点测量激光吸收光谱技术理论分析

陈卫, 伍越, 罗杰, 等. 点测量激光吸收光谱技术理论分析[J]. 光电工程, 2019, 46(10): 180575. doi: 10.12086/oee.2019.180575
引用本文: 陈卫, 伍越, 罗杰, 等. 点测量激光吸收光谱技术理论分析[J]. 光电工程, 2019, 46(10): 180575. doi: 10.12086/oee.2019.180575
Chen Wei, Wu Yue, Luo Jie, et al. Theoretical research of point-measurement laser absorption spectroscopy[J]. Opto-Electronic Engineering, 2019, 46(10): 180575. doi: 10.12086/oee.2019.180575
Citation: Chen Wei, Wu Yue, Luo Jie, et al. Theoretical research of point-measurement laser absorption spectroscopy[J]. Opto-Electronic Engineering, 2019, 46(10): 180575. doi: 10.12086/oee.2019.180575

点测量激光吸收光谱技术理论分析

  • 基金项目:
    国家自然科学基金资助项目(11702312)
详细信息
    作者简介:
    通讯作者: 陈卫, E-mail: chenweikeeping@163.com
  • 中图分类号: O433.1

Theoretical research of point-measurement laser absorption spectroscopy

  • Fund Project: Supported by National Natural Science Foundation of China (11702312)
More Information
  • 点测量吸收光谱技术以饱和吸收为基本原理,可有效克服可调谐二极管激光吸收光谱技术(TDLAS)的线测量缺陷,通过探测光束与一同频的饱和光束交叉来提取交叉位置点处的信息,实现具有毫米级空间分辨能力的点测量。本文对点测量吸收光谱技术进行了详细的理论分析,推导了饱和光束和探测光束在任意交叉角度下的饱和吸收系数,分析了饱和参数对吸收信号的影响。同时提出了一种针对微弱吸收信号的调制方法,推导了饱和光束在高频正弦调制下的探测光一次谐波表达式,并通过数值计算得到了验证。研究还表明,不同阶次谐波信号具有相同的半高宽,并且与无调制时的吸收信号半高宽一致,因此利用多次谐波叠加可进一步提高谱线宽度测量的信噪比。

  • Overview: The tunable diode laser absorption spectroscopy (TDLAS) has been widely applied as a non-intrusive gas detection technology due to its high-sensitivity and convenient operation. However, the 'line-of-sight (LOS)' measurement is the biggest defect of TDLAS, which means that the absorbance signal is the integration along the path from the laser emitter to the receiver. So the directly measured gas temperatures or densities by TDLAS on the non-uniform conditions are not correct. To overcome this defect, the computed tomography (CT) is always employed for reconstructing the 2D spatially distribution. However, the CT combined TDLAS, usually named as TDLAT, needs a lot of laser beams crossing the gas field from different directions, and each beam needs to be received. Sometimes, it is not feasible to deploy so many emitters and receivers, such as the measurement of high-temperature gas in shock wave layer in high-enthalpy wind tunnel. The point measurement laser absorption spectroscopy (PMLAS) studied in this article is another technology to overcome the LOS defect of traditional TDLAS. The PMLAS is the combination of TDLAS and saturated absorption spectroscopy (SAS). It is realized by crossing with two frequency synchronized laser beams: one named probe beam as in traditional TDLAS and the other named saturated beam with higher power. By subtracting the probe beam signal when the saturated beam is off with the probe beam signal when the saturated beam is on, the PMLAS could acquire the absorbance at the crossing point and therefor achieve the 'point' measurement with at least millimeter spatial resolution. In this paper, the theory of PMLAS was firstly analyzed by the theoretical deduction of saturated absorption coefficients with arbitrary cross angles and the numerical calculations of point absorbance under different saturation parameters. Next, a weak signal detection method based on high-frequency sinusoidal modulation of the saturated beam intensity was proposed, in which the first-order harmonic signal was theoretically deduced and verified by numerical demonstration. Furthermore, it is found that the FWHMs (full width at half maximum) of different order harmonics are all the same and equal to the width of the absorption signal without modulation, which implied that the superposition of multi-harmonics could enhance the signal-to-noise ratio (SNR) in measuring the spectrum line-width. The theoretical research of PMLAS including the weak signal demodulation method in this work is the foundation of applying this technology to measure the local temperature and density of gas in dramatic non-uniform and disturbance condition. By the way, due to the very low saturation intensity of the atoms, the PMLAS is easily realized in high-temperature atomic gas measurement.

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  • 图 1  点测量吸收光谱技术原理示意图

    Figure 1.  Schematic diagram of the point measurement absorption spectroscopy

    图 2  一次谐波解调原理示意图

    Figure 2.  The schematic diagram of demodulating the first-order harmonic

    图 3  点测量吸收信号的半高全宽以及幅值相对强度随饱和参数的变化关系

    Figure 3.  FWHM and amplitude curves with different saturation parameters

    图 4  点测量吸收信号半高全宽以及幅值相对强度随光束夹角的变化关系

    Figure 4.  FWHM and amplitude curves with different crossing angles

    图 5  信号解调结果分析。 (a) 原始探测信号与解调的一次谐波信号; (b) 一次谐波积分及其修正与饱和参数的关系

    Figure 5.  The analysis of demodulated signal. (a) The detected signal and its first-order harmonic; (b) Relations of the integration of H1 and its modification with the saturation parameter

    图 6  不同阶次谐波的线宽计算结果。(a) 解调的前五次谐波形式; (b) 前五次谐波叠加后的线宽与温度的关系

    Figure 6.  The linewidths of different order harmonics. (a) The first five harmonics; (b) Relation between FWHM and temperature after multi-harmonics superposition

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出版历程
收稿日期:  2018-11-12
修回日期:  2019-02-21
刊出日期:  2019-10-18

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