01/01/2018
Optical sensor fiber for spatially resolved strain measurement (left) and experimental bridge at BAM with installed sensor fibers (right)

Optical sensor fiber for spatially resolved strain measurement (left) and experimental bridge at BAM with installed sensor fibers (right)

Source: BAM, Division Fibre Optic Sensors

Bridges play a key role in modern transport infrastructure. In the event of damage, potentially human lives are endangered and the failure of a bridge construction always entails considerable economic damage. The condition and the remaining lifespan of most of the existing bridges in Germany is largely unknown after decades of use. In addition, the bridges are subjected to an increasing structural load due to growing transportation demands. The challenge is to determine the actual condition of a structure in order to plan maintenance and remediation measures and to evaluate their effectiveness. One approach is based on the analysis of the vibrational behavior of the structures. Precise measurements of the dynamic strain distribution along the component have the potential to significantly increase the accuracy of the methods.

In recent years, numerous methods have been developed for the dynamic measurement of strain changes along optical fibers (standard silica fibers as they are used for optical communication). Within the BAM project "BLEIB" a novel measuring method was developed, which makes it possible to measure strain changes spatially resolved along the sensor fiber. The method is based on coherent optical time domain reflectometry (C-OTDR): Coherent laser pulses are sent into the fiber and the backscattered light (Rayleigh scattering) of the forward propagating pulse is detected as a function of time and, equivalently, distance. The fibre therefore represents a chain of individual interferometers. Using this "distributed interferometer", it is possible to detect minute strain changes along the sensor fiber with interferometric sensitivity over long distances. The novelty of the demonstrated method is that alternately pulses of different wavelengths are sent into the fiber. By correlating the interferometric power change signals of both pulse wavelengths over progressing time, strain changes can be quantified. In addition, and in contrast to conventional interferometers, the sign of the strain change can be unambiguously determined due to the additional spectral information.

The measurement method was published in Optics Express ("Relative change measurement of physical quantities using dual-wavelength coherent OTDR") and is now used for measurements along sensor fibers that have been applied to a bridge in order to determine precise strain distributions and modal parameters of the structure.

Relative change measurement of physical quantities using dual-wavelength coherent OTDR
Sascha Liehr, Y. S. Muanenda, Sven Münzenberger, Katerina Krebber
Optics Express, Vol. 25, Issue 2, pp. 720-729 (2017)
BAM Department Non-destructive Testing, Division Fibre Optic Sensors