Theoretical And Experimental Researches On The High-sensitivity And High-resolution Methane Gas Molecular Absorption Spectroscopy

The main contents of this thesis are the theoretical analysis and experimental research. High-resolution measurements of absorption linestrengths, pressure induced broadening coefficient and the pressure induced frequency shift coefficient of 2v₃ band R9 manifold of methane by using the external cavity diode laser. The F1,F2 absorption double lines, which is composed of two lines 6MHz apart, was high sensitively detected by use of the wavelength modulation and harmonic detection techniques. The dominating noises in our experiment were analyzed by theory and experiment, and considering these noises factors, the minimum detectable sensitivity was obtained. Besides, by adopting the couple principle between the confocal FP cavity and the laser and the interaction theory between the laser and the molecules, some factors influenced the cavity enhanced spectroscopy and cavity saturation spectroscopy were numerical simulated. The experimental project of cavity saturation spectroscopy was set up.The whole paper is divided into six chapters for above contents. The fist chapter is the introduction, briefly reviewing the existing high-sensitivity and high-resolution techniques, which can be applied to the filed of molecular spectroscopy, and educing the research content of this paper. The saturation absorption spectroscopy theory was given in detail in chapter two. The factors induced the absorption line broadening was analyzed by considering the absorption line broadening and narrowing effects. Finally the common theory of saturation spectroscopy was given. In chapter three, the wavelength modulation and harmonic detection theory was introduced firstly. The influence of laser power fluctuation and cavity resonance effect to the harmonic detection technique were analyzed by theory and we provide the resolving projects. In chapter four, the basic absorption parameters of the 2v₃ band R9 manifold of methane including absorption linestrength, pressure induced broadening and frequency shift coefficients were measured. Firstlythe basic structure, vibration mode, partition function and the NIR spectra of methane were introduced briefly. Finally the stabilization of the frequency of ECDL to the absorption lines of methane was realized. These parameters can give the necessary reference of setting up the device of high sensitivity detection of methane. Based on the theoretical anlysis in chapter three, in chapter five, the influence of power fluctuation to the harmonic detection was eliminated in the experiment. Also the harmonic signals of cavity resonance effect were detected. By measuring the harmonic signal of the low concentration gas closed to the detectable limit and the cavity resonance effects as a function of modulation amplitude, an optimum modulation amplitude was selected to high sensitively detect the methane gas. A minimum detectable sensitivity of our experimental setup was obtained. In chapter six, by analyzing the theoretical model of confocal FP cavity, the cavity transmitted signals with and without absorption medium inside the cavity were calculated out. The saturation spectroscopy of weak molecular absorbance was simulated by adopting saturation theory in the standing wave field. The saturation dip depth and the contrast between the depth and the enhanced Doppler background as functions of pressure, cavity length and the reflectivity of cavity mirror were analyzed, the frequency locking of the laser to the resonance peak of cavity were realized, also the experimental project of cavity saturation spectroscopy was provided. The innovations of this paper:1. The absorption lines of 2v3 band R9 manifold were detected by use of the external cavity diode laser. The linestrengthes of there eight lines and the pressure induced broadening coefficient were measured by direct absorption spectroscopy. Also combining the first harmonic detection the pressure induced frequency shift was measured.2. By experiment analysis, the primary factors of influencing the sensitivity of our setup are the laser power fluctuation and the cavity resonance effect. By theoretical analysis of the influence of power fluctuation to the harmonic signal, the way of eliminating this influence was given, and...