| Tunable Diode Laser Absorption Spectroscopy（TDLAS）technology has the advantages of high sensitivity,fast response and non-invasive.It has been widely used in air pollution gas monitoring,coal-fired boiler and combustion test of aerospace engine.Compared with Direct Absorption Spectroscopy（DAS）,Wavelength Modulation Spectroscopy（WMS）has gradually become the mainstream of TDLAS technology because of its advantages of effectively suppressing the interference of low-frequency noise and enriching the measurement information.This paper mainly focuses on the two key problems encountered in the practical measurement application of WMS technology:（1）the existing Voigt function approximation algorithm can not well take into account the calculation accuracy and efficiency.（2）when gas concentration parameters are detected in complex environments such as aero-engines and coal-fired power plants,the measured light intensity signal contains noises of various types and frequencies due to the influence of strong vibration and radiation in the environment,which can not be used to extract useful information in serious cases,affecting the measurement accuracy.In view of the above problems,the main research work and innovation points of this paper are as follows:1.Aiming at the problems of the existing Voigt function approximation algorithm,this paper proposes a new high-precision and fast approximation method of Voigt linear function with only basic elementary functions through the extension analysis of exponential function expansion.The relative errors of the proposed method are simulated by MATLAB high-precision symbolic computing system.It is found that the relative errors in the computation region of 10-4≤x≤40,000∩10-4≤y≤10-2 are all less than 10-19 and the lowest is 10-26.In the calculation region of 10-4≤x≤40,000∩10-10≤y≤10-4,the maximum relative error is 10-13,and the optimal parameter values of the algorithm under different requirements of calculation accuracy are given.2.Taking the absorption spectrum of CH4 near 6046.95cm-1 as an example,the noise response characteristics of WMS technology were quantitatively studied by numerical simulation.The peak variation of the normalized second harmonic signal extracted by FFT filtering method under different SNR is compared and analyzed.On this basis,the relationship between the standard deviation and the relative error of the second harmonic peak variation and signal SNR is modeled,and the approximate expression between the two and SNR is obtained.The fitted R2 values were 0.99957 and0.99352,respectively.Research results show that when the low signal noise ratio（SNR）,the measured results of change does not accord with normal distribution,by means of the average data cannot fundamentally eliminate noise effects on WMS measurement accuracy,establish standard deviation and relative error of signal-to-noise ratio and measurement index between the mathematical model can to a certain extent,fixed the WMS measurement result under the influence of the noise.3.Based on power spectral density analysis,a high signal-to-noise ratio（SNR）wavelength modulation spectral measurement algorithm is proposed.Compared with the traditional calibration free WMS measurement algorithm based on 2f/1f waveform analysis,the new method only needs to obtain the power ratio of 2 f and 1 f of the measured signal in the frequency domain to calculate the relevant gas parameters.Because of the sparsity of the power spectrum of modulated laser signal,the new method can effectively estimate the noise intensity,and then reduce the influence of noise on the measurement of gas parameters.Numerical analysis results show that the proposed method can improve the accuracy and sensitivity of measurement results,and has a great application prospect in low SNR measurement environment.Further experiments show that when the integral time is 1s,the sensitivity of CH4 gas measurement is improved by 2.64 times compared with the traditional calibration free WMS measurement method. |
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