Measurement of the absolute absorbance based on wavelength modulation spectroscopy

Ratios of the 2nd and 1st harmonics at the line center are very sensitive to absorbance, but not to line profiles when the modulation index is set to 0.94. Based on this characteristic, we proposed a method which uses the 1st and 2nd harmonics to measure the absolute absorbance, and then obtain gas pressure and concentration. Some transitions of CO2 and H2O molecules near 6980 cm−1 are selected to verify this method. The satisfactory agreement between measurement results and theoretical values validates the proposed method.