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 ~7.84 µm or ~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 ~0.029 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%.