Multi-photon lithography of strip-loaded thin-film lithium niobate waveguides

Photonic integrated circuits represent a promising technology for creating compact and cost-effective optical devices with multiple functions on a single chip. The advancement of integrated photonics is driven by research on new material platforms and production techniques. Lithium niobate (LN) is particularly attractive due to its advantageous optical properties like nonlinearity and electro optic effect. Integrated LN waveguides have been historically fabricated using multi-step methods, including lithographic patterning and dry etching. Additive manufacturing, in contrast, allows for rapid single-step production. This paper presents a novel etchless fabrication process using multi-photon lithography to create strip-loaded waveguides on thin-film LN. It demonstrates the reusability of thin-film LN substrates by erasing and reprinting strips, promoting rapid and sustainable photonic chip production. The study includes strip widths between 1 and 5 µm and thin-film layer thicknesses of 300, 600 and 900 nm, with experimental data supported by numerical simulations. We report propagation losses below 1 dB/cm at 1550 nm. Additionally, advantages and disadvantages of different strip-loaded LN waveguide designs for photonic packaging, e.g. higher coupling losses for smaller LN layer thickness, are discussed.