Sensitivity-Enhanced Fiber-Optic Sensor in a Microwave Photonics Fiber Loop Ringdown System

In this article we report a microwave photonics fiber loop ringdown (MWP-FLRD) interrogated fiber-optic sensor with enhanced sensitivity. A section of a single-mode fiber in the fiber loop is macro-bent into a circular ring as the element responsive to displacement. When the circular ring is subjected to external displacement, the transmission loss of the loop changes, resulting in a change in the ringdown time of the MWP-FLRD system. To enhance the measurement sensitivity, a multi-turn structure is proposed and experimentally demonstrated. Compared to the traditional time-domain FLRD system, the MWP-FLRD system employs a frequency domain measurement, showing a high signal-to-noise ratio and high measurement efficiency. The physics of MWP-FLRD, the displacement measurement principle, the sensitivity-enhancement scheme, and the temperature self-compensation capability of the system are discussed in this paper. The experimental results demonstrate that an averaged amplification factor of 0.8221 per turn is achieved for displacement measurement sensitivity using the multi-turn structure. Taking advantage of the mechanical strength of the multi-turn fiber structure with an intact polymer coating, the system is employed to monitor the elbow bending of human body movement in real-time, showing high sensitivity and fast responses. It is envisioned that the concept of the proposed multi-turn structure can be implemented onto other FLRD systems for sensitivity enhancement.