Solid-state laser intra-cavity photothermal gas sensor

Compact, rugged and sensitive laser-based trace gas sensors are in high demand for science and commercial applications. To ensure high sensitivities, laser spectroscopic sensors often use extended interaction paths (e.g. multi-pass cells), which significantly increases their size, weight and susceptibility to misalignment. Herein, we present a novel, miniaturized photothermal gas sensor, where the gas sample is measured inside the resonator of a monolithic microchip solid-state laser operating at 1064 nm. The photothermal-induced gas refractive index variations are directly translated to a solid-state laser frequency shift, which is detected as a beatnote modulation in a heterodyne detection scheme. The system provides high sensitivity to refractive index changes at the level of ∼1.1 × 10⁻¹² within ultra-short intra-cavity interaction path-length of 1.5 mm, which enables trace-gas measurements in a sensing volume of only 4 μl. In a proof-of-concept experiment using dry carbon dioxide as a test sample the sensor reached a minimum detection limit of 350 ppbv for a 100 s averaging time and NNEA = 4.1 × 10⁻⁸ [W cm⁻¹ Hz^−1/2].