Research On High Precision Detection Method Of Wavelength Modulation Absorption Spectrum

Tunable Diode Laser Absorption Spectroscopy（TDLAS）has the advantages of strong environmental adaptability,fast response,non-invasive,and simultaneous measurement of multiple parameters.Wavelength Modulation Spectrocopy（WMS）technology is a typical method of TDLAS,with strong anti-noise interference ability and high sensitivity.It can not only be used for atmospheric environmental protection and trace gas detection,but also has broad application prospects in combustion diagnosis of coal-fired boiler and combustion flow field parameter detection of aero-engine.Based on the theory of WMS,this paper studies the key issues encountered in the field of spectrum measurement:（1）In Scanned Wavelength Modulated Spectroscopy（Scanned-WMS）technology,due to the bandwidth limitation of the optical receiver and the sampling rate limitation of the acquisition device,the interference fringes generated by the high-resolution optical etalon are prone to distortions,resulting in errors in the time-frequency response relationship.The waveform of second harmonics will be deformed due to changes in scanning depth and modulation depth,resulting in poor harmonic fitting effects and large errors in gas parameter measurement values;（2）It is difficult to strictly adjust the optical output center of the tunable semiconductor laser to the center of the spectral line.This makes the signal detected by the fixed wavelength modulation（Fixed-WMS）technology not the peak height information of the absorption peak,which greatly reduces the measurement accuracy of gas parameters.In response to the above problems,the work of this article is as follows:1.The selection principle of optimal cut-off frequency and wide range of optimal modulation coefficients is explored.By simulating the concentration inversion errors corresponding to different cut-off frequencies,the optimal cut-off frequency selection principles of Fixed-WMS technology and Scanned-WMS technology are given respectively;The variation of the optimal modulation coefficient with a single gas parameter was simulated,and then a polynomial model related to the optimal modulation coefficient and spectral line broadening of the second and fourth harmonics are established respectively.2.The scanning characteristics and modulation characteristics of the laser are explored,and the trend of the laser response parameters with the central wavelength of the light emitted under the two working conditions is analyzed.Then,the influence of various parameters on the concentration inversion of the system is simulated.Among them,the scanning depth and modulation depth have a more obvious influence on the concentration.A calibration-free algorithm based on wavelength response parameter correction is proposed,and the scanning depth and modulation depth are used as initial values to participate in the second harmonic fitting process.A set of methane measurement system was built to verify that the algorithm can effectively improve the fitting accuracy of the second harmonic and the measurement accuracy of gas concentration.The relative error decreases from 5%to less than1%.3.A gas concentration measurement method based on Fixed-WMS technology is proposed.By introducing a reference light path with a closed gas cell,the harmonic height is used to calculate the wavelength offset of the laser light center.The loop iterative algorithm is used to realize the effective inversion of the gas concentration to be measured.A CH₄concentration measurement system was built,and the absorption spectrum of CH₄ molecules near 6046.95 cm^-1 was selected for concentration measurement experiments.The corresponding relationship between the second harmonic and the wavelength shift is explored,and the gas concentration error before and after correction is analyzed.The results showed that the maximum relative error of concentration measurement was reduced from 90.51%to 5.204%.The error of less than 70%can be controlled within 2%after correction,which proves that the method can effectively eliminate the influence of wavelength drift on gas concentration inversion.