Resonant Mode Characteristics of Microfiber Knot-Type Ring Resonator and Its Salinity Sensing Experiment

The microfiber knot-type ring resonator is widely used in sensing, and the resonant mode is an essential concern. In this paper, resonant mode characteristics of microfiber knot-type ring resonator are studied theoretically. Results show that resonant mode split occurs when the deviation angle is large enough under the condition that the input is linearly or elliptically polarized light. Moreover, in the case that the input is partially polarized light, the resonant peak positions of split modes are identical with those in the case of inputting linearly or elliptically polarized light under the same deviation angle. The split mode characteristic is further studied by salinity sensing simulation and experiment. The split modes have the same free spectral range, and their resonant peak shifts have the same response to salinity changing. The experimental split mode characteristic is in good agreement with theoretical simulating, which demonstrates the resonant mode split theory of the microfiber knot-type ring resonator. The analysis on resonant mode presented here may offer useful references for sensing applications.