Fiber-Optic Cavity Ring Down Using an Added-Signal for Curvature Sensing

Abstract

This letter presents a fiber-optic cavity ring down (CRD) configuration using an added-signal for curvature sensing. An optical time-domain reflectometer was used to send impulses down into the fiber loop cavity, inside of which a long period grating was placed to act as a sensing device. The added-signal was obtained by the sum of several conventional CRD impulses, thus providing an improvement on the curvature sensitivity when compared with the conventional CRD signal processing. A linear response to applied curvature in the range of 2.2–3.6 $\text{m}^{-1}$ was observed, and a sensitivity of 15.3 $\mu \text{s}/\text{m}^{-1}$ was obtained. This result was found to be 20-fold the one obtained for the conventional CRD signal processing. The added-signal increases the optical power but increases as well the ring-down time due to the sum of the several loops that light travels inside the ring. A ring-down time response of 43.3 $\mu \text{s}$ was attained (versus $23.7~\mu \text{s}$ for the conventional CRD signal processing).