Perpendicular coupler for whispering-gallery resonators.

ABSTRACT

In this Letter, we report on a perpendicular coupler (PC) for whispering-gallery resonators; it is a near field waveguide optimized for high coupling efficiency. The PC provides highly efficient tunneling coupling between the waveguide and microresonator without the need of a phase matching condition, and saves space for integration components. Compared to the Lorentz-shape in the transmission spectrum of the parallel coupler, the reflection spectrum of the PC shows an asymmetric Fano-shape near resonance. Furthermore, we demonstrate that the collection efficiency can be enhanced by near field scatterers, with a maximal efficiency of about 75%. Our simulations show that the PC is not sensitive to most parameters (including the refractive index of the waveguide), which makes the PC optimal for the application of whispering-gallery modes.