Research And Optimization Of Calibration-free Gas Detection Method Based On Wavelength Modulation Spectroscopy Technology

Rapid and accurate gas monitoring can provide significant guarantee for environmental protection and industrial production.Among numerous gas detection methods,tunable diode laser absorption spectroscopy(TDLAS)technology is attracting more and more attention due to its advantages of non-intrusive,high selectivity,high resolution and fast response.This technology has been widely applied in diagnostic analysis of combustion process,industrial process monitoring and control.and trace pollutant detection.The wavelength modulation spectroscopy(WMS)technology in TDLAS can effectively suppress the low frequency noise by superimposing high frequency modulation signal on the scanning signal,thus has higher system sensitivity.WMS is divided into calibration and calibration-free method.The calibration method do not require the etalon and prior information of the spectral database,but need to be calibrated in the known systems in practice.With the development and requirement of gas detection technology.the calibration-free WMS(CF-WMS)becomes the research focus of current gas detection methods.Based on the CF-WMS method,comprehensive and experimental study are carried out from two aspects:expanding the detection scope of the existing methods and improving the signal-to-noise ratio of the system.(1)The laser intensity will be modulated synchronously when the laser frequency is modulated,so as to generate the residual amplitude modulation(RAM)signal.The method of extracting gas parameter information from absorption-dependent RAM signals is called the RAM method.However.the existing RAM method is based on the assumption of weak absorption and small modulation index.which limits the application in practice.(2)The second harmonic signal with higher peak height can be obtained by triangular wave modulation,which is an effective way to improve the signal-to-noise ratio of measurement.However,the calibration-free method combined with triangular wave modulation has not been reported.In order to solve the above problems,the following research work are carried out with methane as the target gas:1.A calibration-free method for measuring gas concentration based on RAM for strong absorbance is studied and it has extended the scope of the existing RAM method.A theoretical model for spectral absorption of gas with arbitrary absorbance and modulation index is established based on Fourier series expansion,and a calibration-free algorithm based on the fitting of background signal normalized first harmonic Y component is proposed.An experimental platform was built,and the CH4 gas absorption spectrum at 1653.73nm was selected to conduct gas concentration measurement experiments under different modulation index.The experiments showed the feasibility of this method.2.A calibration-free measurement method based on the triangular wavelength modulation is proposed and implemented to improve the signal-to-noise ratio of the system.The triangular wave modulation wave is simplified into several sine wave superposition forms,and the time frequency response model of DFB laser under triangular wave modulation is established.An experimental platform was built and CH4 gas measurement experiments under different concentrations were carried out.The fitting results proved the feasibility of the proposed model.Furthermore,the calibration-free measurement of CH4 gas concentration based on triangular modulation is realized.