Based on DFBLD (Distributed Feedback Laser Diode) and harmonic detection technique, a novel fiber-optic methane detection system is constructed. The system can be applied to broad-range concentration detection of methane. Based on the approximation express of the law of Beer-Lambert, detection of methane with various concentration from 0% to 20% is completed using subtraction of background and ratio processing method, as the atmosphere surroundings are treated as background noise. The direct absorption spectra for various concentration is measured using GRIN gas cell, combined with DFBLD. The R5 line of the 2v3 band of methane is selected as the absorption peak. The system is tested online during gas mixing process and the linear relationship between system indication and concentration variation is validated. Also the stability and dynamic response characteristics are confirmed by the experiments. The sensitivity of the system can be adjusted according to the concentration level of various field environments by changing the prism distance using step motor. In the range of 0% to 20% the sensitivity of methane detection can arrive at 0.001%. So the system can be applied to various application fields and adopted as monitoring instruments for coalmine tunnel and natural pipeline.
