| Since the industrial revolution,the concentration of greenhouse gases rapidly rising,including carbon dioxide(CO2),methane(CH4),made the global warming and climate change drastically,which has seriously affected the human production and life and the life and property security.Therefore,it is of high importance for long-term monitoring their concentration variation to guide the energy conservation and emissions reduction,and to understand its source and sink and climate change.With the development of fiber and semiconductor technology,laser heterodyne spectroscopy has gradually been known as its advantages of high detection sensitivity,high spectral resolution,high spatial resolution,small volume of system integration and low cost.It has been gradually applied to the measurement of greenhouse gases in atmospheric study and astronomical observation,which has been a measurment approach complementary to the grounded based fourier transformed spectrometer(FTS).In view of the current needs of multicomponent greenhouse gas detection and the requirements of reliable field measurement,we have in-depth studied laser heterodyne spectroscopy,and developed a passive laser heterodyne detection system with high sensitivity by using three tunable semiconductor fiber lasers near1.277 um(O2)and 1.571 um(CO2)and 1.654 um(CH4&H2O)as the light source.The use of fiber coupler instead of that of the traditional freespace optic coupler improves the long-term stability of the system.In this paper,the quantitative evaluation method of performance parameters of laser heterodyne detection system has been studied.The heterodyne detection efficiencies of the system are calculated to be 36%(1.277um),56%(1.571um),and 76%(1.654um),respectively,by measuring the amplitude of the beat frequency signal and the voltage amplitude of the dc signal using a narrow line width laser as the light source.The result verifies the advantages of long wave detection in laser heterodyne detection.The modulation frequency of the signal light,832 Hz,is optimized by investgating the effects of different chopping frequencies on the background noise of the system,and the performance of the system is improved.The commercial broadband light source is used as the signal light to analyze the noise source and its influence.The instrument lineshape function is precisely obtained by using narrow linewidth semiconductor laser as the light signal to improve the retrieved accuracy.The spectral resolutions of the system are determined to be 0.122 cm-1(1.277 um),0.066 cm-1(1.571 um)and 0.09 cm-1(1.654 um)through the analysis of the linewidth of the measured heterodyne signal combining with Gauss lineshape function.Moreover,the equivalent noise powers of the system corresponding to 5.11 times that of the ideal system equivalent noise power,2.04 times and 5.43 times,respectively,are obtained via analyzing the signal to nosie ratio of the measured heterodyne signal and the known power of the signal lights,which indicates that the laser heterodyne detection with high sensitivity is realized.Secondly,in view of the errors induced by the solar altitude angle calculation and the internal system for the measurement column-averaged dry air mole fraction of the target specie with the reported laser heterodyne radiometer,the retriveral method of column-averaged dry air mole fraction of the target specie for CO2,CH4,H2O with the O2 column abundance as the internal standard function has been investigated.The heterodyne spectrum inversion algorithm has been established to retrieve the column abundants of the target species,which is combined with the line-by-line integration model,the LM(Levenberg-Marquarelt)based optimal estimation method,inversion algorithm and the measured linear function of the instrument.The total amount of the target gas molecular column was accurately retrieved by appling the algorithm to fit the measured spectrum with a normalized residual less than 0.01.The optimal measurement accuracies of the column abundance of the target species are calculated to be 0.5%(CO2),1.0%(O2),1.0%(CH4)and 4.4%(H2O),respectively according to the analysis of the measured results from Hefei city in China.The corresponding measurement accuracy of the dry air averaged column mixing ratios of CO2,CH4 and H2O are 1%,1.3%and 5.1%,respectively.In addition,the continuous measurement results of this system are compared with those of commercial ground-based FTS in hefei area and the observation results of Japanese satellite GOSAT on the average daily column concentration,respectively.The resuls show the reliability of the laser heterodyne detection system applied for the measurement of atmospheric greenhouse gases.Finally,in order to further optimize the measurement method of heterodyne spectrum and reduce the system volume,we have proposed for the first time to apply laser heterodyne spectroscopy based on wavelength modulation spectroscopy to atmospheric measurement.The software based lock-in amplifier is programmed to replace the traditional commercial digital phase-locked amplifier to realize the demodulation of CO2 heterodyne spectrum harmonic signal,which is conducive to the further integration of the system.By buliding the models of the absorption-free heterodyne detected intensity and laser scanning frequency response,the fitting algorithum has been developed based on the self calibrated spectral wavelength modulation spectroscopy.The column abundance of CO2 can be retrieved by fitting the experimental heterodyne R2f/0f spectrum,and atmospheric CO2 column continuous measurements in Hefei have been achieved using this developed system,where the system measurement accuracy is about 0.5%through analizing the noise of the measured results.It also shows the capacity of this method used for simultaneous guarantee of system performance and the reduction of the system volume,which has wide application prospect. |
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