TDLAS波长调制法中调制深度与高次谐波中心幅值关系的研究

可调谐半导体激光光谱技术（TDLAS）是近年来发展十分迅速的光谱检测技术，相较于其他光谱检测技术，它具有高灵敏度、高分辨率、实时监测、便携性好、小型化等优点，在工业环保、医疗检测、气象监测等领域得到了广泛的应用。TDLAS波长调制法中谐波信号易受气压影响，经研究发现气压的影响是调制深度对谐波信号的影响，基于TDLAS技术谐波法的原理，研究了各次谐波与调制深度的关系，通过计算四次谐波与二次谐波中心幅值比，利用调制深度函数推算当前气压环境的调制深度，调整调制频率幅度，使得调制深度接近各次谐波最佳调制深度值，使谐波信号信噪比最佳，提高检测精度。实验通过国瑞智GRZ5031湿度发生器产生固定为1 000 ppm的水汽，调节气阀控制密封箱内不同的气压环境，采用TDLAS水汽检测系统获得了10.2~177.9 kPa气压条件下的二次谐波和四次谐波信号，并进行了仿真与实验分析。仿真结果显示：四次谐波与二次谐波中心幅值比的理论值和仿真值最大相对误差为-1.44%，调制深度的理论值与仿真值最大相对误差为1.78%，说明了仿真下的调制深度函数曲线与理论一致。实验结果显示：根据调制深度函数推算调制深度值，当m=2.226 7时，实测的二次谐波中心频率幅值达到最大值，当m=4.061 0时，实测的四次谐波中心频率幅值达到最大值，与理论结果一致；在30.2 kPa<p<177.9 kPa时，调制深度与气压乘积mp值相对误差较小，最大相对误差不超过±3.2%，说明了此气压条件下的mp值波动不大，通过调制深度函数推算的调制深度值与实际值近似，验证了调制深度函数理论的准确性。

Research on the Relationship Between Modulation Depth and Center of High Order Harmonic in TDLAS Wavelength Modulation Method

Tunable Semiconductor Laser Spectroscopy (TDLAS) has been a rapidly developing spectral detection technology in recent years. Compared with other spectral detection technologies, TDLAS has the advantages of high sensitivity, high resolution, real-time monitoring, good portability and miniaturization, and has been widely used in the fields of industrial environmental protection, medical detection, meteorological monitoring and so on. The harmonic signal in the TDLAS wavelength modulation method is susceptible to air pressure. It is found that the influence of air pressure is the influence of modulation depth on the harmonic signal. Based on the principle of the harmonic method in TDLAS technology, the relationship between each harmonic and modulation depth is studied. The modulation depth of the current air pressure environment is calculated by calculating the center amplitude ratio of the fourth harmonic to the second harmonic. To adjust the amplitude of the modulation frequency so that the modulation depth is close to the optimal modulation depth value of each subharmonic, and the signal-to-noise ratio of the harmonic signal is optimal to improve the detection accuracy. The second and fourth harmonic signals under 10.2~177.9 kPa pressure were extracted by the TDLAS water vapor detection system. The simulation and experimental analysis were carried out. The simulation results show that the maximum relative error of the center amplitude ratio of the fourth harmonic to the second harmonic is -1.44%, and the maximum relative error of the modulation depth simulation to the theoretical value is 1.78%. The experimental results show that the modulation depth value is calculated based on the modulation depth function. When m=2.226 7, the measured central frequency amplitude of the second harmonic reaches the maximum value. When m=4.061 0, the measured central frequency amplitude of the fourth harmonic reaches the maximum value, which is consistent with the theoretical results. When 30.2 kPa<p<177.9 kPa, the relative error between the modulation depth and the pressure product MP value is small. The maximum relative error is not more than ±3.2%. It shows that the MP value under this pressure does not fluctuate much. The modulation depth value calculated by the modulation depth function approximates the actual value, which verifies the accuracy of the modulation depth function theory.