Study On Laser Absorption Spectra Of Water And Carbon Dioxide

A series of disastrous weathers such as global warming, glaciers melting and extreme weather frequently occurs due to human unprincipled carbon emission from the industrial revolution. It is an urgent problem for human, and monitoring the concentration of green-house gas is very important for energy saving and emission reduction.The gas concentration detection is mainly divided into conventional and laser spectroscopy methods. In comparison with the traditional method, laser absorption spectroscopic techniques are characterized by no-required sampling,on-line measurement, simplicity of operation, easy to control and with high sensitivity. Therefore it is the focus of fundamental and applied research. In the thesis, the long optical length cell in association with wavelength modulation technique was employed to study the spectrum of H2 O and CO2, and the differential absorption detection circuit was improved.The relationship between modulation signal and harmonic signal was studied by theory, and the concentration linear-dependence with the harmonic signal was derived through calculations. Free-calibration measurements were achieved by normalized harmonic signal, which eliminated the variation of laser power and the drifting of electric noise.Tuning characteristics of laser temperature and current were tested, we analyzed the effect of the modulation amplitude on harmonic signals, then did experiments under optimal parameters. The direct absorption, 1f and 2f harmonic spectral signals of CO2 at different concentrations were measured using wavelength modulation combined with Herriott-type cell. The acquired data were processed by Origin to remove power background and fit observed lines. Finally, the concentrations of CO2 were obtained by normalizing the measured 2f harmonic signal. And the results were in good agreement with the standard concentration gas. The weak spectra of H2 O were also recorded in the region 12252-12312 cm-1 using same spectroscopic technique. Totally, forty three lines were observed, and the central positions of lines and widths fitted byVogit profile were consistent with the HITRAN database.