光电工程, 2019, 46(10): 180575, 网络出版: 2019-10-01
Theoretical research of point-measurement laser absorption spectroscopy
中国空气动力研究与发展中心超高速空气动力研究所，四川 绵阳 621000
吸收光谱技术 点测量 高空间分辨率 微弱信号检测 谐波 absorption spectroscopy point measurement high spatial resolution weak signal detection harmonic
The point measurement laser absorption spectroscopy (PMLAS) based on saturated absorption theorycould 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 twofrequency synchronized laser beams: one named probe beam as in traditional TDLAS and the other named saturatedbeam with higher power. In this paper, the theory of PMLAS was firstly analyzed by the theoretical deduction ofsaturated absorption coefficients with arbitrary cross angles and the numerical calculations of point absorbanceunder different saturation parameters. Next, a weak signal detection method based on high-frequency sinusoidalmodulation of the saturated beam intensity was proposed, in which the first-order harmonic signal was theoreticallydeduced 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 measuringthe spectrum line-width.