Optical frequency combs, with unique time-frequency characteristics, (OFCs) have enabled significant opportunities for high-precision frequency metrology and high-resolution broadband spectroscopy. Although nonlinear photonics chips have the capacity of frequency expansion for OFCs, most of them can only access the limited bandwidths in the partial infrared region, and it is still hard to satisfy many measurement applications in the ultraviolet-to-visible region. Over the past two decades, nonlinear frequency conversion technologies have been developed to generate broadband spectra. One approach is supercontinuum generation (SCG) based on third-order nonlinearity (χ⁽³⁾) in cubic nonlinear media such as gas-filled photonic crystal fiber (PCF) and non-centrosymmetric crystalline waveguides. The main effects of χ⁽³⁾ causing spectral broadening include self-phase modulation (SPM), four-wave mixing (FWM), high-order soliton fission, and dispersive wave generation. Even with recent advances, the weak nature of χ⁽³⁾ remains constrained to the conversion efficiency and broadening spectral range. Therefore, the SCG usually requires a high-power pump laser to drive spectral expanding.

Compared with χ⁽³⁾, quadratic nonlinear effects (χ⁽²⁾) could provide specific wavelength conversion with lower pump power, but the spectral range is still limited by the phase-matching bandwidth. Through manipulation of various χ⁽²⁾ effects, many investigations and substantial progress have been made to realize several-octave lights. Among them, periodically poled lithium niobate (PPLN, LiNbO₃) crystals with broad transparent spectral range and strong nonlinearities play significant roles in broadband chip-level laser device functionalities. Moreover, LiNbO₃ possesses large χ⁽²⁾ nonlinear coefficients like the other popular materials of GaAs and GaP, supporting various χ⁽²⁾ effects of difference frequency generation (DFG), optical parametric oscillators (OPOs), sum-frequency generation (SFG) , and high-order harmonic generation (HHG). On the other hand, the PPLN exhibits excellent characteristics for processing chirps and waveguide structures, which help to maintain broadband phase-matching spectra and high conversion efficiency. Therefore, lasers based on multi-PPLN chips have significant potential for integrated broadband chip devices.

To generate OFC spanning multiple bands, Wenxue Li et al, from the East China Normal University, demonstrate a compact novelty OFC scheme via the combination of three χ⁽²⁾ nonlinearities in a three-stage PPLN chain, achieving a broadband OFC covering 0.35 to 4.0 μm. Figure 1 presents the schematic of the proposed compact broadband frequency comb with a three-stage cascaded PPLN chain. A near-infrared Er-fiber comb source was delivered to three PPLN chips in sequence, which were designed for DFG, OPA, and HHG, respectively. Relevant research results were recently published in Photonics Research, Volume 12, Issue 9, 2024. [Xiong Qin, Daping Luo, Lian Zhou, Jiayi Pan, Zejiang Deng, Gehui Xie, Chenglin Gu, Wenxue Li, "Frequency comb generation from the ultraviolet to mid-infrared region based on a three-stage cascaded PPLN chain," Photonics Res. 12, 2012 (2024)]

Figure 1. Broadband optical frequency comb generation scheme based on PPLN chain.

In the experiment, they utilized a homebuilt offset-free broadband mid-infrared OFC to generate an ultraviolet to mid-infrared spectra with 1-10 harmonics under the nonlinear effects in PPLN waveguides. The study leveraged three second-order nonlinear effects of PPLN： (1) Employing optical parametric processes to convert near-infrared light to mid-infrared (2.8-5 μm); （2）Amplifying the mid-infrared comb without bandwidth loss through optical parametric amplification; (3) Driving high-order harmonics to generate the ultraviolet light up to 10 harmonics (354 nm). The research employed offset-free mid-infrared pulses as the pump light, which ensured that the generated harmonics also retained the offset-free characteristics, helping to carry out good-quality dual-comb spectroscopy.

Figure 2. Broadband OFC spectra covering from ultraviolet to mid-infrared (the 1^st-10^th harmonics)

Corresponding author Researcher Wenxue Li comment,"In this work, erbium-doped fiber OFC and three-stage-cascaded PPLN chain have been used to extend spectra covering from ultraviolet to mid-infrared. At present, the integrated photonic technology based on PPLN is relatively mature, and it is expected to integrate three-stage-cascaded PPLN chain on one optical chip in the future, so as to realize a single-integrated wide-band frequency conversion device. In addition, combined with the on-chip micro-cavity OFCs, it is highly possible to achieve integrated wide-band OFC sources, serving more applications in complex outdoor environments."

The team will conduct more detailed theoretical analyses of each nonlinear conversion process to guide the tuning of PPLN crystal parameters and structures for higher conversion efficiency. Additionally, they plan to build dual-comb systems covering from ultraviolet to mid-infrared bands for performing high-precision parallel measurements of various substances, and exploring more applications in optical physics and optical chemistry, such as excited-state dynamics and photochemical reaction kinetics.