A fiber-optic sensors for measuring deformation, intended to operate in a harsh environment is provided. The sensor comprises a Fabry-Perot-cavity-based optical measurement head, a linking optical fiber and an expansion reserve case, the case comprising a segment of the linking optical fiber. The inside thickness of the case is comprised between one and several millimeters, the case being flat and of shape referred to as bicorne shape, the shape comprising a convex central portion and two concave symmetric ends, the optical fiber forming, inside the bicorne, one and only one arch, the segment of the optical fiber being, in addition, tangent to the internal surfaces of the reserve case, whatever the temperature conditions.