Mid-infrared frequency comb generation via cascaded quadratic nonlinearities in quasi-phase-matched waveguides

TL;DR

This paper demonstrates a straightforward method for generating mid-infrared frequency combs using cascaded quadratic nonlinearities in lithium niobate waveguides, achieving tunable and broadband spectra with simple, compact setup.

Contribution

It introduces a novel, simple configuration for MIR comb generation leveraging quasi-phase-matched lithium niobate waveguides and cascaded-$ ext{chi}^{(2)}$ nonlinearities, with tunable spectra and validated numerical modeling.

Findings

Octave-spanning supercontinuum in near-infrared

Tunable narrow-band and broadband MIR spectra

Good agreement between experiments and nonlinear envelope simulations

Abstract

We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-$\chi^{(2)}$ nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive-wave generation in the 2.5--3 $\text{\mu}$m region and intra-pulse difference-frequency generation in the 4--5 $\text{\mu}$m region. By engineering the quasi-phase-matched grating profiles, tunable, narrow-band MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment---and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.