Tightly-Confined and Long Z-Cut Lithium Niobate Waveguide with Ultralow-Loss

Journal article, 2025

Lithium niobate (LN) is a promising material for complex photonic-electronic circuits with wide applications in fields like data communications, sensing, optical computation, and quantum optics. There is a great step toward LN photonic integrated circuits (PICs) with the development of dry etching for low-loss LN on insulator (LNOI) waveguides. However, the versatility of the LN waveguide platform for applications like (Formula presented.) nonlinear devices and passive phase sensitive components, has not been fully utilized. Two significant challenges are the difficulty of making highly confined ultralow-loss waveguides and overcoming the strong material birefringence. Here a fabrication technology is developed for an ultralow-loss, tightly-confined, dispersion-engineered LN waveguide. An ultra-low propagation loss of 5.8 dB/m is demonstrated in a decimeter-long LN spiral waveguide. This study is focused on Z-cut LN waveguides with TE mode to avoid the material birefringence. Aiming for (Formula presented.) nonlinear applications, it is demonstrated that the first all normal-dispersion (ANDi) based coherent octave-spanning supercontinuum frequency comb in integrated LN waveguide. This ultralow-loss Z-cut LN long waveguide might be useful in on-chip narrow linewidth lasers, optical delay lines, and parametric amplifiers.