点测量激光吸收光谱技术理论分析

Theoretical research of point-measurement laser absorption spectroscopy

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。

The point measurement laser absorption spectroscopy (PMLAS) based on saturated absorption theory could surpass the defect of 'line-of-sight' measurement in traditional tunable diode laser absorption spectroscopy (TDLAS) and achieve the 'point' measurement with millimeter spatial resolution. 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. 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.