Trident edge coupler on thin-film lithium niobate for optimized coupling of octave-separated wavelengths for nonlinear applications

Abstract

We introduce a trident edge coupler design optimized for the simultaneous coupling of two widely separated wavelengths (2 µm and 1 µm) between a lensed fiber and a 600-nm-thick X-cut lithium-niobate-on-insulator (LNOI) waveguide. These wavelengths are commonly encountered in nonlinear wave mixing applications, representing either the fundamental and second harmonics in second harmonic generation (SHG) processes or the leading and trailing edges of an octave-spanning spectrum generated through broadband nonlinear processes such as frequency comb or supercontinuum generation. Achieving efficient coupling between fibers and strongly confined waveguides in integrated platforms, such as LNOI, can be challenging due to the significant difference in spot sizes between the two wavelengths. Our trident edge coupler offers coupling losses below 1.4 dB for the 2 µm and 1 µm spots simultaneously, showcasing an average transmission enhancement of around 10% compared to the baseline of a single linear taper. Furthermore, it enables a reduction of transmission at 1.5 µm, a typical pump wavelength, with an attenuation of transmission over 10 dB compared to those at the 2 µm and 1 µm wavelengths.