Mode matching of a laser-beam to a compact high finesse bow-tie optical cavity for quartz enhanced photoacoustic gas sensing

We report on the optical characterization of a compact bow-tie cavity composed of two flat mirrors and two concave mirrors, all having a reflectance > 99.99% in the spectral range between 4.8 μm and 5.3 μm, mounted in a stainless-steel enclosure. The cavity was designed for the implementation of an intracavity-quartz enhanced photoacoustic sensor system. The propagation parameters of the intra-cavity beam were determined using the ABCD-matrix method, allowing the analytical formulation of the size of two beam waists occurring inside the cavity. A collimated mid-infrared laser beam was optically coupled and mode matched into the bow-tie cavity via a focusing lens. A cavity finesse of ∼ 2000 was measured at a pressure of 90 Torr inside the cavity, corresponding to an optical power enhancement factor of ∼ 320.