Abstract: Direct absorption spectrum (DAS) can be used to measure the molecular absorptivity function and determine the spectral parameters of the gas by fitting the measured absorptivity function. Wavelength modulation-direct absorption spectroscopy (WM-DAS) is based on DAS and combines with the idea of harmonic analysis in wavelength modulation spectrum (WMS). The measurement accuracy of absorptivity function can be effectively improved by using Fourier transform. In this paper, the absorptivity function of CO R5–R11 near infrared weak absorption line at 1567 nm is accurately reproduced by using the WM-DAS method combined with long optical path gas absorption cell at room temperature and low pressure. The standard deviation of the fitting residual reaches 3 × 10–5, and then the spectral parameters such as collision broadening, Dicke narrowing and speed-dependent collision broadening coefficients are measured in high precision. These parameters are compared with the results from the high sensitivity continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results show that the signal-to-noise ratio of the absorptivity function measured by CW-CRDS is about 2.5 times that of the long-path WM-DAS, and the measured spectral parameters are highly consistent. The relative errors of the collision broadening coefficients obtained by using the Voigt profiles of the two methods are less than 1%. At the same time, the detection limit of CO at 1567 nm based on the WM-DAS method is about 80 ppb, and the corresponding absorption coefficient is 2 × 10–10 cm–1, which is slightly higher than that from the CW-CRDS method. However, the WM-DAS method has the advantages of fast measurement speed, simple system and low cost, and is expected to provide a new method of measuring the weak absorption lines. Keywords:wavelength modulation-direct absorption spectroscopy/ cavity ring down spectroscopy/ absorptivity function/ collision broadening coefficient/ Dicke narrowing coefficient
4.实验结果与分析实验中采用WM-DAS对CO分子1567 nm附近的R5—R11谱线进行了测量. Herriott池有效光程120 m, 气体温度、压力和CO浓度分别为288 K, 18 kPa和0.1% (背景气N2). 激光扫描频率、扫描范围分别为1 kHz, 0.4 cm–1, 单次实验共采集100个正弦波周期(用时0.1 s), 同时采集相应的干涉仪信号(?)进行激光波长标定, 如图2所示, 其中蓝色曲线为透射光强信号, 红色曲线为波长标定结果. 蕴含气体吸收率函数信息的透射光强信号傅里叶系数为Ak和Bk, 将Ak和Bk及通过干涉仪标定的激光波长系数a1, a2和η等参数代入(1)—(3)式中即可复现吸收率函数. 图 2 测量的100个正弦波周期的激光光强及激光相对波长标定结果(FSR为自由光谱范围), 以及蕴含气体吸收率函数信息的透射光强傅里叶变换(FFT)系数 Figure2. Measured transmitted intensities of 100 periods of sinusoidal waves and fitted frequency (FSR, free spectral range), and fast Fourier transform (FFT) coefficients Ak and Bk of transmitted light intensity.
图3(a)展示了WM-DAS方法对R10谱线的测量结果, 为便于与CW-CRDS比较, 将WM-DAS所测吸收率(α)转换为吸收系数(κ). CW-CRDS方法采用步进式扫描激光波长, 间隔约0.002 cm–1, 扫描范围约0.4 cm–1, 共扫描100次, 用时约20 min. 图3(b)展示了CW-CRDS测量结果, 由于测量的吸收系数仅与衰荡时间相关而与光强无关[24-26], 残差波动小, 测量数据更加平滑, 可以清晰看出VP拟合残差中“w”形的精细结构, 其原因在于VP未考虑Dicke收敛效应[31,32], 观察到该精细结构也说明了CW-CRDS具有很高的灵敏度. 与此相比, WM-DAS通过提取周期性正弦信号的整数倍特征频谱来复现气体吸收光谱, 可有效减小或消除振动、电磁等多种噪声干扰[12,13], VP拟合残差中也可清晰地观察到“w”形的精细结构, 这验证了本文WM-DAS测量结果与CW-CRDS相一致. 与VP不同, RP考虑了Dicke收敛效应, 拟合时可以消除“w”形残差. 从RP拟合结果可知, WM-DAS和CW-CRDS两种方法测得的吸收率函数拟合残差标准差相差约2.5倍, 相应的SNR(1σ)相差约2.5倍, 但WM-DAS的测量时间(0.1 s)远小于CW-CRDS (20 min), 在大量的谱线参数标定以及工业现场的气体快速监测中更有优势. 图 3 测量的CO光谱及其最佳Voigt和Rautian线型拟合结果(XCO, c, τ和σSD分别为CO浓度、光速、衰荡时间和残差的标准差) (a) WM-DAS; (b) CW-CRDS Figure3. Measured absorption function of CO and the best fits of Voigt and Rautian profile (XCO, c, τ, and σSD represent the CO concentration, light velocity, ring down time and standard deviation of the residual, respectively): (a) WM-DAS; (b) CW-CRDS.