Broadband light coupling and mode-order conversion on an etchless thin-film lithium niobate platform with bound states in the continuum.

Thin-film lithium niobate (TFLN) has emerged as a promising platform for photonic integrated circuits (PICs) due to its excellent optical properties. Coupled-waveguide devices are one of the most important passive components in PICs, which are used for light coupling or mode-order conversion to construct crucial photonic devices. However, conventional directional couplers usually suffer from high wavelength and structure sensitivity, which makes it challenging to achieve broadband and fabrication-tolerant optical coupling. Therefore, it is still urgent to increase robustness against wavelength variation and fabrication errors. Here, we experimentally demonstrated a TM0-TM0 optical coupler and a TM0-TM1 mode-order converter on an EpoCore-TFLN hybrid platform with bound states in the continuum based on stimulated Raman adiabatic passage. The measured crosstalks of the fabricated coupler and mode-order converter are lower than -15.81 dB and -15.34 dB, respectively, over a wavelength range of 1.50-1.63 µm. The present devices with ultrabroad bandwidth and great robustness are suitable for future on-chip optical interconnect systems.