Advances in Fiber-to-Chip Coupling and Cryogenic Packaging Technologies for Thin-Film Lithium Niobate Photonic Chips

Thin-film lithium niobate (TFLN) has emerged as a core material platform for next-generation integrated photonics, owing to its strong electro-optic effect (γ₃₃ ≈ 30 pm/V), second-order nonlinear optical coefficients, and broad transparency window spanning from the ultraviolet to the mid-infrared. However, the significant mode-field mismatch between sub-micrometer-scale lithium niobate waveguides and optical fibers renders efficient fiber-to-chip coupling a critical bottleneck for practical applications. This challenge is further exacerbated in cryogenic environments, where thermally induced stress causes misalignment at the coupling interface.
This paper systematically reviews the operating principles, performance metrics, and recent progress of the two mainstream coupling schemes on the TFLN platform—edge couplers and grating couplers. For edge coupling, structures including bi-layer inverse tapers, tri-layer inverse tapers, three-dimensional inverse tapers, trident structures, wedge-shaped couplers, double-layer metasurfaces, and multilayer silicon nitride-assisted couplers have achieved coupling losses below 0.5 dB/facet, with bandwidths covering the C+L bands and even extending into the visible range. For grating coupling, chirped gratings, apodized gratings, suspended gratings, hybrid silicon-platform gratings, and vertical gratings with bottom metal mirrors have attained sub-dB coupling losses. This paper also summarizes recent advances in cryogenic packaging technologies and provides an outlook on future development directions.