Research On The Theory And Experiments Of Quasi-Continuous Wave Wavelength Modulation Spectroscopy

The environmental pollution is one of the most severe problems in the world. Inorder to control and improve the atmospheric environment, it is needed to develop gasdetection technologies that with highly sensitive and adaptable for different needs.Tunable diode laser absorption spectroscopy （TDLAS） combined with wavelengthmodulation method is a sensitive trace gas detection technology. Currently, the lightscources of TDLAS are mainly room temperature, continuous wave and single modelasers. In order to use the quasi-continuous wave and multi-mode lasers, which havesimple structures and low costs, in the high sensitive trace gas detection, this thesis ismainly research on the theory, method and experimants of quasi-continuous wavewavelength modulation spectroscopy （QCW WMS）. And the main work includesthree parts as following:1. This thesis built the theoretical system of quasi-continuous wave wavelengthmodulation spectroscopy （QCW WMS）. The QCW modulated wavelength expressionwas acquired. Also, the relationships of QCW WMS harmonic signals and gasabsorption intensities were obtained based on Fourier analysis. Various harmonicsignals of QCW WMS were studied through computer simulations, and then thesignal characteristics with different harmonic orders and different modulationamplitudes were compared and analyzed. These results can provide the theoreticalguidance for the applications of QCW WMS.2. The detection of CO₂with QCW WMS was studied experimentally. In theexperiments, a near-infrared （NIR） DFB diode laser working at QCW mode wasemployed as the QCW light source. The characteristics of the QCW second harmonicprofile was analyzed and compared with traditional continuous wave WMS, the resultshows that, QCW WMS can obtain the same detection sensitivity with continuousWMS. This thesis also studied the multiple frequency, sum frequency and differencefrequency of multiple modulation signals in the harmonics when multiple modulationsignals are super imposed onto the QCW diode laser injection current. Finally itobtained several sum frequency and difference frequency components that havinglarger amplitudes compared to the second harmonic wavelength modulation spectroscopy signal （2f-WMS） commonly used in tunable diode laser spectroscopy,and it may improve the detection sensitivity of QCW modulation spectroscopy.3. The detection of CH₄with QCW multi-mode laser wavelength modulationspectroscopy was studied experimentally. The principle of multi-mode diode laserabsorption spectroscopy was firstly studied through computer simulations. A QCWmulti-mode diode laser emitting at2.2μm, named LD220, was selected as the lightsource in this study. The laser’s tuning characteristics with current and temperaturewere examined experimentally by use of a Fourier transform infrared （FTIR）spectrometer. Finally, combined the QCW WMS studied previously, this thesisrealized the application of QCW multi-mode wavelength modulation spectroscopywith second harmonic to detect the green house gas CH₄, and a50ppm’s sensitivitywas obtained.QCW WMS by use of QCW diode lasers or QCW multi-mode diode lasers, havethe advangtages of low cost and wide wavelength cover range. It can detectmulti-gases or multi-lines of one gas with a single laser simultaneously. And it maypromote the widespread applictions of laser absorption spectroscopy.