In this work, we propose a guideline to design the high Q-factor ring resonators. To keep the optical energy in the ring, the radiation loss of the ring should be reduced. The difference of the propagation constant between the bus waveguide and the ring should be enlarged to prevent from the light in ring to be coupled back to the bus waveguide. The phase difference of the light between the bus waveguide and ring should be adjusted to obtain the destructive interference at the output port (critical coupling). From these points mentioned above, we compare the 6 types of ring resonators. We conclude that the high Q-factor pulley-type ring resonator can be more easily designed. The experimental Q-factor of the pulley-type ring resonators with ring radius of only 4.43μm can be obtained up to 1.73×10~5.
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