Realization of a collinear pump-probe beam configuration for a fiber-optic photothermal trace gas sensing system

We present the realization of a collinear pump-probe beam configuration for photothermal interferometry-based trace gas sensing systems using a custom-made, fiber-coupled and dichroic extrinsic Fabry-Pérot etalon. The system additionally supports the common crossed-beams, as well as an angled-beams configuration. This allows a direct comparison between the configurations and clearly demonstrates the increasing sensitivity with increasing beam overlap. For the collinear alignment, a detection limit of less than 2 ppmv (water vapor) was achieved, representing a fivefold improvement compared to the crossed-beams configuration. Additionally, a competitive normalized noise-equivalent absorption of 1.95×10⁻⁹cm⁻¹W/√Hz was achieved for the collinear configuration. These findings highlight the critical role of beam alignment in optimizing the performance of photothermal interferometry-based sensing systems.