Distortion correction method of line shape based on harmonic detection

When wavelength modulation spectroscopy is used for gas detection in open space, the second harmonic signal is modified due to the interference of optical noise, uncontrollable environmental noise, etc., resulting in the distortion of the line shape, which affects the measurement accuracy. In this study, the harmonic signal distortion is theoretically analyzed, and a distortion correction method of the line shape is proposed based on harmonic detection. By correcting the zero line offset and line shape distortion, the effect of interference is reduced. Furthermore, a monitoring system based on the harmonic technology is established, and a series of experiments are conducted to validate the efficacy of the proposed method. The experimental results reveal that after background noise elimination and zero line offset correction, the maximum fluctuation of the spectral signal in the non-absorption side of the sidelobe is reduced from 0.093 to 0.055 V on the left side and from −0.111 to −0.032 V on the right side. Furthermore, the standard deviation of the peak height ratio before and after the distortion correction is 0.283 and 0.001, respectively, and the standard deviation of the Euclidean distance is 2.601 and 0.007 pm, respectively. The proposed method is compared with traditional 2f concentration inversion and I2f/I1f methods. The mean values for the three methods are 2.80, 2.80, and 2.83 ppm, respectively. The standard deviation of the proposed method is 0.018 ppm, and the Allan analysis result is 2.65 × 10−8 (1 s). These results verify the accuracy of the proposed method.