Fiber Ring Laser Based on Side-Polished Fiber MZI for Enhancing Refractive Index and Torsion Measurement

A fiber ring laser based on a side-polish fiber Mach-Zehnder interferometer (MZI) for effective improving refractive index (RI) and torsion sensing is proposed and investigated. The side-polished fiber MZI sensor can not only enhance the evanescent wave but also break circular symmetry of optical fiber face section, so it can be used for the surrounding RI and torsion sensing. A spectrum 3 dB bandwidth of less than 0.15 nm has been achieved which makes the fiber ring laser torsion sensing system to have higher measurement resolution. Experiment results show that the RI and torsion sensitivity of proposed sensor is dependent on the polish depth: the thicker the polish depth, the higher the sensitivity. For a sensor with side polish depth of 47~\mu {m} , the measured RI sensitivity reaches −81.36 nm /RIU, the torsion sensitivity is as high as −0.019nm/ O , the sensitivity converted to torsional rate is −0.267 nm /( rad . {m} ^{-1} ).