Wavelength-Selective Single-Mode Laser by Joint Use of Parity-Time Symmetry and Vernier Effect in Symmetric and Asymmetric Coupled Microrings

The Control of the gain and loss in microcavities, especially microrings, and discs, plays a crucial role in harnessing the excited modes in these types of microlasers. In multi-microring configurations, the Vernier effect eliminates various excited modes within the system, while simultaneously strengthening and producing many latitudinal modes. The implementation of the spectral Vernier effect involves the utilization of two coupled asymmetric microrings to enhance the effective free spectral range (FSR). By carefully balancing the gain/loss ratio and breaking parity-time symmetry, these latitudinal modes can be extensively suppressed, resulting in the single-mode lasing operation. Here, we have successfully demonstrated that wavelength-selective single-frequency lasing can be achieved in structures consisting of two coupled size-mismatched microrings by simultaneously employing the Vernier effect and broken parity-time (PT) symmetry. By controlling the gain/loss ratio through pump variations, the gain can adapt to changes in single-mode lasing wavelength, while the loss effectively suppresses numerous excited modes, leaving only a dominant mode. Thanks to gap engineering, the single-mode lasing based on broken parity-time symmetry at different wavelengths occurs by changing the air gap between two symmetric microrings. The proposed structures have been thoroughly analyzed both experimentally and numerically. Our findings can contribute to a deeper understanding of the spectral modulation process in coupled microrings lasers, offering valuable insights for future research in this field.