The invention relates to a chemical species optical sensor comprising: a fluorescent source (3) of incoherent infrared rays, including a chalcogenide glass matrix, doped with rare earth ions and combined with a pump source by means of a first optical fiber (2); and at least one infrared ray detector (5), provided with a spectral selection device (50) and set up to detect the rays that are emitted by said fluorescent source and have passed through a detection area (6), said detector including a fluorescent element (510) formed by a chalcogenide glass matrix that is doped with rare earth ions and combined with a second pump source (530) by means of a second optical fiber (520). Such a sensor can be used for differentially detecting a chemical species, and in particular CO2.