Brillouin characterization of slimmed polymer optical fibers for strain sensing with extremely wide dynamic range

To date, most distributed Brillouin sensors for structural health monitoring have employed glass optical fibers as sensing fibers, but they are inherently fragile and cannot withstand strains of >3%. This means that the maximal detectable strain of glass-fiber-based Brillouin sensors was ~3%, which is far from being sufficient for monitoring the possible distortion caused by big earthquakes. To extend this strain dynamic range, polymer optical fibers (POFs) have been used as sensing fibers. As POFs can generally withstand even ~100% strain, at first, Brillouin scattering in POFs was expected to be useful in measuring such large strain. However, the maximal detectable strain using Brillouin scattering in POFs was found to be merely ~5%, because of a Brillouin-frequency-shift hopping phenomenon accompanied by a slimming effect peculiar to polymer materials. This conventional record of the strain dynamic range (5%) was still far from being sufficient. Here, we have thought of an idea that the strain dynamic range can be further extended by employing a POF with its whole length slimmed in advance and by avoiding the Brillouin-frequency-shift hopping. The experimental results reveal that, by applying 3.0% strain to a slimmed POF beforehand, we can achieve a >25% strain dynamic range, which is >5 times the conventional value and will greatly extend the application fields of fiber-optic Brillouin sensing.