Multipoint Gas Sensing Technology Based On FMCW Dispersive Spectroscopy

Optical multi-point gas sensing is of great significance in natural gas pipeline leak detection,landfill gas emission monitoring,underground integrated pipeline gas monitoring and other applications,where quantitative detection and precise positioning of chemical extension distances or areas is indispensable.However,most optical gas sensors at present are single-point sensors,and the application cost is relatively high in large-scale monitoring,and which increases difficulties to mass applications.On the contrary,frequency modulated continuous wave(FMCW)is a frequency division multiplexing technology,which can be applied to multiple sensors sharing expensive optical components,including light sources and detectors,thereby greatly reducing the cost of each sensor.This paper proposed a multi-point gas sensing scheme based on FMCW dispersion spectroscopy,using C₂H₂ gas as the experimental sample,applying three gas cells in parallel to achieve multi-point detection.The following three aspects has been studied:The classical theory of dispersion and absorption is derived from the Lorentz theory of electrons,and the integral expression of the conversion between the real refractive index and absorption coefficient is derived from the k-k relation,so as to realize the theoretical calculation of dispersion spectrum.Based on the basic principle of FMCW,the gas sensing theory based on FMCW dispersion spectrum is deduced and analyzed,and the feasibility of multi-point gas sensing scheme based on FMCW dispersion spectrum is theoretically verified.Simulation based on FMCW dispersion spectrum theory has been carried out.After,working out theoretical calculations on dispersion spectrum,this work applied theoretical dispersion spectrum to simulate the spectral resolution,spectrum leakage and channel crosstalk.Finally the results verified the feasibility of realizing multi-point gas sensing;Combined with FMCW dispersion spectroscopy theory and simulation,multi-point gas sensing experimental research has been conducted.We successfully built the experimental system,to realized multi-point sensing,analysis of the results found close sensing branch has the same phase noise base,so can through the sensor structure design to improve the signal-to-noise ratio of the dispersion spectrum.After phase noise correction,the signal-to-noise ratio has been was increased by 6 times,and the detection limit of 133 ppm and the sensing spatial resolution of 29.41 cm have been obtained,Meanwhile and the system only has reached a crosstalk response only of-31.1 d B.In addition,the performance of the dispersion spectrum has been verified,and the power fluctuation has little effect on the dispersion spectrum,which means that the dispersion spectrum can still be demodulated at a low power of 21 n W;besides the linear response characteristics of the dispersion spectrum has been verified in this experiment,and the dynamic sensing range of the system can reach 1.09×10³ after analysis.