Research And Application On High Sensitive Tunable Diode Laser Absorption Spectroscopy

As the world pays more and more attention to protection of environmental,efficient use of energy,safety of industrial production,and the increasingly rigorous emission standards of air pollutants,fast,calibration-free,sensitive and non-contact optical gas detection technology has become one of the important development directions of the present technology on gas in-situ monitoring.The research of this paper mainly focuses on the existing problems of two methods（direct absorption spectroscopy（DAS）and scanning cavity continuous-wave cavity ring down spectroscopy（S-CRDS））commonly used in optical gas detection technology and the actual demand of wide range gas detection.In order to solve the problems of traditional DAS,such as large uncertainty of baseline fitting and low scanning frequency of triangular wave or sawtooth wave,inspired by the harmonic analysis idea of wavelength modulation spectroscopy（WMS）,wavelength modulation-direct absorption spectroscopy（WM-DAS）was proposed in this paper.This proposed method,which adopts sinusoidal wavelength scanning,uses Fourier series to reproduce the absorptivity function,and simultaneously fits the baseline and absorbance,can effectively solve the problems of periodic noises,low scanning frequency and large uncertainty of baseline fitting,and can improve the measurement accuracy of absorptivity function.The standard deviation of the spectral fitting residual of this method can reach as low as 3×10^-5.The trace gases（CH₄ and CO₂）detection in the atmospheric,the calibration of physical constants of CO spectral lines and the measurement of CO temperature in the CH₄/air pre-mixed flat flame show that WM-DAS was consistent with the measurement results of S-CRDS,and had the advantages of high speed,low detection limit and anti-interference.At the same time,by combining WM-DAS with CRDS,the accurate baseline ring-down time,τ₀,can be calculated by the absorption peak（measured by WM-DAS）and the ring-down time containing gas absorption information（measured by CRDS at the center wavelength of the spectral line）.Therefore,a wide-range,calibration-free tunable diode laser spectrometer was established.Then,through the analysis of Allan variance and power spectrum,the types of jitter noises in traditional S-CRDS were qualitatived and the amplitude of the noises was quantified,inspired by the harmonic analysis idea of WM-DAS,a wavelength scanning cavity ring down spectroscopy based on Fourier transform（FWS-CRDS）was proposed.The proposed method adopts wavelength scanning and fast cavity length scanning.The characteristic frequencies of the periodic ring-down time are extracted after the Fourier transform to recover the absorbance and significantly reduce the noise,especially the troublesome wavelength fluctuations.The calibration of the physical constants of CO and CO₂ spectral lines in different perturb gases and the measurements of OH concentration in the CH₄/air pre-mixed flat flame show that the signal-to-noise ratio of this method is more than 4times higher than that of the traditional S-CRDS.In addition,FWS-CRDS does not need to use the wavelength meter,and has the advantages of anti-interference and high sensitivity.The proposed laser absorption spectroscopy（WM-DAS and FWS-CRDS）of gas detection in this paper can solve the problems of large uncertainty of baseline fitting and low scanning frequency in traditional DAS,as well as the jitter noises of the laser wavelength in S-CRDS,and can effectively improve the measurement sensitivity and anti-interference ability of the two methods.Macroscopically,the work of this paper can provide guidance for the development of productivity,the treatment of environmental pollution and the efficient utilization of energy.Microscopically,the research of this paper can provide theoretical and technical support for the research and development of highly sensitive gas sensors.