Researchers at Sun Yat-sen University and Nanjing University designed and fabricated a thin-film lithium niobate (TFLN) edge coupler that will allow TFLN-based periodically poled lithium niobate (PPLN) devices to achieve efficient, ultrabroadband coupling.

According to the researchers, the edge coupler increases the overall conversion efficiency of current PPLN devices by two to three orders of magnitude compared with current state-of-the-art devices. They said that TFLN-based devices will benefit from the improved coupling efficiency at the pump light and second harmonic wave.

TFLN is a promising platform for nonlinear optics because it has a high refractive index contrast with strong optical properties. However, due to the lack of an efficient broadband coupling scheme, the overall and the on-chip second-harmonic generation (SGH) normalized efficiencies (i.e., fiber-to-fiber) in TFLN-based PPLN devices are too low for many applications.

To address this issue, the researchers designed a coupler that works efficiently at both the 1550-nm (near-infrared) and 775-nm (near-visible) wavelengths.

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A high-efficiency, ultrabroadband edge coupler introduced by a multi-institutional team in China reduces power consumption during the process of nonlinear frequency conversion. The team expects that its work will expand practical use of TFLN-based PPLN devices. The three-dimensional structure of the x-cut edge coupler, consisting of a suspended SiO₂ waveguide and a tri-layer spot size converter, is depicted. Courtesy of Liu et al., doi 10.1117/1.APN.1.1.016001. 

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The coupler consists of a suspended silicon dioxide (SiO₂) waveguide with supporting arms and a spot size converter (SSC) comprising top-, middle-, and bottom-layer tapered waveguides. Light from lensed fiber is coupled into the SiO₂ waveguide and then transferred to the TFLN-rib waveguides through the SSC.

The coupler features low coupling loss and ultrabroad bandwidth. It allows coupling of 1 dB per facet at the 1550-nm band and 3 dB per facet at the 775-nm band.

The researchers found that the experimental coupling efficiency of the coupler in the near-visible light band was 3 dB higher than the simulated efficiency of a traditional two-layer structure. Conventional two-layer couplers do not perform well in the near-visible band because the refractive between the cladding waveguide and the SSC structure is mismatched. The edge coupler’s tri-layer SSC corrects the coupling issues encountered by a conventional two-layer coupler structure at short wavelengths.

Using the ultrabroadband edge coupler in a 5-mm-long PPLN waveguide, the researchers demonstrated an ultrahigh overall SHG normalized efficiency (fiber-to-fiber) of 1027% W⁻¹ cm^− 2, and a corresponding high on-chip SHG normalized efficiency of 3256% W⁻¹ cm⁻².

To date, it is possible to achieve high coupling efficiency in TFLN-based devices at the conventional, or C-band — but an efficient edge coupler that can cover both near-infrared and near-visible wavelengths has not been developed until now.

“Increased fiber-to-fiber SHG efficiency is a critical aspect of almost all photonics demonstrations,” said Sun Yat-sen professor Xinlun Cai. “It is of particular significance to nonlinear and quantum photonic chips, which are often touted as appropriate for use in next-generation photonic systems but suffer from very high coupling losses.”

The high-efficiency, ultrabroadband edge coupler also reduces power consumption during the process of nonlinear frequency conversion, and the team expects that its work will expand practical use of TFLN-based PPLN devices.

PPLN waveguides are used in wavelength conversion, optical parametric oscillation, photon pair generation, and supercontinuum generation.

The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.1.1.016001).

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