Low-loss, highly tunable Sagnac loop reflectors and Fabry-Pérot cavities on thin-film lithium niobate

We present low-loss (<1.5%) and power-efficient Mach-Zehnder interferometers (MZIs) on thin-film lithium niobate. To accurately measure small MZI losses, we develop a self-calibrated method using tunable Sagnac loop reflectors (SLRs) to build cavities. Fabry-Pérot cavities constructed from these SLRs achieve an intrinsic quality factor of 2×10⁶. By implementing thermal isolation trenches, we also demonstrate a >10× reduction in power consumption for thermo-optic phase shifters, achieving a P_π of 2.5 mW. These components are crucial for scaling up complex photonic integrated circuits.