Highly efficient second-harmonic generation in TFLN via compact-footprint spiral PPLN waveguides

Extending the interaction length in thin-film lithium niobate waveguides is a conventional approach to enhancing nonlinear conversion efficiency. However, this strategy is severely limited by phase mismatch induced by intra-wafer thickness nonuniformity. Here, we demonstrate a compact spiral periodically poled lithium niobate (PPLN) waveguide that overcomes this bottleneck. This device tightly folds a long nonlinear interaction path within a sub-millimeter localized space, fundamentally circumventing the constraints imposed by the waveguide length and large-scale intra-wafer thickness gradients. Without the need for complex passive structural compensation or active phase tuning, this design achieves highly consistent quasi-phase matching (QPM) globally. Experimental results show that under low-power and high-power pumping conditions, the device achieves a high normalized conversion efficiency of 3723%/W and an absolute conversion efficiency of 26.75%, respectively. This work realizes ultrahigh nonlinear conversion efficiency within a minimal device footprint, not only overcoming the performance degradation typical of traditional long straight waveguides, but more importantly, offering a highly fabrication-tolerant geometry suitable for scalable wafer-level manufacturing. It provides a scalable and highly manufacturable solution for future high-density on-chip photonic integration and quantum light source networks.