Engineered mode coupling in high-Q microresonators enables deterministic low-repetition-rate soliton microcombs

Low-repetition-rate soliton microcombs (<50 GHz) are essential for applications such as dense wavelength division multiplexing (DWDM), dual-comb spectroscopy, and microwave photonics. However, their realization is hindered by thermal instability, which becomes more severe in long-cavity microresonators requiring high pump power. While dual-mode pumping provides an effective route for thermal compensation, its implementation in low-repetition-rate systems is limited by restricted flexibility in simultaneously engineering the coupling to both modes. Here, we demonstrate a passive approach based on engineered intermodal coupling in racetrack microresonators. By using Euler-bend geometries, moderate coupling between the fundamental and auxiliary modes enables efficient thermal stabilization. We experimentally demonstrate deterministic single-soliton generation in a high-Q (> 10⁷) Si₃N₄ microresonator with a repetition rate of 33 GHz and broadband spectral coverage across the C and L bands. This approach provides a robust and scalable pathway toward low-repetition-rate soliton microcomb generation.