Wavelength modulation spectroscopy (WMS) combined with a multipass absorption cell has been used to measure a weak absorption line of carbon monoxide (CO) at 1.578 µm. A 0.95m Herriott-type cell provides an effective absorption path length of 55.1 m. The WMS signals from the first and second harmonic output of a lock-in amplifier (WMS-1 f and 2 f, respectively) agree with the Beer–Lambert law, especially at low concentrations. After boxcar averaging, the minimum detection limit achieved is 4.3 ppm for a measurement time of 0.125 s. The corresponding normalized detection limit is 84 ppm m Hz–1/2. If the integrated time is increased to 88 s, the minimum detectable limit of CO can reach to 0.29 ppm based on an Allan variation analysis. The pressure-dependent relationship is validated after accounting for the pressure factor in data processing. Finally, a linear correlation between the WMS-2 f amplitudes and gas concentrations is obtained at concentration ratios less than 15.5%, and the accuracy is better than 92% at total pressure less than 62.7 Torr.
Improvement of Concentration Inversion Model Based on Second Harmonic Valley Spacing in Wavelength Modulation Spectroscopy
