Multi-Point Fiber Optic Displacement Sensing System Based on Cavity Length Detection

We propose a macroscopic loss-based olive-shaped single-mode fiber (OSSMF) for displacement sensing in the fiber loop ring-down, which validates the feasibility of displacement sensing. In this work, two systems consisting of single-point and multi-point displacement sensing are built, and the ring-down curves are demodulated using low-cost microcontroller unit and self-developed optical time domain reflectometer (OTDR), respectively. The long-cavity system is initially utilized for single-point displacement sensing to confirm the possibility of multi-point displacement detection with short cavities. The relationship between cavity length and sampling frequency is also discussed, enabling low-frequency, high-precision sampling of the ring-down curve. In the short-cavity multi-point experimental system, different sizes of olive-shaped sensing heads are compared in a 65 m cavity length scenario. It is concluded that the OSSMF with a short axis of 2.1 cm and a long axis of 3.5 cm exhibited the highest sensitivity. The attenuation is −2.11 dB/ (km mm) in the 8--12 mm range and −7.24 dB/ (km mm) in the 12--24 mm range. The displacements are detected in 65 and 125 m fiber loop ring-down (FLRD) cavities, and the ring-down curves are processed using Fast Fourier Transform, this method addresses the issue in multi-point measurements where an extension cord is needed for time-domain differentiation. Different cavity length designs enable distributed measurements of physical quantities, the system is of great significance in the fields of structural engineering monitoring and geological disaster monitoring.