A photoacoustic sensor for spectroscopic gas detection, comprising: - A chamber adapted to contain the gas to be analysed; - An excitation laser source adapted to emit a modulated laser beam, which is absorbed by the gas present in the chamber and creating a pressure wave; - A transducer, the transducer being located inside the chamber, and being adapted to be put into oscillations by the pressure wave, the transducer having a reflective portion; - An interferometer having a first and a second arm and an output, with 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 for interferometer reading, adapted to emit a second laser beam impinging onto the first polarizing beam splitter, the first polarizing beam splitter being adapted to split the second laser beam in the first and the second arm of the interferometer, so that the split beam travelling in the first arm is back-reflected by the reflective portion of the transducer, and the split beam travelling in the second arm is back-reflected by the optical path changer element, so that the resulting back-reflected split beams (50',51') interfere in the interferometer; - A second polarizing beam splitter adapted to split the interfering light at the output of the interferometer in two beams (52,53) which are orthogonally polarized; - A first detector adapted to detect one of the two orthogonally polarized beam exiting the interferometer and to emit a corresponding first interference signal; - A second detector adapted to detect the other of the two orthogonally polarized beam exiting the interferometer and to emit a corresponding second interference signal; - An electronic circuit configured to obtain a difference signal (D2-D1), the difference signal being function of the difference between the interference signal emitted by the first detector and the interference signal emitted by the second detector; - A feedback loop adapted to command the movements of the actuator in order to change 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. - A chamber adapted to contain the gas to be analysed; - An excitation laser source adapted to emit a modulated laser beam, which is absorbed by the gas present in the chamber and creating a pressure wave; - A transducer, the transducer being located inside the chamber, and being adapted to be put into oscillations by the pressure wave, the transducer having a reflective portion; - An interferometer having a first and a second arm and an output, with 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 for interferometer reading, adapted to emit a second laser beam impinging onto the first polarizing beam splitter, the first polarizing beam splitter being adapted to split the second laser beam in the first and the second arm of the interferometer, so that the split beam travelling in the first arm is back-reflected by the reflective portion of the transducer, and the split beam travelling in the second arm is back-reflected by the optical path changer element, so that the resulting back-reflected split beams (50',51') interfere in the interferometer; - A second polarizing beam splitter adapted to split the interfering light at the output of the interferometer in two beams (52,53) which are orthogonally polarized; - A first detector adapted to detect one of the two orthogonally polarized beam exiting the interferometer and to emit a corresponding first interference signal; - A second detector adapted to detect the other of the two orthogonally polarized beam exiting the interferometer and to emit a corresponding second interference signal; - An electronic circuit configured to obtain a difference signal (D2-D1), the difference signal being function of the difference between the interference signal emitted by the first detector and the interference signal emitted by the second detector; - A feedback loop adapted to command the movements of the actuator in order to change 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.