Coupling ideality of standing-wave supermode microresonators

• Min Wang, Yuechen Lei, Zhi-Gang Hu, Cheng-Hao Lao, Yuanlei Wang, Xin Zhou, Jincheng Li, Qi-Fan Yang, and Beibei Li
• received 02/01/2024; accepted 05/14/2024; posted 05/16/2024; Doc. ID 520601
• Abstract: Standing-wave supermode microresonators that are created through the strong coupling between counterpropagating modes, have emerged as versatile platforms for sensing and nonlinear optics. For example, these microresonators have shown potential in nanoparticle sizing and counting, as well as enhancing the single-photon optomechanical coupling rate of stimulated Brillouin scattering. However, it has been observed that the relation between the mode linewidth and on-resonance transmission of the split supermodes differs obviously from that of the non-split modes. This behavior is typically quantified by the coupling ideality (I), which remains inadequately explored for the standing-wave supermdoes. In this study, we theoretically and experimentally investigate the coupling ideality of standing-wave supermodes in a commonly employed configuration involving a SiO2 microresonator side-coupled to a tapered fiber. Our findings demonstrate that, even with a single-mode tapered fiber, the coupling ideality of the standing-wave supermodes is limited to 0.5, due to the strong backscattering-induced energy loss into the counterpropagating direction, resulting in an additional equivalent parasitic loss. While achieving a coupling ideality of 0.5 presents challenges for reaching over-coupled regimes, it offers a convenient approach for adjusting the total linewidth of the modes while maintaining critical-coupled conditions.