Leaky-mode long-period grating on a lithium-niobate-on-insulator waveguide

Lithium niobate on insulator (LNOI) is a promising platform for the construction of high-density integrated photonics circuits. In this paper, we propose compact long-period gratings (LPGs) formed on LNOI rib waveguides for achieving light coupling from the guide mode to the leaky mode. Our LPGs are polymer gratings coated on the surfaces of LNOI waveguides and can be designed to achieve strong mode coupling at specific resonance wavelengths for the TE-like and the TM-like polarization. By means of a numerical analysis, we find that the LPGs exhibit turning points in the phase-matching curves, which promises the realization of broadband wavelength filters and polarizers with such grating structures on the LNOI platform. To experimentally verify the theoretical results and demonstrate the LPG characteristics, we fabricate a number of LPG filters on LNOI rib waveguides, which include two broadband LPGs operating near the turning points: a TE-pass polarizer with a 20 dB extinction ratio over a bandwidth of 270 nm from 1430 to 1700 nm and a TM-pass polarizer with a 20 dB extinction ratio over a bandwidth of 90 nm from 1500 to 1590 nm. The insertion losses for the passing polarization are smaller than 2 dB in the C + L band. Our LPGs can serve as building blocks for the development of a wide range of integrated LNOI devices for application in on-chip signal processing.