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

We present low-loss ($<1.5\mathrm{\%}$) and power-efficient Mach-Zehnder interferometers (MZIs) on thin-film lithium niobate. To accurately measure low MZI losses, we develop a self-calibrated method using tunable Sagnac loop reflectors (SLRs) to build cavities. Fabry-Pérot cavities constructed from these fabrication-tolerant SLRs achieve an intrinsic quality factor of $2\times {10}^6$. By implementing thermal isolation trenches, we also demonstrate a $>10\times$ reduction in power consumption for thermo-optic phase shifters, achieving a $\pi$-phase shift ($P_{\pi }$) with just 2.5 mW. These tunable and efficient components are key for scaling up to complex photonic integrated circuits.