Spatially weighted, grating-based, two-mode, elliptical-core optical fiber vibration sensors

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Abstract
Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers affect the beat length and the sensor sensitivity. Chirped gratings are written by attaching such fibers to cantilever beams positioned in a strained state. We show that fibers with in-line chirped gratings, with the chirp being shaped in the form of a vibration-mode shape, can be used as spatially weighted fiber sensors for vibration analysis. We demonstrate enhanced detection of the first and second modes of vibration of a cantilever beam using this process; vibration-mode suppression of the order of 10 dB is obtained.