Investigation And Design Of Spectral Absorptive Fiber Gas Sensor

Nowadays, with the development of industries such as petroleum, natural gas and coal mining, issues like colliery security, environment pollution have been paid more and more attention to in our society and it is vital to have a high-sensitive detection on noxious gas in the industrial production and our daily life. The spectrum absorptive fiber optical gas sensor, having the merits of high sensitivity, fast response, high resolution, good EMI immunity and conformity to flammable and explosive dangerous areas, gains positive regards coming from both domestic and international scholars, which gives it a great significant to be studied and explored.Based on Lambert-Beer law, the gas concentration is calculated through the measurement of light-intensity attenuation caused by the gas absorption peek in the transmission window of silica fiber. Two schemes, using CH₄ and C₂H₂ as the target gases, were designed and implemented in our research, which resolved the technical problems in the differential detection and the frequency modulation detection, respectively. The main tasks involved in our research are as followings:(1) Studying the principle based on infrared spectrum absorptive fiber gas sensing: on the basis of the gas absorptive characteristics, the light source, optical sensors, the selection of the fiber grating parameters and the couple techniques between them were investigated, a stable low-noise gas cell was designed and the influence on the sensor sensitivity given by the variation of the gas cell length was observed.(2) Studying the approach of gas detection: the fundamentals and existing problems within the present differential and frequency modulation detections were analyzed and discussed, from which the corresponding solutions and detecting systems were proposed.(3) Implementation of the novel time dual differential sensing system:a) A portable, dual optical channel electronic acquisition system specialized in weak signal detection in gas sensing was designed, which gives a balance between the high gain and saturation in the amplifier circuitry.b) The volume of the acquisition system was reduced to a portable size, which settles the big volume problem of the present photoelectric acquisition system.c) Good experimental results were obtained in CH4 detection.(4) Implementation of the frequency modulation detecting system based on FBG and linear filter.a) A novel modulation light source was designed, which served as the narrow-band scanning light source to detect gas; the cost of that kind of light source was reduced while the system additional interference problem caused by the high coherence of the narrow-band light source was resolved at the same time.b) It was the first time that the technical problem that the deterioration of the measurement accuracy due to the instability of the narrow-band light source and the inaccuracy of the center wave-length was settled by the means of taking advantage of the approximate linear section of the gas absorptive curve and the edge filtering characteristics of the linear filter.c) The center wave-length could be traced at the first time.d) A novel weak signal detecting circuitry was designed and implemented, which is specialized in frequency modulation detection.e) Good experimental results were obtained in C₂H₂ detection.(5) Analysis in the experimental results of the system: the procedures and measurement data of the CH₄, C₂H₂ were analyzed in this dissertation.