Research Of Gas Sensing System Based On Near-Infrared Spectrum Absorption Theory

The gas sensor based on near-infrared spectrum absorption has great applications in the atmospheric trace and harmful gas detection. Based on the principle of near-infrared spectrum absorption, three different designs of gas sensing systems were proposed. Gas sensing technology was studied from the perspectives of theory, system and experiment. Based on the theoretical analysis of the principle of near-infrared spectrum absorption, the shape of gas absorption line was derived and fitted. Through experiments, the feasibility of the direct absorption detection and harmonic detection were verified and the measurement of gas concentration was completed, which achieved the system design requirements.This work concentrates on:1. The first gas sensing system is based on the supercontinuum light source. In the system, the principle of supercontinuum is introduced theoretically, and the effect of light source noise and fluctuation is analyzed. The FFT smoothing filter method is used to process the absorption spectrum, and the results fit the acetylene characteristics. This system can be used to sense the acetylene with concentration of 4000ppm.2. The second gas sensing system is based on the broadband ASE light source and F-P type tunable optical filter, which realizes the wavelength sweep for the gas absorption spectrum. In the system, the relationship between driver voltage and transmission wavelength of F-P type tunable optical filter is calibrated, and the influence of different sampling precisions is analyzed for gas sensing. Through measuring the different concentrations of carbon dioxide, the relationship between the absorption intensity and concentration is obtained. The experimental results show that this system can be used to detect carbon dioxide with concentration of 5%-10%, and the measurement error is less than 5%.3. The third gas sensing system is based on the tunable laser and lock-in amplifier, which realizes the gas sensing through detecting the secondary harmonic. The principle of the wavelength modulation and second harmonic demodulation is deduced theoretically, and the relationship between the second harmonic and concentration is obtained. It is worth mentioning that a White Cell is used in this sensing system which can increase the absorption length and the detection sensitivity. In addition, the relationship between the secondary harmonic amplitude and concentration is calibrated through measuring the different concentrations of carbon dioxide. The experimental results show that the system can be used to detect the carbon dioxide with concentration of 1%-5%, and the detection sensitivity is 100 ppm.