Fiber optics pinpoints structural problems

At a recent optical-fiber meeting in California, physicists from Canada’s University of Ottawa reported on a distributed Brillouin sensor that can detect deformation, cracks, and bending under real-world conditions. The DBS system uses optical fibers in contact with the structure under study. A pulse of laser light and a continuous optical beam of a different frequency are sent in opposite directions through the fibers. When the underlying structure is stressed, the resulting phonons slightly change the refractive index in the adjacent part of the fiber through a phenomenon called the Brillouin effect. Careful monitoring of the frequency difference between the counterpropagating light waves provides precise information on the mechanical strains. The system’s 15-cm spatial resolution is determined by the laser’s 1.5-ns pulse width. In one demonstration, the Ottawa researchers glued optical fiber along the length of a section of pressurized natural-gas pipeline subjected to bending and buckling. In another demonstration, the researchers tested the DBS system on a reinforced concrete column subjected to simulated seismic forces. In both cases, problems were detected and located well before the structure failed; such early localization is difficult with current structural-health analysis, which is done on a spot-by-spot basis. (See paper OTuL7 at the meeting website, http://www.ofcnfoec.org , and F. Ravet et al., IEEE Photonics Tech. Lett. 18 , 394, 2006 http://dx.doi.org/10.1109/LPT.2005.862366 .)