Asymmetric spectral responses of microring resonators are analyzed by modeling the coupling region as a two-input/two-output Mach–Zehnder interferometer (MZI). The asymmetric behavior is dictated by the phase difference between the pole and zero, which is a physical result of different optical losses on the upper and lower MZI arms. The coupling region is further modeled as a frequency dependent element by making the MZI arms asymmetric in length. From this model, it is easy to identify the impact on pole and zero magnitudes as well as relative phases, and thus predict the spectral amplitude response over the free-spectral range of the coupler MZI. We verify this model using experimental data and show physical evidence of the asymmetric spectral response and its origin.
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