在利用可调谐激光调制吸收光谱技术进行测量时,气体参量改变将导致线宽变化,从而影响二次谐波信号峰值的大小。针对于Lorentz线型,给出了线宽变化对二次谐波峰值影响的理论分析及利用二次谐波信号峰谷比值进行修正的方法。在10 cm的吸收池内利用1578.22 nm处的吸收谱线对CO2进行了测量,在不同调制幅度下验证了不同工况中线宽对二次谐波信号的影响。结果表明,利用二次谐波峰谷比值确定调制系数进行修正可有效地减小由于工况改变时线宽变化对测量带来的影响,当压力由1.0×105Pa变化至1.5×105Pa时,修正前后的测量相对误差分别为34.3%与1.8%; 当气体体积分数在20%～100%变化时,修正前后的平均误差分别为12.8%与1.8%。

The gas concentration calculated from second harmonic signal would be affected by the variety of linewidth induced by different gas parameters during the measurement by wavelength modulation absorption spectrometry using tunable diode laser. The effect of linewidth variety on second harmonic signal for Lorentz lineshape was analyzed. A correction method using ratio of peak to valley from second harmonic signal was also given here. The experiment on CO2 concentration at 1578.22 nm was carried out in a 10 cm absorption cell with different modulation amplitudes, gas pressures and concentrations to validate the effect of linewidth variey on second harmonic signal. The results show that the relative errors of gas concentration would be decreased from 34.3% to 1.8% by correction method when the pressure rises from 1.0×105 Pa to 1.5×105 Pa, and decreased from 12.8% to 1.8% by correction method when the gas concentration changes from 20% to 100%. The correction method can eliminate the effect of linewidth variety on gas measurement by second harmonic signal effectively.