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 (${\chi }^{\left(3\right)}$) and is difficult to extend to the visible or shorter wavelength regime. Second-order nonlinearity (${\chi }^{\left(2\right)}$), which is orders of magnitude larger than ${\chi }^{\left(3\right)}$, 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 ${\chi }^{\left(2\right)}$-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.