Optical Flow Meter Based on Flow-Induced Bending Loss in an Optical Ring Resonator

We present an optical flow meter based on a fiber ring resonator. The sensor utilizes flow-induced bending loss in the fiber ring resonator for fluid velocity sensing. A tapered segment of the fiber ring is immersed in a flowing fluid, where it undergoes bending in response to the dynamic pressure applied by the fluid. This bending induces optical losses, which are observed as changes in the resonator dip depth, to evaluate the flow velocity. Our study demonstrates that the sensor's performance is significantly influenced by both the ring's coupling regime - determined by the coupling strength of the resonator – as well as by the tapering level of the immersed fiber segment. We demonstrate the sensor operation by measuring water vortex flows in a cylindrical water tank. We show that optimal performance is achieved when the system operates in the over-coupling regime, with a tapered fiber waist diameter in the range of 40–50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m. These findings provide critical design guidelines for enhancing the sensitivity and accuracy of optical flow meters based on ring resonator configurations.