Design And Experiments Of Optical Fiber Methane Gas Sensor

Optical fiber gas sensing technology is a developing and novel technique, which hasa wide applicable prospect in industrial production, environment monitoring and medicalscience. In recent years, with the development of optical fiber sensing technology, theresearch of optical fiber gas sensor has been seriously regarded all over the world. Thesignal detected by optical fiber gas sensors is carried by light wave, which has noinfluence on the measured object. The sensor that has good independence performanceitself can be adapted to all kinds of surroundings. The optical fiber sensing system withthe sensors, which can be connected with computers conveniently, can realize manyfunctions request and intellectualized need. They can match with remote measuretechnology to measure and control in long distance.The gas is one of the sources of the most important universe disaster of the coalmine. Fire damp explosion not only influences the normal production of the mineral well,especially threatens to the life safety of the coal miner seriously, but the maincomposition of the gas in the mineral well is the methane, how can predict the explosioncondition relation graveness of the methane in advance. Aiming at three conditions of themethane explosion, the paper puts forward wavelength modulation technique based onDistributed Feedback Laser Diode (DFB LD), optic fiber sensing technique and weaksignal examination technique, and then designs a new gas detecting system andinstruments. Through experiments the system is proved to have high sensitivity, goodreiteration and fine stability. By improving and making change other accessories slightly,the system can measure other various gas's concentration immediately, having betterapplied foreground at the gas measure field.The main work of the paper is as follows:(1) The conventional method adopts spectrum absorption technique to detect gas'concentration. The gas cell in the system has three forms, those are transmission type gascell, reflecting type gas cell and collimation type gas cell with gradual changingrefractive index lens. The small-scaled gradual changing refractive index lens is used inpractice and optic fiber is used as transmission tool in this paper. The emanative reflexcannot return the former light path. This can reduce the noise and the signal-to-noise canimproved highly.(2) The width of gas absorption spectrum is very narrow. It is only several 1/1000sof the intensity of light source and is smaller than the noise of light source's change. Asthe result, the measure of the gas absorption weak signal is very important. The paperimitates to use the technique of Ring-down cavity to gas cell to decrease secondary noise,increasing the absorption length in effect and raising the measure sensitivity.(3) At present, the source in detecting gas' concentration is laser diode. The power islittle and optic fiber coupling wastage is much. As a result, the precision is small and thesensitivity is low. The wavelength modulation technique used DFB LD as source, thewavelength is 1.65 μm, is adopted in this system. The detecting signal is stabilized andthe precision is improved.(4) Using wavelet noise reduced technique to the system, the spectrum of someinterfering gas is filtered and smoothed. The noise is dealed with. The situation, heightand area of peak value of the spectrum are almost not changed.