Abstract: A photoacoustic sensor for spectroscopic gas detection includes a chamber to contain the gas; an excitation laser source that emits a modulated laser beam; a transducer, having a reflective portion, located inside the chamber; an interferometer having first and second arms and an output, the first arm ending in the transducer, and the second arm including an optical path changer element and an actuator connected to the optical path changer element to change the optical path defined by the second arm of the interferometer; a first polarizing beam splitter; a second laser source; a second polarizing beam splitter; a first detector; a second detector; an electronic circuit; and a feedback loop adapted to command the movements of the actuator in order to change and stabilize the optical path of the second arm, so that the difference signal is kept equal to a desired constant value during the gas concentration measurements.