基于TDLAS的气体检测技术算法
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摘要
可调谐二极管激光吸收光谱(Tunable Diode Laser Absorption Spectroscopy, TDLAS)技术是一种具有高灵敏度、高分辨率的气体吸收光谱检测技术,具有响应快、精度高、单模特性优秀、通用性强等优势。TDLAS直接吸收法通过测量绝对吸收强度来计算待测气体温度和浓度,但容易受到颗粒物浓度、激光强度波动等影响。TDLAS波长调制法采用高频正弦信号对激光器进行调制,使得激光输出频率和强度同时受到调制,具有高信噪比和灵敏度的特点,但是需要通过标定实验或复杂的算法来确定气体参数。因此,通过吸收光谱理论和波长调制理论,推导出蕴含分子吸收信息的谐波通项表达式,并在此基础上分析谐波信号与待测气体绝对吸收强度之间的关系,建立了一种基于谐波信号的绝对吸收强度测量算法。以NH_3分子在1 531 nm附近的谱线为例进行数值分析,发现调制幅度达到a=0.032 cm~(-1)(调制系数m=2)时,仿真结果与理论计算结果(a=0)相对误差不超过2%,进一步验证了算法的可靠性与准确性。
Tunable diode laser absorption spectroscopy(TDLAS) is a high sensitive and high resolution gas absorption spectrum detection technology, with fast response, high precision, excellent single mode characteristics and strong versatility. The temperature and concentration of the detected gas were calculated by measuring the absolute absorption intensity in the direct absorption method of TDLAS, but was easily affected by particle concentration and fluctuation of laser intensity. The laser was modulated through high frequency sinusoidal signal in the wavelength modulation method of TDLAS, making the laser output frequency and intensity modulated simultaneously, with the characteristics of high signal-to-noise ratio and sensitivity, but the gas parameters were determined by calibration experiments or complex algorithms. Therefore, the expression of harmonic terms containing molecular absorption information was deduced through the theory of absorption spectrum and wavelength modulation, and the relationship between harmonic signal and absolute absorption intensity of detected gas was analysed, establishing the measurement algorithm of absolute absorption intensity based on harmonic signal. The absorption spectral line of NH_3 molecule near 1 531 nm was taken as an example for numerical analysis, and the relative errors between theoretical calculation(a =0) and simulation values were no more than 2% when the modulation amplitude reached up to a=0.032 cm~(-1)(m=2), which further proved the reliability of on-line measurement algorithm.
Tunable diode laser absorption spectroscopy(TDLAS) is a high sensitive and high resolution gas absorption spectrum detection technology, with fast response, high precision, excellent single mode characteristics and strong versatility. The temperature and concentration of the detected gas were calculated by measuring the absolute absorption intensity in the direct absorption method of TDLAS, but was easily affected by particle concentration and fluctuation of laser intensity. The laser was modulated through high frequency sinusoidal signal in the wavelength modulation method of TDLAS, making the laser output frequency and intensity modulated simultaneously, with the characteristics of high signal-to-noise ratio and sensitivity, but the gas parameters were determined by calibration experiments or complex algorithms. Therefore, the expression of harmonic terms containing molecular absorption information was deduced through the theory of absorption spectrum and wavelength modulation, and the relationship between harmonic signal and absolute absorption intensity of detected gas was analysed, establishing the measurement algorithm of absolute absorption intensity based on harmonic signal. The absorption spectral line of NH_3 molecule near 1 531 nm was taken as an example for numerical analysis, and the relative errors between theoretical calculation(a =0) and simulation values were no more than 2% when the modulation amplitude reached up to a=0.032 cm~(-1)(m=2), which further proved the reliability of on-line measurement algorithm.
引文
[1] Reid J, Labrie D. Second-harmonic detection with tunable diode lasers-Comparison of experiment and theory[J].Applied Physics B, 1981, 26(3):203-210.
[2] Wang Xuemei, Liu Shi. CO2concentration measurement based on TDLAS[J]. Control and Instruments in Chemical Industry, 2016, 43(11):1158-1161.(in Chinese)
[3] Yuan Zhiguo, Yang Xiaotao, Xie Wenqiang, et al. Research on the online test of diesel NOxemission by TDLAS[J].Spectroscopy and Spectral Analysis, 2018, 38(1):194-199.(in Chinese)
[4] Li Zhe, Zhang Zhirong, Sun Pengshuai, et al. Multi-point full range monitoring of methane based on TDLAS technology[J]. Infrared and Laser Engineering, 2017, 46(9):0917009.(in Chinese)
[5] Xu Lijun, Liu Chang, Jing Wenyang, et al. Tunable diode laser absorption spectroscopy-based tomography system for on-line monitoring of two-dimensional distributions of temperature and H2O mole fraction[J]. Review of Scientific Instruments, 2016, 87:013101.
[6] Li Meng, Guo Jinjia, Ye Wangquan, et al. Study on TDLASsystem with a miniature multi-pass cavity for CO2measurements[J]. Spectroscopy and Spectral Analysis, 2018,38(3):697-701.(in Chinese)
[7] Liu J T C, Jeffries J B, Hanson R K. Wavelength modulation absorption spectroscopy with 2f detection using multiplexed diode lasers for rapid temperature measurements in gaseous flows[J]. Applied Physics B, 2004, 78:503-511.
[8] Sun Lingfang, Yu Hong. Online multi-component gas concentration detection system based on TDLAS technology[J]. Instrument Technique and Sensor, 2017(3):73-77.(in Chinese)
[9] He Xianglin, Pan Yonggang. The development status and application research of TDLAS technology[J]. Journal of Ezhou University, 2017, 24(1):102-104.(in Chinese)
[10] Farooq A, Jeffries J B, Hanson R K. Sensitive detection of temperature behind reflected shock waves using wavelength modulation spectroscopy of CO2near 2.7μm[J]. Applied Physics B, 2009, 96(1):161-173.
[11] Lan L J, Ding Y J, Peng Z M, et al. Calibration-free wavelength modulation for gas sensing in tunable diode laser absorption spectroscopy[J]. Applied Physics B, 2014, 117(4):1211-1219.
[12] Stewart G, Johnstone W, Bain J R P, et al. Recovery of absolute gas absorption line shapes using tunable diode laser spectroscopy with wavelength modulation-part 1:theoretical analysis[J]. Journal of Lightwave Technology, 2011, 29(6):811-821.
[13] Liu Y, Lin J, Huang G, et al. Simple empirical analytical approximation to the Voigt profile[J]. Opt Soc Am B, 2001,18(5):666-672.
[14] Lan Lijuan, Ding Yanjun, Jia Junwei, et al. Theoretical and experimental study of measuring gas temperature in vacuum environment using tunable diode laser absorption spectroscopy[J]. Acta Physica Sinica, 2014, 63(8):97-105.(in Chinese)
[2] Wang Xuemei, Liu Shi. CO2concentration measurement based on TDLAS[J]. Control and Instruments in Chemical Industry, 2016, 43(11):1158-1161.(in Chinese)
[3] Yuan Zhiguo, Yang Xiaotao, Xie Wenqiang, et al. Research on the online test of diesel NOxemission by TDLAS[J].Spectroscopy and Spectral Analysis, 2018, 38(1):194-199.(in Chinese)
[4] Li Zhe, Zhang Zhirong, Sun Pengshuai, et al. Multi-point full range monitoring of methane based on TDLAS technology[J]. Infrared and Laser Engineering, 2017, 46(9):0917009.(in Chinese)
[5] Xu Lijun, Liu Chang, Jing Wenyang, et al. Tunable diode laser absorption spectroscopy-based tomography system for on-line monitoring of two-dimensional distributions of temperature and H2O mole fraction[J]. Review of Scientific Instruments, 2016, 87:013101.
[6] Li Meng, Guo Jinjia, Ye Wangquan, et al. Study on TDLASsystem with a miniature multi-pass cavity for CO2measurements[J]. Spectroscopy and Spectral Analysis, 2018,38(3):697-701.(in Chinese)
[7] Liu J T C, Jeffries J B, Hanson R K. Wavelength modulation absorption spectroscopy with 2f detection using multiplexed diode lasers for rapid temperature measurements in gaseous flows[J]. Applied Physics B, 2004, 78:503-511.
[8] Sun Lingfang, Yu Hong. Online multi-component gas concentration detection system based on TDLAS technology[J]. Instrument Technique and Sensor, 2017(3):73-77.(in Chinese)
[9] He Xianglin, Pan Yonggang. The development status and application research of TDLAS technology[J]. Journal of Ezhou University, 2017, 24(1):102-104.(in Chinese)
[10] Farooq A, Jeffries J B, Hanson R K. Sensitive detection of temperature behind reflected shock waves using wavelength modulation spectroscopy of CO2near 2.7μm[J]. Applied Physics B, 2009, 96(1):161-173.
[11] Lan L J, Ding Y J, Peng Z M, et al. Calibration-free wavelength modulation for gas sensing in tunable diode laser absorption spectroscopy[J]. Applied Physics B, 2014, 117(4):1211-1219.
[12] Stewart G, Johnstone W, Bain J R P, et al. Recovery of absolute gas absorption line shapes using tunable diode laser spectroscopy with wavelength modulation-part 1:theoretical analysis[J]. Journal of Lightwave Technology, 2011, 29(6):811-821.
[13] Liu Y, Lin J, Huang G, et al. Simple empirical analytical approximation to the Voigt profile[J]. Opt Soc Am B, 2001,18(5):666-672.
[14] Lan Lijuan, Ding Yanjun, Jia Junwei, et al. Theoretical and experimental study of measuring gas temperature in vacuum environment using tunable diode laser absorption spectroscopy[J]. Acta Physica Sinica, 2014, 63(8):97-105.(in Chinese)
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