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Key Techniques Of Scattering Differential Absorption Laser Radar

Posted on:2013-11-11Degree:MasterType:Thesis
Country:ChinaCandidate:F TianFull Text:PDF
GTID:2248330374499714Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
The principle of Scattering-Differential Absorption Lidar(S-DIAL) is to transmit the laser pulse to generate scattering return signals basing on the interactions between laser and various components in the atmosphere, thereafter the return signals are collected by a telescope and analyzed to get useful information such as atmospheric visibility, aerosol extinction coefficient and concentration distributions of some target gases. For differential absorption technique, two pulses with slightly different wavelengths (one wavelength is chosen to be strongly absorbed by the target gas, while the other is less strongly absorbed) are transmitted, then the concentration of the target gas can be derived according to the received signal intensity for the two different wavelengths. This technique has many advantages in gas density measurements such as high sensitivity, robustness, etc., for which it has great value in the civilian and military fields, such as air quality test, pipeline leak monitoring, biochemical reagents detection, vehicles and aviation equipment exhaust track detection, etc.Investigations on several key techniques of S-DIAL, such as the laser source technique, the detection and inversion algorithms of scattering signals, the data processing and error control of differential absorption measurement, are of great importance for the set-up of S-DIAL system. In this paper, the basic principles of the S-DIAL were systemically described, and the abovementioned key techniques were investigated through theoretical analysis, numerical calculations as well as experimental studies, which provide a good reference for the development of S-DIAL system.The requirements on the laser source of the S-DIAL system were analyzed. According to the available equipments, tunable Ti:sapphire laser pumped by a frequency doubling Q-switched Nd:YAG laser was chosen to obtain the laser beam for different absorption measurement, and several techniques about these lasers were investigated. The confocal Gaussian variable reflectivity unstable resonator for Q-switched Nd:YAG laser had been designed, and a high quality and pulse energy laser beam with wavelength of1064nm and pulse width of70ns had been realized. The phenomena of self-mode locking in long pulse width Q-switched Nd:YAG laser used for pumping Ti:Sapphire will result in instability of laser pulse peak power output and mangle the optical devices. Only one Fabry-Perot (F-P) etalon with not very high reflectivity was employed to eliminate this phenomenon, and a smooth pulse waveform had been realized. Comparing to other existing techniques (double F-P etalons or a combination of F-P etalon with other technique), this strategy has some features including the lower inserting loss, higher damage threshold, as well as higher pulse energy and so on. A design for the frequency doubling and tripling of the tunable Ti:sapphire laser in a wide wavelength range had been posed, in which BBO crystal mounting on a rotatable platform was chosen to meet the phase-match condition in the wavelength range from700nm to950nm through the rotation of the crystal angel.Theoretical and experimental investigations were carried out on the measurement of atmospheric visibility and aerosol extinction coefficient by scattering method. According to the lidar return signals measured by the experiment, three kinds of inversion algorithms commonly used for scattering lidar equation solving were compared. A new iteration algorithm used for visibility inversion had been posed, which was a combination of the well-known Collis slope method and Klett backward method, and it was applicable to retrieve the average atmospheric visibility under different weather condition. Software which was used for inversing aerosol extinction coefficient by Fernald backward method had been developed, and it was able to process a large number of lidar return signals automatically and draw the two-dimensional diagram of aerosol extinction coefficient distribution to reflect its characteristics directly and clearly.Data processing algorithm used for inversing the concentration distribution of targer sprcies measured by differential absorption technique was studied, and the factors which affect detection sensitivity and accuracy were analyzed, especially the detection error for target species caused by other gases, and the corresponding methods to avoid it had been put forward. In the case of considering all the factors which affect the detection sensitivity, accuracy and errors, a integrated experimental proposal had been proposed for the concentration measurement of several pollutant gases such as SO2, O3and NO2, whose feasibility had been validated through simulation, and it was a good basis for further experimental investigation. Finally, all the research work carried out in this paper had been summarized, and the future activities had been planned and described.
Keywords/Search Tags:lidar, differential absorption, scattering, aerosol, atmosphere pollution
PDF Full Text Request
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