Sensitivity enhancement at higher order exceptional point based on coupled whispering gallery modes in microstructured optical fibers

Abstract

Sensitivity enhancement at higher order exceptional point (EP) was demonstrated based on coupled whispering gallery modes in microstructured optical fibers. Nonlinear wavelength shift response near the third-order EP (EP(3)) could be obtained at four typical refractive indices (RIs) by exploiting the adjustable RI feature of functional materials. The maximum sensitivity near EP reaches about 1.5 × 104 nm/RIU, and the sensitivity at EP approaches infinity. Besides, the sensing characteristics of a second-order EP system and a diabolic point system are compared with the EP(3) system for the same RI measurement range around 1.45 and the calculation results indicate that EP(3) system experiences an increase in RI sensitivity around EP by 6 and 10 times, respectively. In addition, tuning the real part of the medium RI of gain & loss microcavities helps to achieve tuning of the sample material RI with high sensitivity. Finally, another orthogonal state of the eigenvectors has been found by analyzing the eigenvector evolution from Hermitian regime to EP state for the EP(3) system and a lower limit of detection could be obtained as well. These finds would be of great significance for noise suppression in sensing applications based on interrogation of eigenfrequency splitting near higher order EPs.