气体浓度的在线检测在环境保护、能源高效利用、工业生产安全中发挥着十分重要的作用，其中快速、免标定、高灵敏度、非接触的光学气体检测技术已成为当前气体浓度在线检测技术的重要发展方向之一。本文的研究工作主要围绕光学气体检测技术中两种常用的光谱技术（直接吸收光谱（DAS）和扫腔的连续波腔衰荡光谱（S-CRDS））及其应用来展开。1）针对传统DAS中基线拟合误差较大、三角波或锯齿波扫描频率较低的问题，基于波长调制光谱（WMS）谐波分析思想，提出了波长调制-直接吸收光谱（WM-DAS）方法。该方法采用正弦扫描、傅里叶级数复现吸收率函数和基线与吸收率同步拟合的方式，能够有效解决基线拟合误差、扫描频率低等问题，吸收率函数的拟合残差标准差相比传统DAS提高了一个数量级。大气痕量气体CH4和CO2在线监测、CO谱线物理常数标定和平面火焰CO温度测量实验表明该方法与高灵敏的S-CRDS测量结果相一致，并具有测量速度快、稳定性高的优点。2）针对宽量程气体浓度检测需求，提出了宽量程、免标定的气体浓度检测方法。该方法结合WM-DAS和CRDS技术的优点，可测量的CO气体量程跨越4个数量级，并利用WM-DAS测量结果来校准CRDS的基线衰荡时间，实现了基线衰荡时间免标定。3）针对传统S-CRDS中激光波长抖动导致的谱线两翼噪声问题，基于谐波分析与傅里叶变换思想，提出了基于傅里叶变换的波长扫描腔衰荡光谱（FWS-CRDS）。该方法采用波长连续扫描和快速扫腔的方式，利用傅里叶特征频谱重构吸收光谱，能够有效减小谱线两翼处噪声（尤其是周期性噪声）的影响。CO和CO2谱线参数标定结果表明该方法与传统S-CRDS测量结果一致但减小了不确定度，光谱信噪比提升了4倍医生，且平面火焰中OH浓度测量实验表明该方法具有抗干扰、高灵敏、操作简单的优点。本文提出的WM-DAS和FWS-CRDS方法能有效解决传统DAS中基线拟合不确定度较大、扫描频率较低的问题，以及传统S-CRDS中激光波长抖动噪声问题，提升了传统方法的测量灵敏度及稳定性。并基于此提出了宽量程、免标定的气体浓度高灵敏度检测方法，为高灵敏气体检测仪的研发提供了理论和技术支持。
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 4 times 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.
