Research On Optical Fiber Methane Measurement Sensor Based On Harmonic Detection Technology

Coal is the most important energy sources, and occupies 70% of the energy structure in our country. Safety is the main problem which restricts the development of Chinese coal industry. In the recent years, gas explosions occupy more than 80% of coal mine accidents. Methane which is produced in coal mining is a kind of harmful gas, which causes explosions the most directly. Detecting methane gas rapidly and accurately plays a crucial role in the safety of mining, production and life. In the recent years, optical fiber and electro-optical detection technology develop rapidly. In view of that optical fiber can transmit for a long distance. And the power which used by optical fiber sensors is very low, which is very safe. So the probes of sensors can be placed in dangerous environment, such as toxic, hot, flammable and explosive gases. It can monitor scene remotely and detect the flammable and explosive gas in security.In this paper, a new type of optical fiber methane gas detection system is discussed. The infrared absorption of multi-atomic molecular theory is analyzed. According to the molecular structure of methane, the infrared absorption spectra of methane molecule are studied and the appropriate absorption peaks is selected. According to the law of beer-lambert, a mathematical model of harmonic detection technology is established. It can lock wavelength in the gas absorption line center effectively, and can eliminate the errors of light intensity fluctuations caused by common mode noises.Working principle and advantages of DFBLD as the light source is studied. And according to its characteristic, the drive and temperature control circuits are designed. The characteristics of photoelectric detectors are described. A series of signal detection circuits are designed, including sine wave generating circuit, multiplier circuit, pre-amplifier circuit and filter circuit. The structure and working principle of digital lock-in amplifier is studied. Gas chamber and its coupling with fiber is analyzed. Finally the virtual oscilloscope is designed based on the LabVIEW.Experiment of optical path loss, gas absorption and value calibration is carried out. The results of them indicate that the optical fiber methane system designed in this article can measure methane gas effectively.