Bragg gratings have been fabricated by means of sputtering on the end facet of standard multimode optical fibers towards development of refractometers. Tin oxide was selected as the coating material, and argon and oxygen were used as sputtering gases. Changes on the refractive index of the material were achieved by varying the oxygen percentage in the sputtering gas flow. A reflected peak centered at 586 nm with a full width at half maximum of 70 nm has been obtained, which provides a good structure for the development of wavelength modulated optical fiber sensors. The sensitivity of this structure to the refractive index of the surrounding medium has been studied, obtaining a linear sensitivity of 173 nm/RIU, for refractive index varying from 1.333 to 1.442.