渊散射频移量模型为基础,依据相关大气参数模型和美国标准大气(1976)建立基于布里渊散射信号检测的大气探测模型。该模型将布里渊频移量归一为只与大气高度这一单参数相关的函数,详细地描述了低空大气范围内布里渊频移量的连续分布状态。该模型对激光雷达系统参数的设计提供了理论依据,具有很好的指导意义。由于大气中瑞利散射谱和布里渊散射谱混叠在一起,为了能够实现对布里渊频移量的测量,需要将布里渊散射谱从大气瑞利布里渊谱中分辨出来,这也是布里渊激光雷达系统在大气测温应用中最关键性的问题。目前关于如何能够从大气瑞利布里渊谱中分辨出布里渊散射谱尚无成功报道,这严重制约了布里渊激光雷达在大气测温中的应用。文章利用谱线展宽理论对大气中瑞利布里渊散射谱的构成机理进行了研究,探索了大气瑞利布里渊散射频谱线形函数构成,并验证了理论的正确性。给出了测量大气布里渊频移的方法。通过仿真研究表明,大气中的瑞利布里渊散射谱的线形函数可以通过谱线展宽理论来描述,谱线的展宽类型就是由碰撞引起的均匀展宽和多普勒非均匀展宽。散射谱的线形函数是这两种展宽因素造成的综合展宽线形函数,为三个洛伦兹曲线和三个高斯曲线的叠加,其中瑞利散射谱和每一个布里渊散射谱分别由对应的一个洛伦兹曲线和一个高斯曲线叠加而成。文章通过对瑞利布里渊散射谱的理论分析,可以通过模型计算实现对布里渊散射谱的分辨,从而实现对布里渊频移量的准确测量,解决了布里渊激光雷达测温系统中最为关键的问题,为布里渊激光雷达测温系统的实现提供了坚实的理论基础。然后文章依据理论设计了布里渊散射激光雷达测温系统的整体设计方案,进行了激光雷达探测系统的光路设计,并对整个激光雷达探测系统中的几个关键器件进行了选型。其中研究了激光器的单稳频技术指标以及发射功率大小;法布里-珀罗标准具的光谱范围、反射率以及精细度等指标和ICCD的分辨率、响应时间、最小门控等参数指标,并最终给出了符合要求的各器件具体型号。该系统的设计为实际布里渊散射激光雷达测温系统的应用提供了详细的指导。最后通过激光雷达回波信号方程计算了布里渊激光雷达测温系统的探测能力,结果表明布里渊激光雷达测温系统能够满足30 km测量范围的需要。此外还分析了布里渊激光雷达系统测量大气温度的误差,并对各物理量具对温度测量精度的影响进行了详细分析和数值计算。结果表明,使用该方法测量大气温度的不确定度小于0.4256 K。另外,大气温度的误差受布里渊频移量测量精度的影响较大,而受大气折射率和激光入射波长的误差影响较小。更多还原

【Abstract】 Lidar is real-time, sensitive and has high resolving power, so it becomes a new research focus to measure temperature. In present, it has been reported that resonance scattering lidar and Rayleigh lidar can be used to detect temperature in upper atmosphere. But the Mie scattering signals brought from aerosol in lower atmosphere disturb normal temperature measurement, so there is less research on lower atmosphere temperature detecting. A method to detect lower atmosphere temperature by measuring the Full Width of Half Height (FWHH) of atmospheric which is approximately proportional to the square root of the temperature. This is only approximate since the scattering will also depend on the pressure of the gas. So, the accuracy is limited and is only 5K. Although Raman lidar can be used to measure lower atmosphere temperature and has high precision, Raman scattering signals is much less than Mie scattering and Rayleigh scattering for 3-4 levels. It needs high laser power and high precision, high efficiency receiving system, which makes it expensive to use Raman lidar to measure temperature and limits its application. A new lidar for lower atmospheric temperature with high-precision and low-costis urgently needed.A new approach to detect lower atmospheric temperature by measuring the atmospheric Brillouin frequency shift is proposed in the dissertation. The atmospheric Brillouin frequency shift is in direct proportion to the square root of atmospheric temperature. So, the atmospheric temperature profile can be accurately retrieved from this. This approach is based on frequency domain and the disturbance of Mie scattering can be avoided effectively. This approach has a high accuracy because there is a one - to - one correspondence between the atmospheric temperature and Brillouin shift frequency. Furthermore, it only needs an ordinary Rayleigh lidar to detect atmospheric temperature with this method. The measurement cost is reduced greatly.Firstly, the main components and these optical characters of atmosphere are analyzed. And based on Brillouin frequency shift model of laser transmitting through medium and relative atmospheric data, an atmospheric detection model based on detecting Brillouin scattering signal is established. In this model, Brillouin frequency shift is normalized to a function which is only correlative to atmospheric altitude. The continuous distributing state of Brillouin frequency shift at the range of low atmosphere is described detailed.And then, the key is that the Brillouin spectrum is needed to distinguish for measuring the Brillouin frequency shift because the Rayleigh Brillouin spectrum is a complicated mixture. The theory of spectral line broadening is applied to analyze the composing of atmospheric Rayleigh-Brillouin spectrum, and the spectral line models of Rayleigh spectrum and Brillouin spectrum are established. The Rayleigh-Brillouin spectrum was simulated by the S7 model and the simulated data can be used to compare with the spectrum which is calculated by the spectral line models. The results show that the theoretical model is valid for atmosphere. The Rayleigh-Brillouin spectrum is composed of three Gaussian lines caused by natural line broadening and three Lorentzian lines caused by collision line broadening. Base on the theory of spectral line broadening, the Brillouin spectrum can be distinguished and the Brillouin shift can be accurate measured.The Brillouin lidar system is designed based on the above-mentioned theory. The optical path for the lidar system is designed and the key equipments are analyzed which include laser’s monochromatic and stable frequency indexes; F-P etalon spectrum range, reflectance, fineness and ICCD resolution, minimum gate speed and so on. It is provides the guide for the Brillouin lidar practical application.The detection capability of Brillouin lidar is analyzed based on lidar equation and it is approved that the Brillouin lidar can measure the atmospheric temperature in the range of 0-30km. And the uncertainty of measured atmospheric temperature which caused by the wavelength of incident light, reflective index, Brillouin frequency shift was analyzed in detail. The results show that, the uncertainty of measured atmospheric temperature is less than 0.4256K. The atmospheric temperature has a greater influence on the Brillouin frequency shift than the refractive index and the wavelength of incident light.更多还原