Self-mixing in quantum cascade lasers for mid-infrared gas sensing: modeling, simulations, and experiments

This study presents a detector-less mid-infrared gas sensing system using self-mixing (SM) in a quantum cascade laser (QCL), accompanied by a numerical model based on rate equations. By measuring the laser's terminal voltage drop (ΔV_T), the system eliminates the need for external photodetectors, reducing footprint and complexity. Utilizing an experimental hollow-waveguide gas-cell, the system achieved a methane detection limit of 15 ppm. Results demonstrate high spectral accuracy (∼0.1 nm) and strong affine linearity (R² > 0.99) between experimental voltage drop and simulated photon variations (ΔS). This predictive, compact framework offers a robust foundation for cost-effective, field-deployable sensors in industrial and medical applications.