摘要： Laser spectroscopy has proven to be a powerful tool for quantitative and selective measurements of gas samples. Numerous techniques have been developed throughout the recent decades. This progress has been driven mostly by the growing demand on sensitive sensors, also for out-of-lab applications. Here we describe a new gas detection technique, which relies on photo-thermal (PT) effects. In PT spectroscopy the gas sample concentration can be derived by measuring a change in the refractive index (RI) of the excited gas particles. This is usually achieved by exciting the gas particles using a radiation source and by measuring the small variations of the RI (usually in the range of 10-5), e.g. in an interferometric sensor configuration, which is complicated and prone to mechanical and acoustic noise. Moreover, appropriate construction of a PT sensor enables encoding the gas concentration into frequency variations (similarly to dispersion spectroscopy), which provides a baseline-free measurement. Here we propose a novel, patent-pending configuration, which is non-complex, cost effective, miniaturized and offers detection limits at least comparable with other methods, having similar complexity, e.g. quartz enhanced photo-acoustic spectroscopy. Base of the sensor is a standard configuration of a monolithic, solid-state laser based on Nd:YVO4 active crystal and a YVO4 birefringent crystal ensuring single polarization operation of the laser at 1064 nm. To enable implementing PT gas detection an intentional air-gap has been obtained inside the resonator by separating the output mirror with a 2x2x2mm3 quartz crystal. In this particular experiment the air gap was filled with CO2 under ambient pressure and excited with an auxiliary fiber laser targeting a strong absorption line localized at 2003,5 nm. Due to the PT effect the density of the excited gas changes, which results in a change of its RI. A change of RI inside the solid-state laser resonator results in a slight variation of the optical pathlength and hence is directly translated to frequency changes of emission of the solid-state laser. The optical frequency changes are detected in a heterodyne configuration (the reference laser emission was obtained in the same crystal structure to minimise noise; the gas is excited in the path of only one of the emissions). The 2 μm excitation laser was modulated with a f0=1 kHz sinewave function and tuned across the absorption line. The induced PT effect was conveniently filtered-out at 2xf0 using a lock-in amplifier. The schematic of the sensor configuration, along with 2f signal registered for a 1000 ppmv and 200 ppmv CO2 sample is depicted in figure 1.