Sensing enhancement at an exceptional point in a nonreciprocal fiber ring cavity

Sensors operating at an exceptional point (EP) are sufficiently sensitive to small perturbations. In this paper, we demonstrate that an EP enhances sensing in the frame of a fiber ring cavity. An air gap inserted into a fiber ring cavity couples the clockwise and counterclockwise propagating modes, leading to eigenfrequency splitting. The EP was achieved in a nonreciprocity cavity constructed using an isolator. A fiber displacement sensor was realized by monitoring the splitting strength in the spectrum, which depends on the width of the air gap. The experimental results showed that the sensitivity of the sensor at the EP was sufficiently enhanced as compared to that in the reciprocal cavity.