Abstract: There is described a fiber-optic sensor for measuring a light signal from a fluorescible sample comprising heavy metal ions, for example. The fiber-optic sensor comprises an optical fiber having a side surface by which the light signal from the fluorescible sample is inputted. The optical fiber is corrugated to form at least two gratings on the side surface of the optical fiber. Each grating comprises periodically longitudinally spaced-apart valleys on the surface of the optical fiber, and is longitudinally spaced apart from any other grating of the at least two gratings.