High-efficiency spot-size converter for thin-film lithium niobate modulators

Lithium niobate on insulator (LNOI) electro-optic modulator has broad application prospects in rapidly developing largecapacity and high-density coherent optical communication systems, due to its small size, ultra-high bandwidth and low voltage. However, the high coupling loss currently limits its practical application. In this paper, in order to realize efficient coupling property, we propose a high-coupling-efficiency bilayer inverse tapered spot-size converter (SSC) that can be integrated with an LNOI modulator. The spot-size converter consists of three parts: a double-layer inverse tapered waveguide in LNOI, a straight LNOI-based waveguide and a SiON waveguide. We mainly consider end-surface coupling efficiency, abrupt loss and transmission efficiency. The simulation results show that the total coupling efficiency of the chip to the lensed fiber with a mode-spot diameter of 3.2 µm can achieve to 97.51%/96.3% for TE/TM light at a wavelength of 1550 nm. The coupler shows high coupling efficiency and good polarization independence. The 3dB alignment tolerance between fiber and coupler is estimated as ±1.4 μm/±1.4 μm in X/Z direction for TE light, and ±1.45 μm/±1.3 μm in X/Z direction for TM light. Our coupler shows relatively large alignment tolerances that makes the optical packaging of the LNOI modulator more reliable. The proposed high-efficiency spot-size converter in this work is of great significance for realizing the practical application of LNOI modulators.