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In a distributed Brillouin sensor system, it is crucial to keep the pulse energy uniform for a constant signal-to-noise ratio. This means that the variable dc leakage (pulse base) for the electro-optic modulator (EOM) must be locked. We examine two different methods of locking the EOM bias voltage and look at the advantages and disadvantages of each locking method. It is found that the two locking methods, one based on a lock-in amplifier and the other using proportional-integral-derivative control, both have applications in which they excel at locking the pulse base.

A distributed Brillouin fiber sensor has been employed to detect localized pipe-wall buckling in an energy pipe by measuring the longitudinal and hoop strain distributions along the outer surface of the pipe for the first time. The locations of the localized pipe-wall buckling are found and distinguished using their corresponding strain-load data. The formation of the buckling process for the compression and tension characters is studied in the longitudinal and hoop directions. For the pipe with internal pressure, concentric load, and bending load, a localized pipe-wall buckling takes place away from the middle of the pipe on the compressive side and a strain peak with an overall buckling occurs on the tensile side according to the longitudinal strain distributions along the pipe. Different strains on two neutral lines are also observed in the hoop strain distribution, which should be caused by the pipe weld joint.

Abstract-We introduce a novel data analysis approach based on the extraction of peak strain, asymmetric and broadening factors of the Brillouin spectrum measured with the distributed Brillouin sensor (DBS). Such an approach provides simultaneously global and local strains, describing the status of the structure, at the contrary of average strain measurements. These results are confirmed by a trial on a composite column subjected to vertical and bending loads. This demonstrates that the DBS is a powerful tool to give the signature of the structure failure and then to identify early problems in structures that none of existing point sensors can detect.Index Terms-Brillouin spectrum, distributed Brillouin sensor (DBS), form factors, strain sensor, structural health monitoring (SHM).

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