Calibration-free wavelength modulation spectroscopy for gas concentration measurements using a quantum cascade laser

We report the development of an accurate calibration-free wavelength-scanned wavelength modulation spectroscopy system based on the temporal wavelength response of a current-modulated quantum cascade laser (QCL) for gas concentration detections. Accurate measurements and determination of the QCL output intensity and wavelength response to current modulation enabled calculations of 1f-normalized 2f signal to obtain spectroscopic information with and without gas absorption in the beam path. The gas concentration was retrieved by fitting a simulation spectrum based on spectral line parameters to the background-subtracted 1f-normalized 2f signal based on measurements. In this paper, we demonstrate the performance of the developed system for the CH4 detection by applying an infrared QCL (at similar to 7.84 mu m or similar to 1275 cm(-1)) to probe its two infrared transition lines at 1275.042 cm(-1) and 1275.387 cm(-1). The experimental results indicated very good agreements between measurements and modeling, for integrated absorbance ranging from 0.0057 cm(-1) to 0.11 cm(-1) (or absorbance ranging from similar to 0.029 to similar to 0.57). The extracted integrated absorbance was highly linear (R = 0.99996) to the gas sample concentration. Deviations between the nominal sample gas concentrations and the extracted gas concentrations calculated based on HITRAN spectroscopic parameters were within 3.5%.