Spectral Measurement Of Greenhouse Gases By Laser Heterodyne Spectrometer And Retrieval Algorithm

Lase heterodyne is a kind of spectrum measurement technique which based on coherent detection.The technique uses a narrow line-width laser as local oscillator(LO)and mixed with input signal,which converts infrared radiaiton(1013?1014Hz)to radio frequecy(106?108Hz)and it is helpful for high sensitivity and high spectral resolution detection.For those reasons,the technique is widely used for obtaining the total atmospheric transmission,absorption spectrum and greenhouse gases column density.The former study on laser heterodyne made good progress,however,the former established prototype suffered the following shortages:the insufficient signal-to-noise ratio(SNR),the indistinct theory of instrument line shape(ILS)and the retrieval algorithm which limited its application for retrieving the greenhouse gases concentration.In order to enhance the performance of the laser heterodyne spectrometer,the studies on these problems are shown as following:(1)The high spectral resolution laser heterodyne spectrometers based on 3.53?m and 3.29?m distributed feedback inter-band laser(DFB-ICL)have been developed.(2)The SNR of LHS was analyzed based on the theories of equivalent noise temperature and linear two-terminal network,the formula of sensitivity and SNR of LHS were obtained.And the formula of ILS was deduced and presented,which based on the principle of laser heterodyne and the process of signal processing.(3)The SNR and spectral resolution of LHS were promoted based on the analysis of SNR and ILS.The SNR of 3.53?m LHS was improved from 13.9dB to 24.9dB,the spectral resolution was promoted from 0.11 cm-1 to 0.009 cm-1.(4)The field campaigns were performed at Hefei,Dunhuang and Yao'an,and,and the water vapor and methane density were retrieved by optimal estimation(OE)algorithm.The results show that the density of water vapor and methane at Hefei revealed obvious seasonal variations.The correlation coefficient between retrieval results and balloon sounding of Dunhuang is 0.935,and the relative error of water vapor and methane column density between retrieval results and EM27/SUN's results is less than 16%and 5%respectively.And the ozone absorption spectrum and concentration profiles at Hefei were investigated for the first time according to our knowledge.Through the studies in this thesis,several achievements were obtained.Above all,the results are very helpful for the application of LHS based for measuring greenhouse gases density profile retrieval and other atmospheric studies.