Mode control for microring resonators with inner-wall gratings

Abstract

Microring resonators with angular gratings distributed along the inner wall are proposed to achieve single-mode microlasers. Based on a numerical investigation using the finite element method, we find that the inner-wall gratings create a periodic perturbation and couple the whispering-gallery modes into low-angular-momentum components, resulting in an enhancement of radiation loss. By properly arranging the inner-wall grating protrusion number as a nontrivial divisor of 2ν, we can have the mode with azimuthal mode number ν as the only high Q mode for single-mode operation. The impacts of grating depth, microring width, and grating duty cycle are investigated by 2D simulation for the optimization of mode coupling and radiation loss. Finally, the mode selection mechanism is verified based on a 3D simulation.