| Summary: | By selectively breaking parity-time (PT) symmetry for the specific whispering-gallery mode, the single mode lasing has been experimentally demonstrated in a single microring resonator. In this paper, we analyze the modal discrimination to show the robustness of the single mode operation in two different types of PT symmetric single microring lasers: one with the gain/loss grating, and the other with the gain/index/loss grating. We show that they both can exhibit much higher modal discrimination than the conventional microring resonator. The single mode operation is robust even if the gain and loss distribution is quasi-symmetric, i.e., the unbalanced perturbation amplitudes or duty cycles. Therefore, the PT symmetric single microring lasers can provide very stable single-mode operation.
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