Breaking dense integration limits: inverse-designed lithium niobate multimode photonic circuits

Despite the growing interest in thin film lithium niobate (TFLN) as a material platform for photonic integrated circuits (PIC), its moderate refractive index, CMOS-incompatible fabrication processes and inherent material anisotropy still raise questions about achieving dense integration comparable to mature platforms like silicon photonics while preserving the superior properties of lithium niobate. Here we show a photonic inverse design method to enable miniaturization and dense integration of lithium niobate PIC components. As proofs-of-concept, we experimentally demonstrate ultra-compact mode-division (de)multiplexer (19 × 25 μm²), multimode waveguide crossing (15 × 15 μm²), and waveguide bends (30 μm bending radius). The fabricated components are used to construct multimode photonic circuits for large-capacity data communications, demonstrating dense integration of over 10 waveguide elements within a 0.06 mm² chip area. By integrating electro-optic modulators on the same chip, high-speed data modulation is demonstrated with 120 Gbps data rate per channel alongside multimode signal transmission. This work is expected to advance 10-fold higher area density of passive components and optical path design in TFLN.