Direct High-Frequency Modulation of a Quantum Cascade Laser for High-Sensitivity Molecular Dispersion Spectroscopy at Ambient Pressure

Heterodyne phase-sensitive dispersion spectroscopy has emerged as a promising new approach to gas-phase chemical sensing. The most common implementation of this method uses direct modulation of the injection current of a quantum cascade laser (QCL). The modulation produces a three-tone optical signal, which, when passed through the gas sample, allows the profile of the refractive index, from which the gas concentration can be derived to be measured. However, the frequency of this modulation signal must be in the range of the linewidth of the target molecular resonance, which for many compounds of interest is in the few gigahertz at ambient pressure. This challenging requirement prevents the technique from operating in free space with good sensitivity. Nevertheless, in this article, we present a novel QCL package that, together with a simple and reliable heterodyne phase-sensitive dispersion spectroscopy (HPSDS) instrument design, demonstrates a very convincing solution to overcome the performance limitations of current systems at ambient pressure, hence enabling high-performance gas analysis in free space. Various experiments targeting methane at 1311.5 cm−1 show excellent linearity over a concentration range of four orders of magnitude and a limit of detection (LOD), which is well into the sub-ppm $\cdot $ m