In this paper, we theoretically show, using a generic model based on the universal description of the device spectral characteristics and a coupled-mode theory treatment of the coupling section, that access-waveguide-coupled resonators can exhibit a wideband critical-coupling bandwidth when their constitutive resonator and access waveguides are different i.e. when they are phase-mismatched. To illustrate this, we have calculated the spectral response of Si3N4/SiO2 racetrack resonators and have found that, when the coupler beat-length becomes achromatic, the device critical-coupling bandwidth is expanded by more one order of magnitude compared to their phase-matched counterpart.
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C. Arlotti, A. Monmayrant, O. Gauthier-Lafaye, N. Gutierrez, A. Fernandez, O. Llopis, S. Calvez, "Wideband critically-coupled resonators," Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105180I (16 February 2018);