In this paper, a 2 µm broadband emission under 796 nm laser diode excitation in low phonon energy GeO₂-Ga₂O₃-BaO glass system is co-doped with 0.7Tm₂O₃/(0.07-0.7)Ho₂O₃ (mol%). The widest emission band (where the Tm³⁺ → Ho³⁺ energy transfer efficiency is 63%) was obtained for 0.7Tm₂O₃/0.15Ho₂O₃ co-doped glass from which a double-clad optical fiber was realized and investigated. Optimization of Tm³⁺/Ho³⁺ concentration enabled the acquisition of broadband amplified spontaneous emission (ASE) in double-clad optical fiber with a full width at half maximum (FWHM): 377 nm and 662 nm for 3 dB and 10 dB bandwidth, respectively. ASE spectrum is a result of the superposition of (Tm³⁺: ³H₄ →Η³F₄) 1.45 µm, (Tm³⁺: ³F₄ → ³H₆) 1.8 µm and (Ho³⁺:⁵I₇ → ⁵I₈) 2 µm emission bands. Hence, highly rare-earth co-doped germanate glass is characterized by a remarkably broader ASE spectrum than silica and tellurite fibers showed promising lasing properties for their further application in tunable and dual wavelength lasers.

The ErF₃-doped oxyfluoride phospho-tellurite glasses in the (40-x) TeO₂-10P₂O₅-45 (BaF₂-ZnF₂) -5Na₂O-xErF₃ system (where x = 0.25, 0.50, 0.75, 1.00, and 1.25 mol%) have been prepared by the conventional melt-quenching method. The effect of erbium trifluoride addition on thermal, structure, and spectroscopic properties of oxyfluoride phospho-tellurite precursor glass was studied by differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR), and Raman spectroscopy as well as emission measurements, respectively. The DSC curves were used to investigate characteristic temperatures and thermal stability of the precursor glass doped with varying content of ErF₃. FTIR and Raman spectra were introduced to characterize the evolution of structure and phonon energy of the glasses. It was found that the addition of ErF₃ up to 1.25 mol% into the chemical composition of phospho-tellurite precursor glass enhanced 2.7 µm emission and upconversion. By controlled heat-treatment process of the host glass doped with the highest content of erbium trifluoride (1.25 mol%), transparent erbium-doped phospho-tellurite glass-ceramic (GC) was obtained. X-ray diffraction analysis confirmed the presence of BaF₂ nanocrystals with the average 16 nm diameter in a glass matrix. Moreover, MIR, NIR, and UC emissions of the glass-ceramic were discussed in detail and compared to the spectroscopic properties of the glass doped with 1.25 mol% of ErF₃ (the base glass).