Compact Optical Delay Line with Large Group Delay Enabled by Photonic Crystal on Thin-Film Lithium Niobate

Optical time delay lines constitute key building blocks for realizing true time delay functions in optical communications and microwave photonics. However, conventional optical time delay lines based on thin-film lithium niobate typically suffer from limited group delay density and large device footprints, hindering their integration in compact photonic circuits. In this paper, we demonstrate a compact optical delay line on thin-film lithium niobate, employing defect-engineered one-dimensional photonic crystal waveguides to achieve an exceptionally large group delay. Our experimental results reveal a peak group delay density of 27.80 ± 0.69 ps/mm at specific wavelengths, representing a four-fold improvement over conventional thin-film lithium niobate rib waveguides, while achieving a 75% reduction in footprint. Over a broad 18-nm bandwidth, the device maintains an average group delay density of 17.68 ± 0.46 ps/mm, still significantly higher than that of conventional designs.