Technical And Experimental Study On Methane Detection System Based On Absorption Spectroscopy

Methane is the dominant ingredient of coal mine tuunel gas and natural gas, which is flammabel and explosive. Therefore, in the event of spill in our enviroment its early and accurate detection by means of reliable sensor is vital for chemical facilities, gas plants, mines and residential areas.Gas spectrosclpy is a selective and sensitive technque for evaluating the concentrations of gaseous species. Fiber optics enable gas spectroscopy to be performed remotely using the near infrared band of spectrum. The fiber-based remote sensing method for methane detection in coal mine tunnel has advantages over the conventional pellistor sensors employ a catalyst head, such as fully optical, non-hazardous, easy calibration, little disturbance owing to interference gases and electromagnetic noise.The dissatation starts with molecular structure and spectrum, and gave a detailed theretical analysis on the subject of vibration, rotation and vibration-rotation spectra. Different broadening mechanisms and lineshapes on isolate absorption line were illustrated.Key issues, such as frequency and amplitude modulation, fringe generation and supression, system detectability, have been explained in detail. The relation between phase-locked amplifier harmonic output and Fourier decomposition on lineshape and electric signal form photodevice has also been interpreted.The equivalent absorption coefficient theory was improved on comb-shaped light source, and photo-electric pre-amplifier and LabVIEW data acquisition system were proposed. Methane concentration information can be deduced from that of other gas ingredient. Absorption and fluorescence spectra of SO2 have been measured in experiment. Multi-component gas detection system employing absorption and fluorescence measurement has been designed. The feasibility of hollow-core photonic crystal fiber(HC-PCF) used as absorption-fluorescence measurement sensor was disscussed. Sensor network based on fiber splitter and fiber laser was analysed.A novel fiber methane detection system was constructed based on integration of prism gas cell and harmonic detection technique. The system can be applied to broad-range concentration detection and its sensitivity can be adjusted by changing the prism distance using step motor; A novel fibre-optic methane detection system was proposed based on sampled fiber bragg grating (SFBG) , tandem Fabry-Perot filtering technique. By means of SFBG and tandem F-P, Near-infrared equal-spaced multi methane absorption lines were detected simultaneously and difference absorption detection theory was developed; In hollow-core photonic bandgap fibres, long interaction pathlengths between light and the gas confined in the core are achieved, which enables sensitive detection of methane even at its weak absorption bands. It is proposed a novel optical fiber gas sensor based on cascade of HC-PCFs. The diffusion of gas inside of the hollow core is also analysed; A novel gas leakage detection sensor based on self-heated effect of fibre optic is presented. The optical energy leaking out from the fibre is absorbed by the metallic coating, which alter the grating pitch of the fibre Bragg grating. When gas leaking out from the pipeline blow on the fibre grating, its temperature changes accordingly. The gas leaking velocity can be determined by monitoring the change of resonance wavelength.