An optical sensor is provided. The sensor includes an optical fiber having a free extremity on which a polymer layer is deposited normal to the longitudinal axis. A light source injects a analytical light beam in the fiber, which is reflected by the polymer layer. The reflected beam is analyzed by a spectrum analyzer, which determines the thickness of the polymer layer based on the Fabry-Perot effect. This thickness is related to a substance to be detected. An optical nose made from a plurality of such sensors is also provided, and may be used to detected a variety of substances.