Multipoint And Large Dynamic Range Gas Detection Based On Frequency Modulated Continuous Wave

Gas detection has been widely used in environmental pollution monitoring,gas pipeline detection,atmospheric science research and other fields.Due to the high cost of optical devices,the gas sensor based on laser spectroscopy is usually a single point sensor.A set of equipment can only detect one position.The cost of installing a large number of single point sensors in wide range or long distance applications is extremely high.In addition,the absorption path length of the conventional gas sensor based on absorption spectrum is fixed,and the dynamic range of its concentration measurement is also limited.Therefore,it is significant to study innovative gas detection solutions for reducing the cost of gas detection and improving the dynamic range of gas detection.Frequency modulated continuous wave(FMCW)technology has high spatial resolution and high sensitivity.It also has the characteristic of sweeping optical frequency,which could cover the absorption peak of gas.Therefore,FMCW technology can simultaneously realize absorption localization and spectrum measurement.In this dissertation,the frequency division multiplexing characteristics of FMCW technology is used to invert the gas transmission spectrum signals at different locations in space domain.Three aspects was mainly carried out.First,spectrum inversion process based on FMCW is studied by simulations.Spectral resolution is studied theoretically.The oscillation noise of the spectral signal induced by spectrum leakage is studied and Savitzky-Golay filter is introduced to eliminate the noise.Second,multi-point gas detection based on FMCW is studied by both simulations and experiment.Structure of the theme is first proposed.The process of multi-point acetylene gas transmission spectrum inversion is simulated.The crosstalk effect between adjacent channels is simulated.In the experiment.Three point acetylene detection is carried out whose concentration is 1120 ppm,0 ppm,9250 ppm respectively,with a sensing spatial resolution of 30 cm,a sensitivity of 55 ppm and a measurement range of 52 m.The crosstalk effect between the sensors is less than-30 d B.Third,large dynamic range gas detection within multi-pass cell(MPC)based on FMCW is studied by both simulations and experiment.A multiplexing absorption paths gas detection is realized with spectrum inversion of different reflectors within MPC,which can extend the dynamic range of concentration detection.High,middle and low concentration spectrum inversion is simulated.In the experiment,acetylene gas of five different concentration gradients of 98000 ppm,9250 ppm,1120 ppm,95 ppm and 9.3 ppm is measured,and dynamic range of 1.91×105 is realized.On the premise of basically not changing the FMCW system structure,the direct absorption spectrum signal of the MPC is measured at the same time,and the dynamic range of concentration detection is further extended to 1.88×106.We innovatively proposed a multi-point gas detection scheme and a large dynamic range gas detection scheme both based on FMCW,the feasibility of which are verified by both simulations and experiment.It provides new solutions for large range multi-point gas detection and large dynamic range gas sensing.