ABSTRACT Microresonators are essential in integrated photonics, enabling optical filters, modulators, sensors, and frequency converters. Their spectral response is governed by bus‐to‐resonator coupling, typically classified as under‐, critical‐, or over‐coupling. Conventional single‐bus designs inevitably link the conditions for critical coupling, a transmission zero, and maximum intra‐cavity power, preventing independent control of these phenomena and restricting the ability to engineer coupling regimes and resonance lineshapes. Here we propose and experimentally demonstrate a dual‐bus racetrack resonator that breaks this constraint. Our design demonstrates complementary channel‐specific coupling regimes and enables wavelength‐dependent Lorentzian‐to‐Fano lineshaping. We model the device using three‐waveguide coupled‐mode theory and pole‐zero analysis, which reveals that transmission zeros are decoupled from cavity‐defined critical coupling and maximum intra‐cavity power. Furthermore, the dual‐bus scheme operates broadband, spanning visible to mid‐infrared across all four transmission channels, highlighting its spectral richness and platform independence. These results establish a general framework for multi‐port spectral engineering in integrated photonics, with broad implications for tunable filters, modulators, sensors, and nonlinear optical systems.
Kim et al. (Fri,) studied this question.