High-Q Thin-film Lithium Niobate Asymmetric Microring Resonator with Different Etching Depths and a Multimode Interferometer Coupler

As a key device of photonic integrated circuits (PICs), the microring resonator (MRR) shows excellent performance in sensing, switching, laser, high-speed electric-optical modulation, generation of optical frequency comb, and wavelength division multiplex due to its unique properties in wavelength selectivity. In this paper, we propose an asymmetric MRR based on thin-film lithium niobite (TFLN) with different etching depths inside and outside the ring. The proposed microring with different etching depths can reduce the transmission loss which is one of the major limitations of the quality factor (Q factor) of the ring resonators. The key parameters of the MRR, such as the radius and the etching depths of the ring, the width of the single-mode waveguide (SMW), and the width and the length of the multimode interferometer (MMI) coupler are calculated and optimized by the 3-dimension finite-difference time-domain (FDTD) method. Using FDTD method, the bending loss of the ring and the coupling efficiency between the bus waveguide and the ring are also analyzed. For the X-cut LN-on-insulator (LNOI) wafer with a 600-nm-thick LN film, the optimal radius of the microring is 50μm, the inner and the outer etching depths of the microring are 300nm and 450nm, respectively. According to the simulation results, the bending loss of the microring can be reduced to less than 0.1dB/cm. Compared with the traditional ring structure, the proposed asymmetric microring resonator can significantly improve the Q factor with small device footprint. Such asymmetric MRR with superior performance can help improve the performance of on-chip devices and further promote the development of photonic integration devices.