A novel fiber-optic sensor has been developed based on a newly found principle, the 'Doppler effect in a curved light waveguide'. Excellent performance of the circular loop sensor has been obtained. The circular loop sensor can detect the sum of principal strain rate with extremely high sensitivity and very wide frequency range. The sensitivity of the sensor was experimentally examined and the theoretical value was confirmed. The sensor was applied to damage detection in a reinforced concrete structural model of a railway girder. The protected sensor system was developed in order to improve the survivability during the structural test. In total 25 sensors were applied to the structural model. The developed fiber-optic sensors have shown almost equivalent sensitivity to conventional PZT sensors and much wider frequency range than resonance type PZT sensors. Initiation and propagation of cracks in the reinforced concrete have been analyzed by using the new fiber-optic vibration/acoustic sensing system.

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Optical fiber sensors are promised candidates as sensor elements in structural health monitoring (SHM). Especially fiber-optic distributed strain sensors that return a strain value as a function of linear position along an optical fiber have been attractive for people in the field of SHM. We have developed a distributed strain sensing system using long-length fiber Bragg gratings (FBGs), based on optical frequency domain reflectometry (OFDR). We employ long-length FBGs whose length is about 100 mm and the sensing region, in other words the gauge length, can be expanded up to more than 1 m by serially-cascaded long-length FBGs. This sensing system has the high spatial resolution of less than 1 mm. In this paper the distributed sensing system with OFDR and its application to SHM are described.

In the field of fiber-optic sensing technology, distributed sensors that return a value of the measurand as a function of linear position along an optical fiber are regarded as a promising sensor which can be applied to structural health monitoring (SHM). We have developed a distributed strain sensing technique using long gauge fiber Bragg grating (FBG) based on optical frequency domain reflectometry (OFDR). FBGs functioning as mirrors with wavelengthselective reflectivity have been used as strain or temperature sensors. OFDR is a technique designed to measure backreflections from optical fiber networks and components. In our system, we use a longer gauge FBG whose length is ordinarily more than 100 mm and we can measure strain at an arbitrary position along the FBG. Therefore, we can obtain continuous strain data along the FBG. Furthermore, since the spatial resolution in strain measurements is less than 1 mm, it enables us to measure the strain distribution of stress concentrated area, such as welded and bonded joints, precisely. In this paper, we describe the principle of the distributed sensing technique based on OFDR and the applications to strain monitoring of a bonded joint and a wing box structure.