Lanthanum Fluorogermanate Glass for Telecom Nonlinear Photonics: fs-Laser-Written Waveguides

Several rare-earth-based photonic devices rely on materials that combine wide transmission windows, favorable nonlinear properties, high ion solubility, and compatibility with waveguide inscriptions. While silica dominates current technologies, this work investigates a novel trivalent europium-doped lanthanum fluorogermanate glass. Such glasses stand out as promising candidates because of their optical transparency and thermal stability. Nevertheless, the suitability of LFG glasses for waveguide-based all-optical devices remains unclear, as optical parameters governing mode confinement, Kerr response, and laser damage threshold are still insufficiently characterized. Here, we investigate sub-bandgap femtosecond-laser processing of fluorogermanate glass, determining surface micromachining parameters and inscribing waveguides within the bulk. Incubation effects under 515 and 1030 nm irradiation are studied, and differences are discussed in terms of the ionization mechanisms. Also, photoluminescence spectroscopy indicates an increase in the local centrosymmetry around the Eu³⁺ sites following laser irradiation. Finally, we determine optical parameters for evaluating the performance of the material: the refractive index spectrum and group-velocity dispersion indicate a low-dispersion character of the glass. The nonlinear refractive index is determined via nonlinear ellipse rotation, reaching values up to three times higher than those of silica in telecommunication bands. Together with the three-photon figure of merit, our findings highlight the potential of fluorogermanate glass for waveguide-based all-optical devices for telecom photonics.