NOVELTY - The photo-acoustic sensor, to detect the presence of a gas, has a laser (1) as a source of radiant energy to deliver excitation energy at the monitored zone where the selected gas is collected. The laser beam has a maximum wavelength absorption band for the gas. The assembly has a modulator (2) to modulate the excitation energy, and a transducer (10,11) in the measurement zone to detect acoustic signals. The measurement zone is a resonant dish (4), where the resonant frequency is adapted to the modulation frequency of the optical excitation.    USE - Detection of a gas, with simultaneous measurement of two or more gases.    ADVANTAGE - The sensor assembly has an increased sensitivity to detect the presence of a gas in very small concentrations. The apparatus is easily adapted to detect all types of gas.    DETAILED DESCRIPTION - The sensor to detect a gas has a resonant dish with two transducers within its walls which, together, form a Helmholtz resonator with the walls in phase opposition. The output of the transducer at one wall, illuminated by the laser beam, is linked to the positive input of a differential amplifier (8). The output of the other transducer is linked to the negative input of the differential amplifier. The dish walls are measurement tubes, with a capillary tube at each end. The first capillary takes an entry tube and a second capillary has an outlet tube, with a faucet at each capillary tube. If the assembly is mounted at a vehicle, the entry tube is linked to the ambient air for the detection of any gas present. The laser wavelength can be set as a function of the gas to be detected. The modulator has a mechanical action. The optical unit to modulate the excitation is set electronically by changes in the command current to the diode laser.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the gas detection apparatus.    diode laser (1)    mechanical excitation energy modulator (2)    resonant dish (4)    differential amplifier (8)    transducers (10,11)