Octave‐Spanning Second‐Harmonic Generation in Dispersion‐Engineered Lithium Niobate‐on‐Insulator Microwaveguide

Broadband lasers, e.g., ultrashort lasers, optical supercontinuum, and frequency combs, are revolutionary coherent light sources, which enable a plethora of state‐of‐the‐art applications ranging from precision spectroscopy to optical clocks. However, the spectral broadening of these coherent light sources mainly relies on the third‐order nonlinearity () and is difficult to extend to the visible or shorter wavelength regime. Second‐order nonlinearity (), which is orders of magnitude larger than , becomes a powerful tool for the frequency translation if its broadband operation is well addressed. Herein, an octave‐spanning second‐harmonic generation scheme is experimentally demonstrated beyond an extremely large frequency range of 135 THz and high conversion efficiency of 1% for sub‐100 pJ for the near‐infrared picosecond supercontinuum in a fiber–waveguide–fiber configuration. The process relies on ultrabroadband birefringence phase matching in the dispersion‐engineered lithium niobate‐on‐insulator ridge microwaveguide. The mode area of microwaveguide well matches with single‐mode lens fiber, reducing coupling loss and ensuring easy packaging. The method provides a new approach to span the wavelength range of coherent light with ‐based wavelength translation for supercontinuum or frequency combs into the visible regime. The result would find applications in spectroscopy, astrophysics, atomic optics, optical synthesis, etc.