We observe multimodal interference in single-mode-multimodesingle-mode sensors comprising short polymer optical fibers (POFs) with lengths from 100 mm down to 7 mm. Characteristic spectral peaks/dips are observed not in the conventionally used telecom band but around 1000 nm. We find that the dip wavelength depends on temperature but that the sensitivity is much lower than those obtained in the longer wavelength range when longer POFs are used. We discuss the possibility that, even with the reduced sensitivity, our success in observing multimodal interference in millimeter-long optical fibers will be a basis toward the combined use of frequency and intensity information in conventional fiber-optic single-point sensors for discriminative measurement of multiple physical parameters.
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.
For Brillouin-sensing applications, we develop a method for mitigating the Fresnel reflection at the perfluorinated-polymer-optical-fiber ends by covering them with an amorphous fluoropolymer (CYTOP, fiber core material) dissolved in a volatile solvent. Unlike the conventional method using water, even after solvent evaporation, the CYTOP layer remains, resulting in long-term Fresnel reduction. In addition, the high viscosity of the CYTOP solution is a practical advantage. The effectiveness of this method is experimentally proved by Brillouin measurement.
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