Noncritical birefringence phase-matched second harmonic generation in a lithium-niobate-on-insulator micro-waveguide for green light emission.

Lithium niobate on insulator (LNOI) holds great potential for frequency conversion, where a variety of high-performance nonlinear devices based on different structures has been demonstrated. Here, we report on second harmonic generation (SHG) in MgO-doped LNOI ridge micro-waveguides for efficient green light emission, via an exact type-I noncritical birefringence phase matching (BPM). The LNOI micro-waveguide has a cross section of ∼3×4 m, featuring low coupling loss with lens fiber. The normalized conversion efficiency from a continuous-wave (cw) pump to its second harmonic is measured to be 37%/Wcm in a single-pass configuration. The device shows both relatively high efficiency and a void of periodic poling, offering a potential solution for efficient and scalable green light sources and frequency converters.