Spectral engineering of the transmission characteristics of racetrack resonators

Integrated whispering gallery mode resonators constitute a family of compact devices which has been shown to be attractive to perform selective filtering, low-threshold emission or nonlinear conversion functions [1]. In the latter case, and for Kerr frequency comb generation in particular, the performance is controlled by the ability to achieve critical coupling over a wide spectral range. In this paper, we study the influence of the coupler design on the spectral response of verticallycoupled racetrack resonators and draw out design rules to achieve wideband critical coupling.