A strain sensor includes an optical fiber with at least one optical fiber, a reflector body with a reflective surface, a housing affixed to the optical fiber probe and to the reflector body. The reflective surface is spaced apart at a distance d from the ends of the probe's fibers and receives light from the end of the fiber and to reflect at least a portion of the light into the end of the fiber. The housing is attached to the fiber probe at a first end of the housing and attached to the reflector body at a second end of the housing. The housing is affixed to the material to be measured, and in the material causes a change in gap between the fiber end and the reflective surface, modulating the amount of light received in the receiving fiber, detectable by a photodetector connected to the receiving fiber.