Controlling resonance lineshapes of a side-coupled waveguide-microring resonator

Resonance lineshapes of a side-coupled waveguide-microring resonator (MRR) is crucial for the performances of MRR-based on-chip photonic devices. Much efforts have been made to modify the resonance lineshapes to other types, such as asymmetric Fano profiles. However, complex photonic structures are required to integrate with waveguide-MRR. Here, we model the light propagation in a waveguide-MRR into the interactions of a discrete resonance mode and a continuum waveguiding mode and propose the phase delay between the two states plays great roles in controlling the resonance lineshape into symmetric Lorentzian dips, Lorentzian peaks, and Fano lineshapes with arbitrary asymmetric factors. We experimentally verify this by fabricating silicon waveguide-MRR with an air-hole inserted in the bus-waveguide section coupled with the MRR, where the air-hole with varied dimensions could control the phase delay. The results not only have potentials to strengthen the performances of MRR-based devices, but also provide a simple strategy to control resonance lineshapes in other optical resonators, including photonic crystal cavity, microtoroid, etc.