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波长调制-直接吸收光谱(WM-DAS), 同时具有直接吸收光谱(DAS)的免标定、可测量吸收率函数的优点和波长调制光谱(WMS)高信噪比、抗干扰能力强的优点. 本文利用免标定、高信噪比的WM-DAS方法结合长光程Herriott池, 在常压常温条件下, 对大气中CH4 (6046.952 cm–1)和CO2 (6330.821 cm–1)分子两条近红外吸收谱线的吸收率函数进行了测量, 其光谱拟合残差标准差可达到5.6 × 10–5. 随后, 采取WM-DAS方法结合Herriott池, 对大气中CO2和CH4浓度进行了连续监测, 并将其与高灵敏度的连续波腔衰荡光谱(CW-CRDS)测量结果进行比较. 实验结果表明: 本文采用的长光程WM-DAS与CW-CRDS方法测量结果一致, 两组数据线性拟合相关性达到0.99, 其中基于WM-DAS方法的CO2和CH4的检测限分别达到170 ppb和 1.5 ppb, 略高于CW-CRDS检测限, 但其测量速度远高于CW-CRDS, 并且具有系统简单、对环境要求低、可长期稳定运行等优点.
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关键词:
- 波长调制-直接吸收光谱 /
- 腔衰荡光谱 /
- 痕量气体在线检测 /
- 免标定 /
- CH4
Wavelength modulation-direct absorption spectroscopy (WM-DAS) integrates the advantages of measuring absolute absorbance profile from calibration-free direct absorption spectrum (DAS) with the enhanced noise rejection and high sensitivity of wavelength modulation spectrum (WMS). This method can be used to precisely recover the crucial absorbance profile via the extraction of the characteristic frequency of the modulated transmitted light. In this paper, the WM-DAS method with non-calibration and high signal-to-noise ratio is integrated with a Herriott cell (about 128 m). Under the condition of atmospheric pressure and room temperature, the absorptance functions of two spectral lines of CO2 (6330.821 cm–1) and CH4 (6046.964 cm–1) in air are measured, and their standard deviations of spectral fitting residual are 5.6 × 10–5 and 7 × 10–5, respectively. Subsequently, the concentration of CO2 and CH4 in air are monitored on-line by the WM-DAS method integrated with the Herriott cell, and compared with those by the highly sensitive continuous wave cavity ring down spectroscopy (CW-CRDS). The experimental results show that the measured results of the long optical path WM-DAS method are consistent with those by the CW-CRDS method, and the linear correlation between the two methods is above 0.99. The detection limit of CO2 and CH4 by the WM-DAS method are 170 ppb and 1.5 ppb respectively, which are slightly higher than those by the CW-CRDS. However, the measurement speed by WM-DAS is much higher than that by CW-CRDS, and possesses the advantages of simpler operation, lower environmental requirements, long-term stability, etc.-
Keywords:
- wavelength modulation-direct absorption spectroscopy /
- cavity ring down spectroscopy /
- trace gas monitoring /
- calibration free /
- CH4
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图 1 WM-DAS与CW-CRDS的系统原理图(LC, 激光电流和温度控制器; FI, 光纤隔离器; AOM, 声光调制器; APD, 雪崩光电二极管; PD, 光电二极管; DDG, 数字延迟发生器; PZT, 压电换能器; RF, 射频发生器; DAQ, 数据采集系统; WM, 波长计; MFC, 质量流量计)
Fig. 1. System schematic diagram of WM-DAS and CW-CRDS. LC, laser current and temperature controller; FI, fiber isolator; AOM, acousto-optic modulator; APD, avalanche photodiode; PD, photodiode; DDG, digital delay generator; PZT, piezoelectric transducer; RF, radio frequency; DAQ, data acquisition system; WM, wavelength meter; MFC, mass flow controller.
图 2 激光频率标定以及FFT滤波 (a) 干涉仪信号(黑色实线), It (蓝色实线), 测量(黑色实心圆)以及拟合的相对频率(红色实线), 拟合残差(红色空心圆); (b) It的傅里叶系数的实部A (红色)与虚部B (蓝色), 低频噪声(0.35, 0.45 kHz)和高频噪声(499.7, 514.1 kHz)
Fig. 2. Laser wavelength calibration and FFT filtering: (a) Etalon signal (black solid line), It (blue solid circle), measured (black solid circle) and fitted relative frequency (red solid line), fitting residual (red hollow circle); (b) real part and imaginary part of Fourier coefficients of It, and low frequency (0.35, 0.45 kHz) and high frequency (499.7, 514.1 kHz) noise.
图 3 采用WM-DAS方法在298 K, 100.9 kPa下所测的CO2 (红色)和CH4 (蓝色)吸收光谱, 以及Voigt拟合, 用时约1 s (为方便与CW-CRDS比较, 将吸收率转换为吸收系数(cm–1))
Fig. 3. Absorption spectra of CO2 (red) and CH4 (blue) measured by WM-DAS in about 1 s at 298 K and 100.9 kPa, and the best fit of Voigt profile. In order to compare with CW-CRDS, the absorptance is converted to absorption coefficient (cm–1).
图 4 采用CW-CRDS在298 K, 100.9 kPa下测量的CO2 (红色)和CH4 (蓝色)两条谱线(用时约24 min) (a) 衰荡时间与电流的关系; (b) 吸收率函数及其Voigt拟合
Fig. 4. The absorption spectra of CO2 (red) and CH4 (blue) measured by CRDS in about 24 min at 298 K and 100.9 kPa: (a) The relationship between the ring down time and the current; (b) the absorption function and the best fits of Voigt profile.
图 5 (a) 两种方法的CH4量程对比; (b)两种方法在不同CH4浓度下的直方图
Fig. 5. (a) Comparison of measuring range of CH4 between the two methods; (b) histograms of two methods at different concentrations of CH4.
图 6 WM-DAS与CW-CRDS方法联合测量的时序图、激光电流(黑色实线)、透射光强It (蓝色实线)、衰荡时间(红色实线)
Fig. 6. Time sequence diagram of WM-DAS and CW-CRDS, laser current (black solid line), transmitted light It (blue solid line), ring down time (red solid line).
图 7 (a) 两种方法测量的大气中CO2浓度; (b) 两种方法测量数据的线性拟合
Fig. 7. (a) CO2 in atmosphere measured by the two methods; (b) linear fitting of the data measured by the two methods.
图 8 (a) 两种方法测量的大气中CH4浓度; (b) 两种方法测量数据的线性拟合
Fig. 8. (a) CH4 in atmosphere measured by the two methods; (b) linear fitting of the data measured by the two methods.
图 9 两种方法测量的Allan方差 (a) CO2; (b) CH4
Fig. 9. Allan variance measured by the two methods: (a) CO2; (b) CH4.
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[1] Adámek P, Olejníček J, Čada M, Kment Š, Hubička Z 2013 Opt. Lett. 38 2428
Google Scholar
[2] Goldenstein C S, Spearrin R M, Jeffries J B, Hanson R K 2016 Prog. Energ. Combust. 60 132
Google Scholar
[3] Witzel O, Klein A, Meffert C, Schulz C, Kaiser S A, Ebert V 2015 Proc. Combust. Inst. 35 3653
Google Scholar
[4] McManus J B, Zahniser M S, Nelson D D, Shorter J H, Herndon S, Wood E 2010 Opt. Eng. 49 111124
Google Scholar
[5] McManus J B, Zahniser M S, Nelson D D, Shorter J H, Herndon S C, Jervis D, Agnese M, McGovern R, Yacovitch T I, Roscioli J R 2015 Appl. Phys. B 119 203
Google Scholar
[6] Zhao G, Tan W, Jia M, Hou J, Ma W, Dong L, Zhang L, Feng X, Wu X, Yin W, Xiao L, Axner O, Jia S 2016 Sensors 16 1544
Google Scholar
[7] Pogány A, Klein A, Ebert V 2015 J. Quant. Spectrosc. Radiat. Transfer 165 108
Google Scholar
[8] Witzel O, Klein A, Meffert C, Wagner S, Kaiser S, Schulz C, Ebert V 2013 Opt. Express 21 19951
Google Scholar
[9] Hanson R K 2011 Proc. Combust. Inst. 33 1
Google Scholar
[10] Kostinek J, Roiger A, Davis K J, Sweeney C, DiGangi J P, Choi Y, Baier B, Hase F, Groß J, Eckl M, Klausner T, Butz A 2019 Atmos. Meas. Tech. 12 1767
Google Scholar
[11] Pal M, Maity A, Pradhan M 2018 Laser Phys. 28 105702
Google Scholar
[12] Maity A, Pal M, Banik1 G D, Maithani S, Pradhan M 2017 Laser Phys. Lett. 14 115701
Google Scholar
[13] Bayrakli I 2018 Appl. Opt. 57 4039
Google Scholar
[14] McHale L E, Martinez B, Miller T W, Yalin A P 2019 Opt. Express 27 20084
Google Scholar
[15] 孙丽琴, 陈兵, 阚瑞峰, 李明星, 姚路, 魏敏, 何亚柏 2015 光学学报 35 0930002
Google Scholar
Sun L Q, Chen B, Kan R F, Li M X, Yao L, Wei M, He Y B 2015 Acta Opt. Sin. 35 0930002
Google Scholar
[16] Coburn S, Alden C B, Wright R, Cossel K, Baumann E, Truong G, Giorgetta F, Sweeney C, Newbury N R, Prasad K, Coddington I, Rieker G B 2018 Optica 5 320
Google Scholar
[17] Fdil K, Michaud-Belleau V, Hébert N B, Guay P, Fleisher A J, Deschênes J D, Genest J 2019 Opt. Lett. 44 4415
Google Scholar
[18] Du Y J, Peng Z M, Ding Y J 2018 Opt. Express 26 9263
Google Scholar
[19] Sun K, Chao X, Sur R, Jeffries J B, Hanson R K 2013 Appl. Phys. B 110 497
Google Scholar
[20] Peng Z M, Ding Y J, Jia J W, Lan L J, Du Y J, Li Z 2013 Opt. Express 21 23724
Google Scholar
[21] Lan L J, Ding Y J, Peng Z M, Du Y J, Liu Y F 2014 Appl. Phys. B 117 1211
Google Scholar
[22] Li J D, Du Y J, Peng Z M, Ding Y J 2019 J. Quant. Spectrosc. Radiat. Transfer. 224 197
Google Scholar
[23] Mazurenka M, Wada R, Shillings A J L, Butler T J A, Beames J M, Orr-Ewing A J 2005 Appl. Phys. B 81 135
Google Scholar
[24] Halmer D, Basum G, Hering P, Mürtz M 2004 Rev. Sci. Instrum. 75 2187
Google Scholar
[25] Du Y J, Peng Z M, Ding Y J 2018 Opt. Express 26 29550
Google Scholar
[26] Gordon I E, Rothman L S, Hill C, et al. 2017 J. Quant. Spectrosc. Radiat. Transfer 203 3
Google Scholar
[27] Morville J, Romanini D, Kachanov A A, Chenevier M 2004 Appl. Phys. B 78 465
Google Scholar
[28] Zimnoch M, Necki J, Chmura L, Jasek A, Jelen D, Galkowski M, Kuc T, Gorczyca Z, Bartyzel J, Rozanski K 2019 Mitig. Adapt. Strat. Gl. 24 1051
Google Scholar
[29] Savi F, Bene C D, Canfora L, Mondini C, Fares S 2016 Agr. Forest Meteorol. 226 67
Google Scholar
[30] Tang J, Li B C, Wang J 2019 Atmos. Meas. Tech. 12 2851
Google Scholar
[31] Allan D W 1966 Proc. IEEE 54 221
Google Scholar
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