Ultrahigh-Q lithium niobate microring resonator with multimode waveguide.

Difficulty in etching lithium niobate (LN) results in a relatively high propagation loss, which necessitates sophisticated processes to fabricate high-quality factor (Q) microresonators. Here, we fabricate a multimode microring resonator with an intrinsic Q of 6 × 10, which exhibits a propagation loss 50 times lower than that of a single-mode LN microring fabricated under the same process. Notably, the excitation of higher-order modes in the multimode microring is effectively suppressed by utilizing the Euler bend. The highly regular transmission spectrum of the resonator demonstrates a free spectral range (FSR) of 56 GHz. Based on this microresonator, we implement a bandpass microwave photonic filter with an ultra-narrow 3 dB bandwidth of 47.5 MHz and a large tuning range of 2-26.5 GHz. It can be anticipated that the combination of existing advanced etching techniques with this work will drive the propagation loss of a LN waveguide closer to the material absorption loss, significantly facilitating the optimization of performance in applications requiring ultrahigh-Q LN microresonators, such as frequency combs, frequency conversion, electro-optic modulation, and quantum photonics.