Novel Folded Structure TFLN Recycling Phase Modulator Enabling Large Low‐Vπ Bandwidth and Efficient Microwave–Optical Velocity Matching

Abstract Electro–optic phase modulators (PMs) are critical components for modern communication, sensing, and microwave photonics. A recycling PM has been demonstrated to effectively reduce the half‐wave voltage ( V π ) while exhibiting resonant behavior for the driven microwave signals [Nature 612, 252 (2022)]. However, the significant optical delay of the loop‐back waveguide reduces the low‐ V π bandwidth to 1.5 GHz. Herein, a novel folded structure is proposed for recycling PM fabricated on a thin‐film lithium niobate (TFLN) platform. The low‐ V π bandwidth can be expanded by significantly reducing the optical loop‐back delay. Additionally, efficient velocity matching can be achieved by aligning the time delay between the microwave and optical signals. The proposed folded recycling PM, with a 0.6 × 12 mm 2 footprint, is characterized by the lowest V π of 2–3 V in each modulation period across 3.5 to 40 GHz and a doubled low‐ V π bandwidth of 3 GHz. As an application demonstration, such a folded recycling PM is employed for tunable optical frequency combs (OFCs) generation. Flat OFCs consisting of 15, 21, and 13 comb lines are generated with flexible comb spacings of 29.6, 31.3, and 33 GHz, respectively. The proposed novel modulator can find distinct applications in communication, sensing, and microwave photonics.