Mid-Infrared Gas Sensor Based on Mutually Injection Locked Quantum Cascade Lasers

We present experimental investigations of a novel mid-infrared gas sensor relying on a laser system consisting of two quantum cascade sources mutually coupled with each other. The free-space path between the two lasers contains the sensing cell which is filled with the gas of interest. This laser system proves to be a very interesting dynamical system which can offer enhanced detectivity when compared to a conventional direct absorption spectrometer utilizing a laser, the gas cell, and a detector. We present comprehensive investigations of the operation regimes under which the laser system behaves as an efficient sensor and demonstrate its dependence on critical parameters such as the coupling strength between the two lasers and their bias currents. The experimental study is supported by numerical simulations, which provide a theoretical understanding of the underlying mechanisms. A six fold improvement of the sensitivity in a N
₂
O absorption spectroscopy experiment is demonstrated in comparison to a conventional direct absorption spectrometer. This sensitivity improvement is exhibited for a wide range of N
₂
O concentrations.