Non-Hermitian tunable exceptional point system based on coupled fiber-optic Fabry-Pérot microcavities for broadband microwave signal generation

Abstract

Appearance of exceptional points (EPs) in photonics arises enormous attention as a result of the counterintuitive phenomena occurring around EP. In particular, adiabatically encircling EP contributes to accumulation of Berry phase, leading to mode switching and strong chirality. Therefore, it would be of significance to develop a photonic system with tunable EP states, which helps to implement versatile functionalities for various applications. Here we propose a non-Hermitian tunable system based on coupled fiber-optic Fabry-Pérot microcavities with distributed gain/loss to host quasi-static EP-encirclement and parity-time (PT) symmetry schemes. Analytical calculations for the case of EP encircling along the path in the vicinity of EP are compared with their counterparts on the boundaries of the parameter space based on quasi-adiabatic theory, revealing mode switching and chirality relevant to the system singularity. And moreover, tunable coupling between microcavities enables tunability of EPs under PT symmetry regime, based on which generation of broadband microwave signals ranging from 0 to 75GHz could be achieved and the maximum frequency tuning sensitivity up to the magnitude of 106GHz/RIU could be reached for specific refractive index ranges, providing clues for manipulating EPs in non-Hermitian fiber-optic systems and designing novel EP-based photonic devices.