Fiber-optic Gas Sensor Based On M-Z Interferometer

Along with the development of oil and natural gas industry, the coal mine explosion and gas poisoning accidents increased everyday. With the scientific and technological progress, the traditional gas sensor gradually exposed their weakness. Therefore, developing the advanced gas sensor becomes an urgent task. In recent years, the fiber-optic gas sensors with the unparallel advantages traditional gas sensors have developed rapidly, representing an important direction of gas sensor developing.Firstly, the paper presents the research status and applications of fiber optic sensor interferometer. Secondly, the basic principle of the M-Z interferometer based on phase modulation is studied, by analyzing theoretically and simulating numerically by MATLAB with scattering matrix method and interference analysis method, respectively. The results show the coupler splitting ratio of the M-Z and the difference of two arms length will cause the output optical power and interferometer pattern change, which provides theoretical basis for phase detection by using of interferometer. The influence of the stress,strain and temperature on M-Z interferometer is analyzed.The current research status of domestic and foreign fiber-optic sensors is summarized in this paper. And a kind method of phase modulation fiber-optic gas sensor based on M-Z interferometer is adopted. The M-Z interferometer having the characteristics of suppressing source noise and mode noise has been paid more and more attention in high-precision measurements.The sensing arm of the interferometer is coated with gas-sensitive film, which is sensitive to the gas to be detected. The M-Z interferometer is put into gas chamber. When the gas is input into chamber, it will lead to the changes of refractive index of fiber core and phase difference of two arms, which cause the change of interferometer's output light intensity and the shift of interference patterns, and the variation of the shift is corresponding to the gas concentration.The gas concentration is got by analyzing the interference pattern.On this basis, schematic diagram of gas sensor device is designed. Take detection H₂S gas for example, DFB LD is chose as light source, because absorption lines of H₂S gas in the near infrared region is 1.578μm. The sensing arm of M-Z interferometer is coated the film. When the sensing device is placed in gas chamber, interference patterns will shift. Collect interference patterns by the CCD camera, process by computer, and get the shift number and direction of the stripes, which corresponding to the gas concentration. According to the requirements of the experiment, a special gas chamber is designed, and the airtight of the chamber meet the demand of test requirements. By theoretical analysis, the method the paper proposed is feasible.