Glass and ceramic materials doped with rare earth (RE) ions have gained wide interest in photonics as active materials for lasers, optical amplifiers, and luminescent sensors. The emission properties of RE-doped glasses depend on their chemical composition, but they can also be tailored by modifying the surrounding active ions. Typically, this is achieved through heat treatment (including continuous-wave and pulsed lasers) after establishing the ordering mechanisms in the particular glass–RE system. Within the known systems, silicate glasses predominate, while much less work relates to materials with lower energy phonons, which allow more efficient radiation sources to be constructed for photonic applications. In the present work, the luminescent and structural properties of germanate glasses modified with phosphate oxide doped with Eu³⁺ ions were investigated. Europium dopant was used as a “spectroscopic probe” in order to analyze the luminescence spectra, which characterizes the changes in the local site symmetries of Eu³⁺ ions. Based on the spectroscopic results, a strong influence of P₂O₅ content was observed on the excitation and luminescence spectra. The luminescence study of the most intense ⁵D₀→⁷F₂ (electric dipole) transition revealed that the increase in the P₂O₅ content leads to the linewidth reduction (from 15 nm to 10 nm) and the blue shift (~2 nm) of the emission peak. According to the crystal field theory, the introduction of P₂O₅ into the glass structure changes the splitting number of sublevels of the ⁵D₀→⁷F₁ (magnetic dipole) transition, confirming the higher polymerization of fabricated glass. The slightly different local environment of Eu³⁺ centers the results in a number of sites and causes inhomogeneous broadening of spectral lines. It was found that the local asymmetry ratio estimated by the relation of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) transitions also confirms greater changes in local symmetry around Eu³⁺ ions. Our results indicate that modification of germanate glass by P₂O₅ allows control of their structural properties in order to functionalize the emissions for application as luminescent light sources and sensors.

Bismuth-germanate glasses with low hydroxide content co-doped with Ho³⁺/Yb³⁺ ions have been investigated in terms of structural and spectroscopic properties. To reduce OH^- ions content and improve transmittance value at the wavelength of 3.1 µm, the glass synthesis has been carried out in low vacuum conditions (45–65 mBar). The composition of the host glass based on heavy metal oxides affects the maximum phonon energy (hω_max = 724 cm⁻¹), which low value has a positive impact on the mid-infrared emission parameters. Emission band at the wavelength of 2.87 µm was observed in glass co-doped with mol% 0.25 Ho₂O₃/0.75 Yb₂O₃ under 980 nm high power laser diode wavelength excitation. Lifetime measurements of the Yb³⁺:²F_5/2 quantum level indicate efficient Yb³⁺ → Ho³⁺ energy transfer (η = 61%). The developed active bismuth-germanate glass was used as the core of optical fibre operating in the mid-infrared region.