We report near-infrared to ultraviolet (UV) upconversion emissions in triply-doped Gd3+–Tm3+–Yb3+ fluoride–phosphate glasses. Emission at 310 nm, originated from the Gd3+:6P7/2 → 8S7/2 transition, was observed for the first time in glasses. The high-purity glasses prepared exhibit extended transparency in the UV down to 200–250 nm. The mixed fluoride–phosphate environment of the rare-earth ions was characterized by means of NMR techniques using scandium as a diamagnetic mimic for the luminescent species, for which the ligand distribution was quantified by 45Sc­{31P} rotational echo double-resonance NMR. Both the intensity of the Gd3+ emission as well as those of the UV emissions at 290, 347, and 363 nm increase with increasing ratio of fluoride to phosphate ligands coordinating to the rare-earth ion.

We report a detailed structural investigation of a series of fluoride-phosphate glasses with different phosphate/fluoride ratios in the system xSr(PO)-(100 - x)[AlF-CaF-SrF-MgF] with x = 5, 10, 20, 40. Raman and multinuclear solid NMR spectroscopies confirm that the polyphosphate network structure is successively transformed to a structure dominated by Al-O-P linkages with increasing AlF content. Average numbers of Al-O-P linkages have been quantified by Al/P NMR double-resonance techniques. The majority of the fluoride species are found in an alkaline earth metal/aluminum rich environment. The local environments for rare-earth ions have been characterized by EPR spectroscopy of Yb ion spin probes and by photoluminescence experiments on Eu dopant ions, including the D → F and D → F transition intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and the lifetime of the D excited state. The results indicate clear correlations between these parameters as a function of composition, and confirm that even at the highest fluoride levels, there is still some residual rare-earth phosphate coordination.

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a b s t r a c tIn this study, we report on the fabrication process of highly pure step-index fluorophosphate glass optical fibers by a modified crucible technique. High-purity fluorophosphate glasses based on 10 mol% of barium metaphosphate and 90 mol% of metal fluorides (AlF 3 eCaF 2 eMgF 2 eSrF 2 ) have been studied in order to produce step-index optical fibers transmitting in the deep-ultraviolet (DUV) region. The characteristic temperatures, viscosity around softening temperature and optical transmission in the UVevisible region of the prepared bulk glasses were characterized in a first step. The selected glass compositions were then used to prepare core-cladding optical preforms by using a modified built-in casting technique. While uncontrolled crystallization of the fiber was observed during the preform stretching by using the conventional method, we successfully obtained crystal-free fiber by using a modified crucible technique. In this alternative approach, the produced core-cladding preforms were inserted into a home-designed fused silica crucible assembly and heated at 643 C to allow glass flowing throughout the crucible, preventing the formation of crystals. Single index fluorophosphate glass fibers were fabricated following the same process as well. The optical attenuation at 244 nm and in the interval 350e1750 nm was measured on both single index and step-index optical fibers. Their potential for using in DUV applications is discussed.