Theoretical Modeling And Implementation Of Optical Fiber CO Sensor

Electronical gas sensors are advanced, but this type of sensors have some shortcomings such as causing explosion, sensitive material poisoning. Since optical fiber gas sensors adopt optic as information carrier, this type of sensor has special advantage in security. And this type of sensor is fit for working in bad conditions. Therefore research and development of optical fiber sensor is gradually become the highlight of gas sensors'research. Optical fiber gas sensors are studied systematically in this dissertation, which include theoretical modeling, developing and testing of prototypes.Based on Principle of absorption spectrum and Beer-Lambert law, mathematical model of gas sensor based on difference absorption was analyzed. And harmonic detection mathematical model of optical fiber gas sensor based on narrowband optical source was analyzed. Then improved harmonic detection mathematical model of optical fiber gas sensor was presented. We call this model as difference-harmonic-detection model. Simulation research on harmonic detection optical fiber gas sensor based on narrowband optical source was processed. Using the difference between referenced light intensity to metrical light intensity, we can counteract the fundamental wave component. Detecting 2nd harmonic of difference signal, we can obtain the gas concentration. By using this method, system's ability of detecting 2nd harmonic component is improved, and system's resolving power is increased too.According to analysis above, structure of difference-harmonic-detection optical fiber sensor system was designed. In optical design, we present a method that using cascaded GRIN Lens couples to compose a gas cell. Mathematical model of transmission type of gas cell that based on cascaded GRIN Lens couples was analyzed. And structure of gas cell is designed and developed. If the other condition is fixed, with increasing of optical path length, sensor's sensitivity is enhanced. For transmission gas cell based on GRIN lens, the same method is usually adopted. However, for some of sensors, the formal size is limited. Especially, the length is limited. To shorten sensor's size and length, and enhance sensor's sensitivity, a method that using cascaded GRIN Lens couples to compose a gas cell is presented. With this method, the optical path length is increased and the detection sensitivity of the gas cell is greatly increased. According to principle of Photoelectricity Weak Signal Detection and mathematical model of difference-harmonic-detection optical fiber sensor, the system of Photoelectricity Weak Signal Detection is designed. The circuit of Photoelectricity Weak Signal Detection, preamplifier circuit, difference circuit, modulation circuit, optical source thermal control and current drive circuit etc. are designed.At last, based on the research above, the prototype of an optical fiber gas sensor is developed. Experimental results show that optical fiber gas sensor based on difference- harmonic-detection and structure of cascaded GRIN Lens couple has good detection effects.