Long-period grating on thin-film lithium-niobate waveguide for polarization mode conversion

We design and fabricate a long-period grating (LPG) on a thin-film lithium-niobate (TFLN) rib waveguide for achieving effective conversion between two orthogonally polarized modes. Our experimental device, which employs a 330-µm long dual SiO 2 grating structure, achieves a mode conversion efficiency of ~99% at 1550 nm with a 10-dB bandwidth of ~20 nm, in good agreement with our theoretical results. This compact polarization mode converter, or polarization rotator, can serve as a basic component in TFLN-based photonic circuits. Our study extends the conventional functions of LPGs as mode converters for the same polarization and thus opens up new possibilities in the development of polarization-controlling devices for applications in high-density integrated photonics.