Low-loss multilayer silicon nitride-assisted edge coupler on thin-film lithium niobate platform by photolithography

Thin-film lithium niobate (TFLN) has rapidly emerged as a highly promising platform for photonic integrated circuits (PICs) due to its unique and advantageous material properties. However, efficient coupling of light between optical fibers and integrated TFLN waveguides is still a challenge, hindering the TFLN platform's wider adoption and seamless integration into optical systems. Although many spot-size converter designs have been proposed to reduce the coupling loss, the minimum critical dimensions are patterned by electron beam lithography (EBL), which is not efficient for massive production. To address this critical challenge, we propose and experimentally demonstrate a novel, to the best of our knowledge, multilayer silicon nitride (SiN)-assisted SSC integrated onto the TFLN platform via photolithography. The use of multilayer SiN waveguides not only substantially enhances the SSC's robustness against manufacturing tolerance but also allows for greater design flexibility of the mode field diameter. The coupling efficiency can be maintained over 85% even under the TFLN waveguide width deviations of ±200 nm and height variations of ±30 nm. The fabricated SSC achieves a coupling loss of 1.2 dB per facet and a wavelength-dependent loss below 0.4 dB per facet across the entire C-band. The misalignment of the coupling loss increase of 3 dB is ±3 µm, which greatly increases the tolerance and stability of packaging. This work presents a practical, robust, and fabrication-friendly solution towards the TFLN PICs.