Measurement of absorption line profile function based on wavelength modulation technology
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摘要: 在可调谐二极管激光吸收光谱技术中，直接吸收法通过对透射光强拟合可直接得到线型函数，但目前具有较高信噪比的波长调制法不能对其进行有效测量。提出一种基于多次谐波的线型函数测量理论及方法，通过谐波通项表达式推导出谱线中心频率处二次与四次谐波比值仅与线型函数和调制系数有关，当调制系数m为2.492 8时，无论线型函数中Gauss线宽和Lorentz线宽所占比例如何，二次与四次谐波比值均为2.186 2，根据该点的特征可首先得到谱线半宽，然后将其应用于弱吸收条件下2f/1f气体浓度免标法测量。实验中采用2 326.82 nm处谱线对CO浓度进行了测量，其结果与传统直接吸收法测量结果误差小于2%。文中为波长调制法中线型函数的精确测量提供了重要的理论依据，进一步完善了2f/1f免标法。Abstract: In tunable diode laser absorption spectroscopy technology, the direct absorption method can directly obtain absorption line profile function through fitting the transmitting light intensity. But at present, the wavelength modulation method with higher signal-to-noise ratio can't measure it effectively. A measurement theory and method of absorption line profile function based on multiple harmonics was proposed. Through the analysis of harmonic expressions, the ratio between the 2nd harmonic and 4th harmonic at the center frequency was only related to the absorption line profile function and modulation index. When the modulation index m was 2.492 8, the ratio between the 2nd harmonic and 4th harmonic was fixed to 2.186 2 no matter what the ratio of Gauss linewidth and Lorentz linewidth was. According to the characteristics of this point, the spectral line half width can be obtained firstly, and then it can be applied in 2f/1f calibration-free measurement of gas concentration. In the experiment, CO concentration has been measured using 2 326.82 nm spectral line through the proposed method. The error between the proposed method and traditional direct absorption method was less than 2%. An important theoretical basis for the measurement of absorption line profile function in wavelength modulation method has been provided by this proposed method, and the traditional theory of 2f/1f calibration-free method has been further improved.
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