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Direct Detection Doppler Wind Lidar Based On Aerosol Backscattered Signal With Twin-channel Fabry-Perot Etalon

Posted on:2007-10-17Degree:MasterType:Thesis
Country:ChinaCandidate:H Y XiaFull Text:PDF
GTID:2178360185478430Subject:Optics
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The societal, economic and scientific impact and the history of Doppler Wind Lidar (DWL) are reviewed in Chapter 1. DWL leads to improved long-range weather forecasting, more accurate hurricane landfall prediction, improved climate models, better battlefield environmental predictions for military operations, more accurate upper /lower stratospheric winds for Air Force operations, and potential chemical-biological release trajectory prediction for homeland security operations. Many research groups have built ground-based DWL all over the world. There are also numerous studies on the design and feasibilities of spaceborne DWL.Chaper 2 deals with the basic concepts of DWL. DWL is an active remote sensing instrument that uses aerosol and/or molecular backscatter to measure wind. DWL instrument concepts can be divided into two main categories: Direct detection and coherent detection. The direct detection method, which uses high-resolution optical devices such as a Fabry-Perot etalons, detects the Doppler shift directly from spectrum of light backscattered from atmospheric air molecules and aerosols. The other approach is optical heterodyning, or coherent detection, which measures Doppler shifts by beating the backscattered laser light with a laser source from a stable local oscillator.The edge technique is a direct detection method for measuring Doppler shift of an atmospheric backscattered signal from a pulsed laser by differential measurement of atmospheric echo and the outgoing laser pulse. The double-edge technique is a variation of the edge technique. It has the same basic advantages as the edge technique but with new capabilities. It improves the measurement accuracy and allows the Rayleigh and aerosol components of the signal to be separated.Optimization of the operating parameters of Fabry-Perot interferometers for the direct detection Doppler wind lidar based on double-edge technique is considered in detail. Simulation of the the lidar signal to evaluate the performance of DWL is also discussed briefly.System parameters, timing diagram and structure are introduced in chapter 3. With a CCD camera positioned behind the Fabry-Perot interferometer (FPI), we find that the shape of the illumination pattern to and intensity distribution on the FPI affect its transmission curves obviously. These phenomena result in a large system error in the inversion of wind velocity. However, by setting a mode scrambler on the transfer fiber and introducing intensity distribution function in the data processing, these effects can be reduced. A feedback technique to locate the laser frequency on the steep edge of the FPI is also presented.In chapter 4, the system control method and typical experimental results are presented. A Doppler radar was used to assess the performance of DWL. The resulting comparisons of wind profiles are shown.A portable verifying attachment for convenient calibration of DWL system is designed in chapter 5. The designed relative error of the verifying attachment is 1%, the dynamic range of velocity is greater than ±50m/s, the maximum spectrum broadening corresponds to the designed hard target has a width of 0.7MHz, which is approximate to the aerosol circumstance. Lidar experimental results show that an accuracy of 0.6m/s can be obtained for 2000 detected photons.
Keywords/Search Tags:Lidar, Doppler Wind Lidar, Direct detection, Fabry-Perot Etalon
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