the present invention relates to an optical fiber for a fiber optic sensor , comprising a first optical grating adapted to operate over a first range of wavelengths ; and at least one set of further gratings adapted to operate over a second range of wavelengths , each grating being adapted to operate over a portion of the second range ; wherein , each grating within said set has an operating range that partially overlaps with at least one other such grating operating range . the invention also extends to a sensor system , and method , using such an optical fiber .