Study On The Detection Of SO₂ And NH₃ Based On The THz-TDS

Air environmental pollutions will lead to a series of global problems, how to monitor those pollutions and make early-warning has attracted people’s widespread attention. As a new coherent spectral technique, terahertz time-domain spectroscopy(THz-TDS) has a broad application prospect in many fields such as material inspection due to many advantages of the fingerprint spectral characteristics, wide frequency range and high signal to noise ratio. In this thesis, a multi-channel pump probe system was designed. Moreover, based on the algorithms for THz time-domain spectroscopy system, the THz spectral characteristics of two kinds of gases(sulfur dioxide and ammonia gas) were studied, and the pressure on refractive index and absorption coefficient of samples was also analyzed. The main contents and results are described as follows: 1. The pure rotational spectra and rotational transition frequency interval of SO2 and NH3 gases were theoretically calculated, and indicated that their pure rotational spectrum lies in the terahertz range. Moreover, the results show that in terahertz range, SO2 will have a series of absorption peaks, while there will be not a series of absorption peaks because the transition interval of NH3 is very larger. In addition, an algorithm to extract optical parameters(refractive index and absorption coefficient) of the gas was introduced, and the terahertz spectrum of high-density polyethylene material is used to validate this method. 2. A multi-channel pump-probe system(including OPOP, OPTP and THz-TDS) is set up based on THz-TDS system devices. By using LabVIEW, the multi-channel pump-probe system software was prepared, so as to provide the quick, rich and diverse study methods for samples, and to also deepen the understanding and application of THz-TDS. 3. Based on the algorithms for THz time-domain spectroscopy, the high-resolution optical parameters of SO2 and NH3 at the different pressures were studied. The results show that the refractive index and absorption intensity of SO2 increase with increasing the pressure. However, the positions of the absorption peaks and the two-adjacent-peak interval with about 0.1 THz are not sensitive to the pressure, which is consistent with the theoretical analysis. 4. According to the fitting equation given by the Least Squares, both the refractive index and the average absorption peak intensity of SO2 are linear with pressure, however both the refractive index and the average absorption peak intensity of NH3 are nonlinear with pressure. The pressure is converted into a concentration, and the concentration fitting equation on the refractive index or absorption peak can be obtained by a least squares fit, which can be used to quantitatively detect the pollution gas.