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In continuous-wave cavity ring-down spectroscopy (CW-CRDS), the measurement sensitivity is seriously affected by the multimode excitations in the ring-down cavity. The using of an intracavity aperture is a common way to restrain the excitation of high-order modes, thus leading the laser power to additionally lose and the signal-to-noise ratio to degrade. In this paper, two numerical methods, named “trigger threshold method” and “curve fitness method”, are proposed for selecting the mode in which the decays excited by the high-order modes can be removed. The laser coupling efficiency between the incident laser and the oscillating fundamental or high-order modes is studied in a misadjusted ring-down cavity. It is found that with a misadjusted ring-down cavity, the laser energy is partially coupled into the high-order modes, and the coupling energy increases with the extent of the cavity misadjustment increasing. In this case, the ring-down decaying traces excited by these high-order modes are different from and much shorter than those excited by the fundamental mode, which are respectively called “bad decays” and “good decays” in this paper. Both the fundamental mode and the high-order modes can reach the threshold in the case of low triggering threshold selection and result in the components of both good and bad decays in the output ring-down curves. When the trigger threshold rises, the bad decays are effectively restrained by the deficient coupling into the high-order modes. Thus raising the trigger threshold is an effective method to restrain bad decays for the mode selection. Another approach is to consider the time spent on turning off the laser injection since the fitting goodness of good decays is better than that of bad decays. In this paper this characteristic is also used to separate the good decays from the bad ones. These two methods are demonstrated in the CW-CRDS experiments. The results show that the sensitivity of the CW-CRDS instrument can be greatly improved by one order of magnitude in the trigger threshold method with the minimum of Allan deviations gradually approaching to a constant, while the acquisition rate of the ring-down decays slows down with the increase of the trigger threshold. The results also explain the relationship between single sampling and averaged sampling, which presents an answer to the question about the sequence choice between averaging and fitting. A numerical model is proposed to estimate the probability of good decays versus the trigger threshold, which can be used to choose appropriate trigger threshold for CW-CRDS experiment. The applicable conditions and the limitations of these two methods in CW-CRDS for trace gas detection are also discussed in the paper.
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Keywords:
- cavity ring-down spectroscopy /
- cavity misadjustment /
- multimode decay /
- measurement sensitivity
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Hu C D, Yan J Y, Wang Y, Liang L Z 2018 Spectrosc. Spect. Anal. 38 346
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Tan Z Q, Wang Z G, Long X W 2007 Acta Photon. Sin. 36 60
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图 1 腔误调时高阶模功率耦合占比情况
Figure 1. Proportion of higher-order cavity mode excitement in a misadjusted CRDS system.
图 2 受有限的关断时间影响的衰荡曲线仿真
Figure 2. Simulation of ring-down curves affected by the finite shutdown time.
图 3 CW-CRDS实验装置图
Figure 3. Schematic of CW-CRDS experimental instrument.
图 4 四次平均模式下的(a)衰荡曲线、(b), (c)衰荡时间及(d)拟合度分布
Figure 4. (a) Typical decays, (b), (c) distributions of ring-down time, and (d) decay curve fitness in four times averaged mode sampling.
图 5 衰荡腔内存在多模激发
Figure 5. Multimode excitation in ring-down cavity.
图 6 单次采样的典型衰荡曲线
Figure 6. Typical decays in single sampling.
图 7 优衰荡出现频率随触发阈值的变化
Figure 7. Variance of the frequency of good decays with trigger thresholds.
图 8 不同阈值单次采样下的Allan方差图 (a) 10.7 mV; (b) 14.7 mV; (c) 18.7 mV; (d) 22.7 mV
Figure 8. Allan deviations in single sampling with different trigger thresholds: (a) 10.7 mV; (b) 14.7 mV; (c) 18.7 mV; (d) 22.7 mV
图 9 不同阈值下40个衰荡过程的Allan方差
Figure 9. Allan deviations of 40 decays with different trigger thresholds.
图 10 不同阈值的噪声等效吸收系数及测量灵敏度
Figure 10. Noise equivalent absorption coefficients and sensitivities under different trigger thresholds.
图 11 单次采样模式下的衰荡时间(a)及曲线调整拟合度(b)分布
Figure 11. Distributions of ring-down times (a) and curve fitness (b) in single sampling.
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[1] McCarren D, Scime E 2015 Rev. Sci. Instrum. 86 103505
Google Scholar
[2] 胡纯栋, 焉镜洋, 王艳, 梁立振 2018 光谱学与光谱分析 38 346
Hu C D, Yan J Y, Wang Y, Liang L Z 2018 Spectrosc. Spect. Anal. 38 346
[3] Liu Y S, Zhou L X, Tans P P, Zang K P, Cheng S Y 2018 Sci. Total Environ. 633 1022
Google Scholar
[4] Miles N L, Martins D K, Richardson S J, Rella C W, Arata C, Lauvaux T, Davis K J, Barkley Z R, McKain K, Sweeney C 2018 Atmos. Meas. Tech. 11 1273
[5] Li Z Y, Hu R Z, Xie P H, Chen H, Wu S Y, Wang F Y, Wang Y H, Ling L Y, Liu J G, Liu W Q 2018 Opt. Express 26 A433
Google Scholar
[6] Tan Y, Wang J, Zhao X Q, Liu A W, Hu S M 2017 J. Quant. Spectosc. Radiat. Transf. 187 274
Google Scholar
[7] Chen J, Hua T P, Tao L G, Sun Y R, Liu A W, Hu S M 2018 J. Quant. Spectosc. Radiat. Transf. 205 91
Google Scholar
[8] Cui H, Li B C, Han Y L, Wang J, Gao C M, Wang Y F 2017 Chin. Opt. Lett. 15 053101
Google Scholar
[9] Cui H, Li B C, Xiao S L, Han Y L, Wang J, Gao C M, Wang Y F 2017 Opt. Express 25 5807
Google Scholar
[10] Wahl E H, Tan S M, Koulikov S, Kharlamov B, Rella C R, Crosson E R, Paldus B A 2006 Opt. Express 14 1673
Google Scholar
[11] Karpf A, Qiao Y, Rao G N 2016 Appl. Opt. 55 4497
Google Scholar
[12] Berden G, Peeters R, Meijer G 2000 Int. Rev. Phys. Chem. 19 565
Google Scholar
[13] O’Keefe A, Deacon D A G 1988 Rev. Sci. Instrum. 59 2544
Google Scholar
[14] Romanini D, Kachanov A A, Stoeckel F 1997 Chem. Phys. Lett. 270 538
Google Scholar
[15] Romanini D, Kachanov A A, Sadeghi N, Stoeckel F 1997 Chem. Phys. Lett. 264 316
Google Scholar
[16] Huang H F, Lehmann K K 2007 Opt. Express 15 8745
[17] 崔立红, 赵维宁, 颜昌翔 2015 64 224211
Cui L H, Zhao W N, Yan C X 2015 Acta Phys. Sin. 64 224211
[18] Lehmann K K, Huang H F 2009 Frontiers of Molecular Spectroscopy (Amsterdam: Elsevier Science) p638
[19] Grand Y L, Taché J P, Floch A L 1990 J. Opt. Soc. Am. B 7 1251
[20] Sayeh M R, Bilger H R, Habib T 1985 Appl. Opt. 24 3756
Google Scholar
[21] Wójtewicz S, Cygan A, Domyslawska J, Bielska K, Morzyński P, Maslowski P, Ciurylo R, Lisak D 2018 Opt. Express 26 5644
Google Scholar
[22] 谭中奇, 汪之国, 龙兴武 2007 光子学报 36 60
Tan Z Q, Wang Z G, Long X W 2007 Acta Photon. Sin. 36 60
[23] Wang J D, Yu J, Mo Z Q, He J G, Dai S J, Meng J J, Liu Y, Zhang X, Yi H 2019 Appl. Opt. 58 2773
Google Scholar
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