Self-mixing detection of methane and carbon dioxide using mid-infrared quantum cascade lasers

We explore the use of self-mixing schemes based on Quantum Cascade lasers (QCLs) for the identification of greenhouse gases such as Methane (CH₄) and Carbon Dioxide (CO₂) in the mid-infrared (Mid-IR) region. In self-mixing, a QCL plays the role of the excitation source and the detector, thus eliminating the need for a dedicated cooled Mid-IR detector, reducing the cost and power consumption of the system. The experiments were performed within the spectral windows of 7669-7678 nm (1303.95-1302.42 cm^-1) for CH₄ and 4317-4322 nm (2316.42-2313.74 cm^-1) for CO₂. Measurements conducted for CH₄ concentrations of 1300 and 480 ppm, and for CO₂ concentrations of 430 ppm, 300 ppm and 200 ppm. After the calibration on our system, experiments took place that indicated a minimum detectable concentration of 45 ppm for CH₄ at the 7673.83 nm absorption line and 9 ppm for CO₂ at the 4319.78 nm absorption line, which are comparable to detector-based schemes. Additionally, a separate validation experiment demonstrated measurement accuracy of at least 90%.