Large free spectral range, high Q-factor microring resonator in thin-film lithium niobate photonics.

A microring resonator (MRR) with a large free spectral range (FSR) and a high quality factor (Q-factor) is highly desirable in many fields, such as optical communication and sensing. However, realizing such an MRR in thin-film lithium niobate (TFLN) photonics is a significant challenge due to the low refractive index contrast between the shallow-etched core waveguide and the cladding. Here, we experimentally demonstrate that such an MRR can be successfully achieved in TFLN photonics by implementing wide MRR waveguides and weakly tapered gap waveguides, along with optimizing the coupling ratio between the bus waveguide and the MRR. The proposed all-pass MRR features an FSR of 6.5 nm and a Q-factor of 3.11 × 104. Moreover, using the same method, we also experimentally demonstrate an add-drop MRR with an FSR of 4.2 nm and a Q-factor of 6.69 × 105.