Thin-film lithium niobate (TFLN) is a versatile nanophotonic platform that has recently emerged. TFLN-based periodically poled lithium niobite (PPLN) devices outperform their legacy counterparts in both non-linear optical efficiency and device footprint due to their advantages of high optical confinement, enhanced light-matter interaction, and flexible dispersion control.
How to achieve efficient and broadband off-chip coupling is a major challenge for TFLN-based PPLN devices. The overall and on-chip second-harmonic generation (SGH) normalised efficiencies (fiber-to-fiber) of TFLN-based PPLN devices are too low for many practical applications due to the lack of an efficient broadband coupling scheme. To date, high coupling efficiency at the C-band has been achieved, but an efficient edge coupler that can cover both near-infrared (1550 nm) and near-visible (775 nm) wavelengths has not been developed.
Sun Yat-sen University and Nanjing University researchers designed and built an ultrabroadband and efficient TFLN edge coupler, according to Advanced Photonics Nexus. Due to a refractive index mismatch between the cladding waveguide and the spot size converter (SSC) structure, they discovered that the conventional two-layer coupler does not work well in the 775-nm band.
To address this issue, they created an efficient coupler that operates at both 1550 and 775 nm. It is made up of a suspended SiO2 waveguide with supporting arms and a tri-layer SSC with top, middle, and bottom tapers. Light from the lensed fibre is coupled into the SiO2 waveguide before being transferred to the TFLN-rib waveguides via the SSC. At short wavelengths, the tri-layer SSC solves the coupling problem of the conventional two-layer coupler structure. At 1550 nm, the measured coupling loss is 1 dB/facet, and at 775 nm, it is 3 dB/facet.
The work also shows the benefits of the designed coupler in nonlinear applications. With a fiber-to-chip coupling scheme, they achieve a record high overall SGH normalised efficiency, as well as a high corresponding on-chip second harmonic efficiency. When compared to cutting-edge devices, the overall normalised efficiency is said to be two to three orders of magnitude higher.
Xinlun Cai, senior author and professor at Sun Yat-sen University’s School of Electronics and Information Technology, says, “Increased fiber-to-fiber SHG efficiency is critical in nearly all photonics demonstrations. It is especially important for non-linear and quantum photonic chips, which are frequently touted as suitable for use in next-generation photonic systems but suffer from extremely high coupling losses.” The researchers hope that their work will broaden the practical applications of TFLN-based PPLN devices.