Ultra‐Compact and High‐Speed Thin‐Film Lithium Niobate Tunable Optical Delay Lines

High‐speed tunable delay lines are vitally important in various applications. Thin‐film lithium niobate (TFLN), with its excellent electro‐optic effect, holds great potential for applications in fast‐switching tunable delay lines. However, its relatively weak optical mode confinement typically necessitates large bending radii, posing challenges for high‐density photonic integrated circuits. Here, based on TFLN platform, high‐speed and ultra‐compact tunable delay line is demonstrated that leverages ultra‐low‐loss broadened photonic waveguides and multi‐mode multiplexing techniques. Here, three guided‐modes within the multimode delay line are involved to achieve cumulative delays in a compact area. A 5‐bit tunable delay line is successfully designed and fabricated with maximum delay of 9.92 ns delay, a delay step of 320 ps, and a switching speed of 32 ns. The fabricated device features a compact footprint of 1.5 cm2, with measured transmission losses of 0.2, 0.4, and 0.8 dB cm−1 for the three guided‐modes. This work paves the way toward scalable, low‐loss, and high‐speed tunable delay lines for advanced integrated photonic applications.