The invention relates to a fiber-optic sensor comprising an optical waveguide having at least one first core and a cladding surrounding the first core, wherein the first core extends substantially over the entire length of the optical waveguide, wherein the sensor has at least one second core which is at least partly surrounded by the cladding, wherein the longitudinal extent of the second core is less than the total length of the optical waveguide and at least one Bragg grating is introduced into the second core. Furthermore, the invention relates to a use of the fiber-optic sensor.