Thin-film lithium niobate (TFLN) has recently emerged as a versatile nanophotonic platform. With the advantages of high optical confinement, enhanced light-matter interaction, and flexible dispersion control, TFLN-based periodically pooled lithium niobite (PPLN) devices outperform their older counterparts in both nonlinear optical efficiency and device footprint.
A major challenge of TFLN-based PPLN devices is to realize an efficient and broadband off-chip link. Due to the lack of an efficient broadband interconnection scheme, the general and on-chip second harmonic generation (SGH) normalized efficiencies (fiber-to-fiber) are too low for many practical applications of TFLN-based PPLN devices. To date, it is possible to achieve high coupling efficiency on the C-band, but an efficient edge coupler that can cover both near-infrared (~1550nm) and near-visible (~775nm) wavelengths has been until now not yet developed.
As reported in Advanced Photonics Nexus, researchers from Sun Yat-sen University and Nanjing University have designed and fabricated an ultra-wideband and efficient TFLN edge coupling. They found that the conventional two-layer coupler does not work well in the 775 nm band, due to the refractive index mismatch between the mantle waveguide and the spot size converter (SSC) structure.
To address this problem, they designed an efficient coupling that operates at both 1550nm and 775nm. It consists of a hanging SiO2 waveguide with wishbones and a three-layer SSC, including tapered top, middle and bottom layers. The light from fiberglass with a lens is coupled to the SiO2 waveguide and then transferred through the SSC to the TFLN rib waveguides. The three-layer SSC solves the coupling problem of the conventional two-layer coupler structure at short wavelengths. The measured coupling loss is 1 dB/facet at 1550 nm and 3 dB/facet at 775 nm.
The work also demonstrates the advantages of the designed coupling in nonlinear applications. They achieve record high overall SGH normalized efficiency with a fiber-to-chip coupling scheme and high corresponding on-chip second harmonic efficiency. Compared to the state-of-the-art devices, the overall normalized efficiency is said to be two to three orders of magnitude greater.
Senior author Xinlun Cai, professor at Sun Yat-sen University’s School of Electronics and Information Technology, notes: “Increased fiber-to-fiber SHG efficiency is a critical aspect of nearly all photonics demonstrations. It is of particular importance for nonlinear and quantum photonic chips, which are often touted as suitable for use in next-generation photonic systems, but suffer from very high coupling losses.” The team expects their work to expand the practical applications of TFLN-based PPLN devices.
Xiaoyue Liu et al, Ultra-broadband and low-loss edge coupler for highly efficient second harmonic generation in thin-film lithium niobate, Advanced Photonics Nexus (2022). DOI: 10.1117/1.APN.1.1.016001
Quote: Research team develops ultrabroadband edge coupler for highly efficient second harmonic generation (2022, June 30) Retrieved June 30, 2022 from https://phys.org/news/2022-06-team-ultrabroadband-edge-coupler-highly.html
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