All-Euler-bend lithium niobate microresonators with intrinsic Q-factors exceeding 10 million

We report an ultra-low-loss thin-film lithium niobate microresonator composed of all-Euler-bend geometries. This design allows the use of a multimode waveguide to reduce sidewall scattering loss while suppressing higher-order-mode excitation. Unlike racetrack designs, our microresonator contains no intermediate straight sections, yielding a lithography-friendly footprint of 0.9 mm × 1.7 mm that supports a free spectral range of ∼28.5 GHz and a high intrinsic quality factor of up to 1.2×10⁷. By self-injection locking a distributed-feedback laser to the resonator, we obtained an integrated linewidth of 45.6 kHz within 100 ms, showing a 305-fold improvement over the free-running laser. We further observed low-noise Raman frequency combs at boosted pump powers. These results highlight the potential of all-Euler-bend microresonators for integrated photonic applications.