Distributed measurements of structures by fiber optic sensors

Continuous monitoring of civil structures is very useful to evaluate the structural health status. Fiber optic sensors can give the sensing solution for conducting continuous monitoring because these sensors are going to be multiplexed with various sensors on one line of an optical fiber and have the durability to harsh environments, and also are not affected by electromagnetic effects. In this study, we studied on the development of two kinds of sensors: One is an optical loss based fiber optic sensor which could be multiplexed by optical time domain reflectometry, and another is a fiber optic BOTDA (Brillouin optical time domain analysis) sensor for distributed temperature measurement. To measure the strain by optical loss based fiber optic OTDR (optical time domain reflectometry) sensor, the optical loss of an optical fiber was designed to be sensitive to the strain. By performing beam deflection test, the strains of the beam was measured by these sensors, and compared with the strains of electrical strain gages at the same location. From the results, we could confirm these sensors had a good sensitivity to monitor the large civil structures. In order to measure the distributed temperature, A fiber optic BOTDA sensor system, which has a capability of measuring the temperature distribution, attempted over several kilometers of long fiber paths. This simple fiber optic sensor system employs a laser diode and two electro-optic modulators. The optical fiber of the length of 1400 m was installed on the surfaces of the building. The change of the distributed temperature on the building construction was well measured by this fiber optic sensor. The temperature changed normally up to 4°C through one day. Also, The temperature distribution of spring season was compared with that of early winter season.