The invention relates to a fiber-optic sensor (1) comprising at least two cores (10) from a first material having a first refractive index (n1) and a sheathing (11) of a second material having a second refractive index (n2), which sheathing surrounds the cores. At least one fiber Bragg grating (12) is integrated into each core (10) and the fiber Bragg gratings (12) are arranged at different locations along the longitudinal extension of at least one first longitudinal section (21) of the fiber-optic sensor (1). The invention further relates to a corresponding method for detecting mechanical state variables.