Research On Fiber Optic Sensing System Of C₂H₂ Based On Near-infrared Spectrum Absorption

With the increase of population and industrial production, the environmental problem is facing an unprecedented challenge. And correspondingly, gas sensors being applied in the environmental monitoring and industrial waste gas detection are developped rapidly.Acetylene gas is one of the wast gases produced by car engine and malfunction gas of tranformer used in oil well. So it is very significant to detect acetylene gas concentration accurately and fast in environmental protection and production safety. Near-infrared fiber optic gas sensores have been widely applied in the field of gas detection due to its fast response, high sensitivity and high identification ability. In this paper, the key technology about C2H2 gas fiber optic sensing based on the near-infrared spectrum absorption theory is investigated.First, based on the theory of selective absorbtion of gas, the character of P9 absorption lines of C2H2 in its near-infrared spectrum are analyzed according to HITRAN database, a wavelength of 1530.37nm(6534.3636cm-1) is selected and the corresponding parameters are determined; according to Lambert-Beer law, gas concentration is analyzed by detecting the intensity of light coming from reflective or transmissive gas cavity; according to wavelength modulation and harmonic detection technology, the influence of light intensity fluctuation on each harmonic signal are simulated and the relationship between first and second harmonic and wavelength modulation coefficients is discussed using the frequency modulation signal.and by analyzing the relationship between harmonic signals and gas concentration, this paper provides a detection solution to eliminate light-intensity variation via the first harmonic ratio.Second, based on the characters of reflective and transmissive gas cavities, this paper analyzes the basic working principle about improving the detection sensitivity using off-axis integral cavity output spectrum technology to and introduces the main noise of cavity membrane. Meanwhile, combining with wavelength modulation technique, the noise level of integral cavity output spectrum technology is analyzed.Finally, this paper presentes the overall design scheme of fiber optic sensing system of C2H2 by near-infrared spectrum absorption. According to theoretical analysis, an appropriate laser source is selected and the corresponding driving circuit is disigned, while the amplify and filter circuit of weak signal is given. Based on the related detection principles, this paper uses the balanced modem AD630 as a phase-locked amplifier to extract the harmonic information. Through signal acquisition and data processing, the result of gas concentration is obtained. The feasibility of the system is verified through experiments.