Lithium niobate micro-waveguides for efficient supercontinuum generation and frequency comb self-referencing

Optical frequency combs (OFCs) and supercontinuum generation (SCG) facilitate a plethora of important applications in metrology, spectroscopy, optical clocks, etc. Recent advances in integrated photonics offer an attractive avenue to implement compact or chip-integrated comb sources. However, the prevalent method based on nano-waveguides usually exhibits low output power and large coupling losses during the SCG process. Here, we experimentally demonstrate over two-octave-spanning SCG with high output power in dispersion-engineered magnesium-oxide-doped lithium-niobate-on-insulator (MgO:LNOI) micro-waveguides using a high-repetition-rate femtosecond fiber oscillator laser. The micro-waveguide can deliver high-power SCG due to its excellent fiber compatibility, lower insertion loss, and higher optical damage threshold. The designed waveguide also allows for birefringence phase matched second-harmonic generation (SHG), realizing simultaneous f -2 f interference in a single-pass configuration. Measurement and stabilization of carrier-envelope offset frequency ( f CEO ) are achieved with over 36-dB signal-to-noise ratio for OFC self-referencing. Furthermore, a nonlinear envelope equation model is established to simulate the spectral broadening and frequency conversion process regarding cascaded χ (2) and χ (3) nonlinearities. This work provides a compact and convenient solution for f -2 f self-referencing frequency combs using photonic micro-waveguide technology.