中红外谐波检测背景信号漂移的修正方法

可调谐半导体激光吸收光谱（TDLAS）由于具有高灵敏度、高分辨率、非侵入及实时检测等特点，被广泛应用于燃烧诊断、痕量气体监测、工业过程控制等领域中。波长调制光谱（WMS）的二次谐波（2f）检测是最常用的TDLAS气体传感方法之一。激光器作为TDLAS-WMS在线检测系统中最核心的部件之一，在长期运行过程中会由于其工作温度等因素变化，引起输出激光波长漂移和2f背景信号基线变化，从而导致气体浓度反演的精确度和TDLAS-WMS在线检测系统的稳定性降低。针对上述问题，根据NO气体分子在中红外波段5.176~5.189 μm的基频吸收特性，选择峰值发射波长位于5.184 μm的分布反馈式连续波量子级联激光器（DFB-CW QCL），分析了输出激光中心波长对应的峰值采样点位置随采样时间变化的漂移规律和2f吸收及其背景信号的漂移特性。基于上述分析，提出了以2f信号平均峰峰值替代2f信号峰值建立气体浓度反演模型以修正2f背景信号基线漂移，并结合以信噪比最优为2f背景信号波长漂移修正原则的2f背景信号漂移综合修正方法，以消除TDLAS-WMS在线检测系统长期连续检测过程中2f背景信号漂移对气体浓度反演结果的不利影响。研究结果表明，2f信号平均峰峰值随配置的NO样气浓度的增加而增大，这两者呈现较好的线性关系，其拟合曲线的线性拟合度R2达到了0.999 9。在使用体积浓度为20×10-6 NO气体样品开展的连续60 min监测实验中，波长漂移修正后，反演浓度的标准偏差由波长漂移修正前的0.19×10-6下降到了0.07×10-6，反演浓度的最大相对误差由波长漂移修正前的6.30%下降到了3.85%，相对误差均方值由波长漂移修正前的24.39%下降到了9.99%。结果显示，该2f背景信号漂移综合修正方法可以有效地抑制2f背景信号漂移对气体浓度反演结果的影响，显著提高了TDLAS-WMS在线检测系统连续监测的灵敏度、精确度和稳定性。

Research on Correction Method of Background Signal Drift in Mid-Infrared Harmonic Detection

Tunable Diode Laser Absorption Spectroscopy(TDLAS)is widely used in the combustion diagnosis,measuring trace gases and industrial process control and so on due to its high sensitivity,high resolution,non-intrusive and real-time detection.The second-harmonic(2f)detection technology of wavelength modulation spectroscopy(WMS)is one of the main methods for TDLAS gas sensing.The laser is one of the core components of TDLAS-WMS online detection system and its output wavelength and baseline of 2f background signal will drift due to changes of the laser’s operating temperature and other factors during long-term operation,resulting in reduced accuracy of gas concentration inversion and stability of TDLAS-WMS online detection system.Aiming at the above problems,according to the fundamental absorption properties of NO gas molecules in the mid-infrared band of 5.176~5.189μm,a distributed feedback,continuous-wave quantum cascade laser(DFB-CW QCL)with the peak wavelength of 5.184μm will be chosen to analyze the drift variation of the laser center output wavelength corresponding to the sampling points and the characteristics of the 2f absorption and background signal drift.Based on the above analysis,the average peak-to-peak value of 2f signal is used to replace the peak value of 2f signal to establish the gas concentration inversion model to correct baseline drift of 2f background signal as well as combining with the signal-to-noise ratio is optimized as the principle of wavelength drift correction,and the 2f background signal drift correction method is finally established to eliminate the adverse effect of the 2f background signal drift on the gas concentration inversion results in the long-term continuous detection of TDLAS-WMS online detection system.The results showed that the average peak-to-peak value of the 2f signal increased with the concentration of NO gas.The average peak-to-peak value of the 2f signal and the concentration of the NO sample showed a good linear relationship and linear fit R2 of the fitted curve reached 0.9999.In the 60 min continuous monitoring experiment which was conducted by using NO sample with a volume concentration of 20×10^-6,the standard deviation,the maximum relative error and the relative error mean square value of the inversion concentration decreased from 0.19×10^-6 to 0.07×10^-6,decreased from 6.30%to 3.85%and decreased from 24.39%to 9.99%after the drift correction,respectively.The method can effectively suppress the influence of 2f background signal drift on the gas concentration inversion result,and significantly improve the sensitivity,accuracy and stability of the TDLAS-WMS online detection system continuous monitoring.