Low-loss spot-size converter for thin-film lithium niobate photonic integrated circuits at 780nm with simple fabrication process

The advancements in integrated thin-film lithium niobate on insulator (LNOI) platform have significantly enhanced the performance of various integrated electro-optic devices, including modulators, external cavity diode lasers, and optical frequency comb generators. Additionally, the development of LNOI has facilitated applications at shorter wavelengths, due to its wide transparency window. The coupling efficiency between laser diodes (LDs) and LN chips is commonly enhanced by designing spot size converters (SSCs). However, achieving high-efficiency SSCs is more challenging at shorter wavelength due to the smaller mode area and the increased confinement of the optical field in LN waveguides. In this study, we present a spot size converter based on hybrid SiN-LN structure, for low-loss light coupling between a III– V gain chip and a LNOI waveguide at 780nm. The parameters of SiN waveguide, LN taper and SiO₂ spacing layer has been optimized in order to enhance the matching of effective refractive indices. The entire SSC structure can be fabricated with two steps of photolithography and etching, demonstrating high fabrication tolerance. Simulations indicate that the coupling losses between the output mode of the LD and the fundamental mode of the LN waveguide are 0.41dB/facet for TE mode and 0.55dB/facet for TM mode at a wavelength of 780nm. Our design is intended to offer efficient light coupling from LD to LNOI chips at short wavelength range, characterized by its simple process and high fabrication tolerance.