Fig. 1. Physical model of standard ding-shaped microring resonator

Fig. 2. The model of the coupler

Fig. 3. Electric field distribution and transmission spectrum of standard ding-shaped microring resonator

Fig. 4. Electric field distributions under different coupling regions

Fig. 5. Output spectra under different numbers of coupling regions. (a) A pair of coupling regions; (b) two pairs of coupling region; (c) three pairs of coupling region