Anisotropic Thermo‐Optic Mach–Zehnder Interferometer on LNOI for Polarization Handling and Multiplexing

Abstract Thin‐film lithium niobate (LN) on insulator (LNOI) is emerging as a promising integrated photonic platform. Thermo‐optic (TO) Mach–Zehnder interferometers (MZIs) on LNOI are demonstrated to be a viable and often preferred alternative to their electro‐optic counterparts for low‐speed and static applications, where stability and repeatability are crucial. Harnessing the unique and strong anisotropic TO effect of LN, a novel and versatile anisotropic TO MZI on x ‐cut LNOI for polarization handling and multiplexing is proposed and experimentally implemented. Each MZI arm consists of a two‐section anisotropic TO phase shifter along the y ‐ and z ‐directions of the LN crystal, leading to anisotropic temperature dependence of the effective refractive indices for the transverse‐electric (TE) and transverse‐magnetic (TM) polarizations. A polarization‐insensitive switch and a polarization beam splitter are implemented with the MZI device, which features low excess loss of ≈0.2–1.8 dB and a high extinction ratio of ≈20–45 dB in the telecom C‐band (1530–1565 nm). More intriguingly, arbitrary splitting ratios and even arbitrary combinations of unitary transmission matrices can be further implemented for both polarizations simultaneously. The versatile configurations of this anisotropic TO MZI have important implications for large‐scale photonic computing and interconnect.