Broadband-bandwidth coherent light sources, e.g., ultrashort lasers, supercontinuum generation (SCG), and optical frequency combs (OFCs), are critical in various fundamental physics and important applications, such as precision metrology, spectroscopy, and time keeping. Octave-spanning SCG and OFCs in the near-infrared (NIR) range have been achieved in highly nonlinear and photonic crystal fibers, as well as on various integrated photonics platforms. In the last decades, the SCG and OFC range has been well extended to NIR and longer wavelengths. But direct generation of broadband coherent emission in the short wavelength of visible or even UV range remains a huge challenge due to strong material dispersion. Until now, the spectral broadening still mainly relies on the third-order nonlinearity. A promising method for wavelength extension or frequency translation is using three-wave-mixing processes such as second-harmonic generation (SHG) to translate an existing SCG or OFC to its harmonic regime. The advantage is that it relies on nonlinear wave mixing mediated by the large second-order nonlinearity, which is orders of magnitude larger than the third-order nonlinearity. Still, dispersion renders it difficult to achieve efficient nonlinear conversion in a wide range for long interaction lengths. SHG generally exhibits tradeoff between the conversion bandwidth and efficiency, restricted by the stringent phase-matching condition. Thus, mitigating or solving the phase-matching problem is critical to the advancement of SCG and OFCs, as well as other nonlinear light–matter interaction in a broader manner.

Here, we show that lithium niobate-on-insulator (LNOI) ridge micro-waveguides, with proper dispersion management, offers an approach for engineering broadband phase matching and high efficiency of three-wave mixing processes in a compact and natively fiber-compatible manner. We experimentally obtain octave-spanning birefringence phase-matching (BPM) quadratic interaction, i.e., ultrabroadband SHG of supercontinuum light from NIR to visible range, in dispersion-engineered micro-waveguides on the micrometer-thick LNOI platform. An extremely large frequency range of 135 THz in the NIR range and high conversion efficiency of 1% for sub-100 pJ is achieved in a fiber–waveguide–fiber manner with low insertion loss. Our system provides a flexible way to directly convert broadband light from the NIR range into the visible range. The scheme provides a promising paradigm for the construction of octave-spanning SCG and OFCs to shorter wavelengths for a variety fundamental sciences and high-technology applications.

Fig. 1. Schematic of LNOI micro-waveguide for ultrabroadband frequency conversion. Conceptional illustration of octave-spanning SHG in the LNOI micro-waveguide, translating NIR SCG or OFCs into visible range. Inset: SEM image of the micro-waveguide and the simulated mode profile at 1550 nm. The optical axis of LN is along the z-axis.

Experimentally, we adopt a supercontinuum source as the pump to generate octave-spanning SH and investigate the second-order nonlinear frequency conversion process, whose schematic is illustrated in Figure 2a. The experimental setup is depicted in Figure 2b. Figure 2c shows the experimentally obtained octave spanning SH spectra corresponding to waveguides with different widths.The spectra are in good agreement with the theoretical prediction. The NIR supercontinuum light is frequency doubled into a range of 550–1100 nm (spanning 270 THz). As the waveguide width increases from 4.4 to 7.2 μm, the generated broadband SH spectra also broaden in both directions of longer and shorter wavelengths, respectively, in consistent with theory prediction.The bandwidth at 30 dB below the peak of the visible spectrum is ≈550 nm, the BPM spectrum is equivalent to be Δλ ≈ 1100 nm or Δf ≈ 135 THz. Correspondingly, the bandwidth–length product is Δλ L ≈ 22 000 μm2. To further convince the broadband BPM mechanism, we also carry out the SHG experiments using continuous-wave (cw) and ultrashort laser. The fiber-to-fiber conversion efficiency is about 3.5% for the 7 mW pulsed light input or 17.5% conversion efficiency in the waveguide.

Fig. 2. Octave-spanning SHG in LNOI micro-waveguides. a) Schematics of spectral translation to visible regime from a NIR supercontinuum. b) SHG experimental setup. c) Experimental and theoretical SHG spectra from a set of LNOI micro-waveguides with different widths. Insets: Photographs of the glowing waveguide chip under pumping.

This work is published at “Yongzhi Tang, Tingting Ding, Yuting Zhang, Wenjun Ding, Yiwen Huang, Jiayu Wang, Hao Li, Shijie Liu, Yuanlin Zheng, and Xianfeng Chen Octave-spanning second-harmonic generation in dispersion engineered lithium niobate-on-insulator micro-waveguide, Advanced Photonics Research, 2400051 (2024)”.

Link: http://doi.org/10.1002/adpr.202400051