Research On Spectrum Absorption Based Fiber Gas Sensing Theory And Applications

With the development of science and industry, gas sensors have been widely used in environmental monitoring, industrial process monitoring, mine safety monitoring, atmospheric sciences and bio-respiratory diagnosis. However, conventional gas sensors have great limitations when they are applied for many fields, due to the disadvantages of short-lifetime, significant cross-sensitivity and high cost of networking. Optical fiber gas sensors, as a new type of gas detection method, have the advantages such as intrinsically safety, exclusion of cross-sensitivity impact, long lifetime, and easy to be multiplexed to reduce the cost of sensor networks. For these reasons, optical fiber gas sensor become a very important research direction in gas sensing field.This thesis conducts theoretical analysis on optical fiber gas sensing system based on spectrum absorption, and presents the experimentals results as well as the further optimization. Firstly, the gas spectral absorption theory is introduced, and the relationships between gas concentration and the spectrum absorption is established, which provides the references for the selection of gas absorption line. Subsequently, methane gas sensing which has a strong absorption spectrum in the optical fiber communication window is studied. Based on the principle of differential absorption, the methane gas detection system is designed and experimental demonstrated. The achieved detection accuracy is as high as 271 ppm. Further, a hybrid SDM/TDM based multi-point gas detection system assisted with optical-switch is proposed and investigated. Experimental system of 7-points gas sensor network is set up and tested. Moreover, the application range and limitation of the system are also discussed.For the gas with weak optical absorption, it's hard to achieve high measurement accuracy by using differential optical absorption spectroscopy. Therefore, this thesis further studies the high sensitive gas sensor based on tunable diode laser and harmonic detection. Firstly, the theoretical model to analyze the relationship between harmonic signal and the gas concentration is set up. In order to optimize the design parameters of the system, a simulation in MATLAB is conducted to analyze the influence of the scanning and modulation signal on the sensitivity. Thereafter, circuit design and experimental system construction are accomplished to conduct CO2 gas detection and verify the theory and simulation results. Through calibrating the gas concemtration and optimizating the detected signal, a detection limitation of 68 ppm and a high sensitivity of 10 mV/ppm are achieved, which makes significantly improvement compared with other researches.