The optical sensor generally has a frame having a deformable member mounted to the frame, and a sensing optical fiber being fixedly attached to a portion of the deformable member, the sensing optical fiber having at least one π-phase-shifted fiber Bragg grating inscribed thereon, the at least one π-phase-shifted fiber Bragg grating of the sensing optical fiber deforming together with the deformable member when the frame is subjected to a force.