Circular Bragg resonators (CBRs) are analyzed in both the frequency domain and the time domain based on the scattering matrix method and the numerical model. The CBR with the same size as a dielectric ring can be designed to have denser resonant mode distributions in the frequency domain, and the expansion of the slow light band is imposed by the combination of multiresonant modes. Thus the expansion is independent of group velocity and is not limited by the delay-bandwidth product constraint in static photonic structures, which is deduced for a single resonant mode. Hence, the CBR can store more bits than a dielectric ring. For certain parameters, clockwise (CW) and counterclockwise (CCW) modes in the CBR are quite sensitive to dielectric perturbations, which are weak enough that they have little effect on the CW mode and CCW mode in a dielectric ring. When light propagates along a line waveguide coupled with the CBR, and if there are weak dielectric perturbations in the CBRs, extraordinary reflections could be produced and there exists strong coupling and conversion between CW and CCW modes in the CBR. The optical property indicates that extremely weak dielectric perturbations in the CBR play an important role in mode conversion. These unique properties of CBRs may find applications in the design of practical optical delay line buffers, and they also provide a new method to achieve light control by mode conversion in passive optical resonators.
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