Optical frequency combs (OFCs) are a set of teeth-shaped spectral lines with equidistant frequency spacing and phase coherence, which connects optical frequencies to radio frequencies with extremely precise frequency intervals, and have broad application prospects in high-precision metrology fields such as astronomical spectral calibration, optical atomic clocks, and frequency measurement. Significant spectral broadening via supercontinuum generation (SCG) represents a pathway to acquiring ultra-broadband coherent comb sources and also is crucial for achieving self-referencing and forming stable OFCs. The generation of a coherent octave-spanning supercontinuum allows for the determination of f_CEO through self-referenced f-2f interference, thereby enabling the realization of ultrastable frequency combs. Recent advances in integrated photonics offer an attractive avenue to implement compact or chip-integrated comb sources. However, prevalent methods based on nano-waveguides usually exhibit low output power and large coupling losses with fiber.

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. 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 second- and third-order nonlinearities. This work provides a compact and convenient solution for f-2f self-referencing frequency combs.

Fig. 1. (a) Schematic of LNOI micro-waveguides for SCG and frequency comb self-referencing. (b) SEM image of the waveguide cross section. (c) Optical microscopic image of the micro-waveguides.

Figure 2(a) presents the results of SCG and harmonic generation experimentally observed in a 2-cm-long MgO:LNOI microwaveguide. A 1550-nm femtosecond pulsed laser is injected into the waveguide from the left side through a lens fiber. The bright visible light is generated by SHG, SFG and THG processes in the waveguide. Figure 2(b) shows bright white light collected at the coupled fiber output. As can be seen from the supercontinuum spectrum in Fig. 2(c), the pulsed light has achieved sufficient spectral broadening in the waveguide. A wideband supercontinuum spectrum with a pulse energy of 628 pJ is experimentally obtained, covering the near-infrared to visible light bands, and the 30-dB spectral bandwidth exceeded two octaves (600-2500 nm).

Fig. 2. (a) Photograph of SCG experiments in the MgO:LNOI micro-waveguide. (b) Bright white light of SCG at the coupled fiber output. (c) Experimental SCG spanning two octaves.

This work is published in “Lithium niobate micro-waveguides for efficient supercontinuum generation and frequency comb self-referencing, Photonics Res. 13, 3332 (2025)”.