This article describes a fiber-optic interrogation device based on the pulsed time-of-flight technique. The apparatus is capable of measuring time delays between wideband reflectors, such as connectors, along a fiber path with a precision of about 280 fs (rms value) and a spatial resolution of about 3 ns (0.30 m) in a measurement time of 25 ms. Potential application areas include measuring integral strain and its derivatives such as cracks, deflections, and displacements, particularly in large civil engineering and composite structures. The operation and basic blocks of the measurement system are presented in detail together with measurement results obtained in laboratory and field conditions. It is shown that by using a fiber loop sensor with a reference fiber, it is possible to achieve a strain precision below 1 microstrain and a measurement frequency of 4 Hz. System performance proved adequate for the study of both static and dynamic phenomena in a bridge deck.
Monitoring of fuel tanks and toxic liquid containers with embedded long optical fibres makes it possible to determine their condition and to detect probable environmental risks. We present here measurements of microbending loss in long, single-mode and multi-mode telecommunication fibres installed inside the walls of composite containers during the manufacturing process. The measurements were performed by the optical through power technique and using a commercial optical time domain reflectometer. The measurement data indicate that the optical sensing fibres can detect the end of the curing process in the composite and the occurrence of external loading likely to damage the structure.
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