The present disclosure provides a novel fiber optic sensing device using ultra-weak, terahertz-range reflector structures. A fiber optic sensor device for distributed measurements (strain/temperature) includes an optical fiber detection arm having an inner core extending along a length of the optical fiber, an outer cladding surrounding the inner core, and at least one ultra-weak, terahertz-range reflector structure. Each reflector structure is comprised of two or more ultra-weak range reflectors (gratings) written at a spacing corresponding to the terahertz range and formed along a length of the inner core of the optical fiber. A narrow bandwidth, tunable laser interrogation system interrogates the optical fiber and measures changes in reflections and interference patterns caused by physical changes in the optical fiber.