Research On Atmospheric Temperature Measured System Based On Brillouin Scattering Lidar

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.