Large-dynamic-range fiber loop ringdown strain sensor using Frequency-shifted Interferometry

A fiber loop ringdown (FLRD) sensor combined with frequency-shifted interferometry (FSI) for large strain measurement was proposed and experimentally demonstrated. The FSI technique was employed to realize the space-domain FLRD sensing in which only continuous-wave light source and slow detection were required, and thus the sensing system cost were greatly reduced. A tapered multimode fiber was used as the sensor head for large strain measurement. By measuring the ringdown distance, the strain was monitored and a high strain sensitivity of 0.34 km−1 me−1 was obtained, corresponding to a strain detection limit of 48.2 μɛ. As a benefit from good mechanical strength of multimode fiber, a dynamic range of up to 5 me was achieved. The experimental results support that the proposed FSI-FLRD strain sensor has advantages of low cost, high sensitivity and good stability, which is suitable for health monitoring of concrete structures.