Fiber optic distributed sensor for structural monitoring

We describe a new optical fiber sensor designed to measure strain in structures. Extensive studies have already been conducted on fiber optical sensors for the Brillouin optical time domain reflectometer (BOTDR) to be installed in concrete structures for measuring distributed strain and temperature [I]. Previous studies have focused only on BOTDR for communication fiber cables. The fiber cables incorporate soft material in their design to prevent excessive strain from being transferred to the fiber core. These characteristics are favorable for communication cables but not for sensor systems in which the strain should be correctly transmitted to the core. Our newly developed optical fiber sensor can overcome this problem and yet have a strong protective layer enabling it to be embedded in concrete structures without any additional reinforcement. We examined the optical performance of the sensor under several loading conditions and for several material parameters. The power loss associated with the resulting strain was acceptable for practical use. In addition, the strength and durability of the sensor were sufficient for field applications far concrete structures. The strain measurement accuracy was calibrated by a series of tests and was in excellent agreement with the real strain values.