Investigation of the TeO2/GeO2 Ratio on the Spectroscopic Properties of Eu3+-Doped Oxide Glasses for Optical Fiber Application

This study presented an analysis of the TeO2/GeO2 molar ratio in an oxide glass system. A family of melt-quenched glasses with the range of 0–35 mol% of GeO2 has been characterized by using DSC, Raman, MIR, refractive index, PLE, PL spectra, and time-resolved spectral measurements. The increase in the content of germanium oxide caused an increase in the transition temperature but a decrease in the refractive index. The photoluminescence spectra of europium ions were examined under the excitation of 465 nm, corresponding to 7F0 → 5D2 transition. The PSB (phonon sidebands) analysis was carried out to determine the phonon energy of the glass hosts. It was reported that the red (5D0 → 7F2) to orange (5D0 → 7F1) fluorescence intensity ratio for Eu3+ ions decreased from 4.49 (Te0Ge) to 3.33 (Te15Ge) and showed a constant increase from 4.58 (Te20Ge) to 4.88 (Te35Ge). These optical features were explained in structural studies, especially changes in the coordination of [4]Ge to [6]Ge. The most extended lifetime was reported for the Eu3+ doped glass with the highest content of GeO2. This glass was successfully used for the drawing of optical fiber.


Introduction
Mid-infrared (MIR) lasers have received significant attention in the past because of their applications, e.g., eye-safe laser radar, monitoring air pollution, military, remote sensing, and surgery [1]. Up to now, the mid-infrared emission of active ions has been mainly focused on germanate [2], tellurite [3], fluoride [4], and chalcogenide [5] glasses. The optical properties of rare-earth (RE) ions have an important influence on mid-infrared applications. They are related to the host matrix's phonon energy, the solubility of lanthanides ions, and the ligand field around rare-earth ions. Reducing the host glass phonon is a condition to achieve high-efficiency mid-infrared luminescence [6]. Among glass hosts, heavy metal oxides (HMO) glasses and especially the tellurite and germanate glasses have drawn growing interest as alternative hosts for mid-infrared emitting optical centers [7]. Tellurite glasses offer a wide transmission window from 0.4 to 6.5 µm, low maximum phonon energy (~750 cm −1 ), high rare-earth solubility, T g (~350 • C), and high refractive

Materials and Methods
Tellurite and tellurite-germanate glasses with general formula (64.60-x)TeO 2 -xGeO 2 -35(Ga 2 O 3 -ZnO-BaO-Na 2 O)-0.4Eu 2 O 3 , in mol% (where x = 0, 5, 10, 15, 20, 25, 30, and 35) were prepared by the melt-quenching technique using metal oxides of high purity 99.99%, Sigma-Aldrich, St. Louis, MI, USA. The glasses samples were denoted according to the GeO 2 amount (Te0Ge, Te5Ge, Te10Ge, Te15Ge, Te20Ge, Te25Ge, Te30Ge, and Te35Ge). Each batch of 10 g was melted in a non-covered Pt/Ir crucible to 1100 • C with a 10 • C/min in ambient air. After reaching the maximum temperature, the melt was cast onto a preheated stainless steel and next annealed at 10 • C below the transformation temperature of 24 h. The annealed samples were divided into two parts. The first one was cut and polished (15 mm × 10 mm × 2 mm) for refractive index, Raman, PLE (photoluminescence excitation), PL (photoluminescence) spectra, and time-resolved spectral measurements. The second part was ground to the powder form for DSC (differential scanning calorimetry), XRD (X-ray diffraction), and MIR (mid-infrared) spectra investigations.
X-ray diffraction studies were conducted using the X'Pert Pro X-ray diffractometer (PANalytical, Almelo, The Netherlands) with Cu K α1 radiation (λ = 1.54056 Å) in the 2θ range of 10-90 • . The thermal investigation, i.e., glass transition (T g ) and crystallization (T c ) temperature measurements, was carried out on Jupiter DTA STA 449 F3 thermal analyzer (NETZSCH Thermal Analysis, Selb, Germany). The glasses' mid-infrared (MIR) absorption spectra were measured using the Fourier spectrometer (Bruker Optics-Vertex70V, Rheinstetten, Germany) at 128 scans, and the resolution of 4 cm −1 . Raman spectra of glasses were recorded using a LabRAM HR spectrometer (HORIBA Jobin Yvon, Palaiseau, France). The excitation wavelength of 532 nm was used, and the diffraction grating was 1800 lines/mm. The MIR and Raman spectra were decomposed using Fityk software (0.9.8 software, opensource (GPL2+)) into Gauss maxima. The refractive index of all samples was measured at 632.8 nm by using a Metricon Model 2010/M prism coupler. The photoluminescence excitation (PLE), photoluminescence (PL) spectra, and luminescence decays were recorded at room temperature on a Horiba Jobin Yvon FluoroMax-4 spectrofluorimeter (Horiba Jobin Yvon, Longjumeau, France) supplied with a 150 W Xe lamp. The PLE and PL spectra were recorded with ±0.1 nm resolution and decay curves with ± 0.2 µs accuracies. The Eu 3+ doping the glasses allowed analyzing the R (fluorescence intensity ratio), defined as the quotient of the 5 D 0 → 7 F 2 (red) to 5 D 0 → 7 F 1 orange intensity bands. The 5 D 0 → 7 F 2 emission is derived from the magnetic dipole transition, called a hypersensitive one. In the last step, the glass fiber was manufactured using SG controls drawing tower. Glass preform was fed into the tube furnace with 2-3 mm/min speed. The required diameter was regulated by the drum and the feed speed of the fiber preform. The drawing temperature was in the range of 700-800 • C.
The glass-forming region of the TeO 2 -GeO 2 -Ga 2 O 3 -BaO-ZnO-Na 2 O-Eu 2 O 3 system was obtained to our knowledge for the first time. The glass-forming domain of the tellurite and tellurite-germanate glasses was plotted in the TeO 2 -GeO 2 -RO + R 2 O + R 2 O 3 diagram as shown in Figure 1 Figure 1-left sidediffraction patterns) might be observed. This is characteristic of amorphous materials [39]. The phase separation was not observed in all glasses (inset in Figure 1-right side-SEM photo for Te35Ge glass).

Thermal Properties and Refractive Indexes DSC Curves of Glasses
The thermal behavior of prepared Eu-doped glasses with various molar ratios of the TeO 2 /GeO 2 (Te0Ge to Te35Ge) was investigated in the temperature range from 100 to 800 • C ( Figure 2). Additionally, the values of onset glass transition (T g ) and crystalization in maximum (T x ) temperatures were given in Table 1. It is seen that with the substitution of TeO 2 with GeO 2 , the value of T g temperature increased from 356 • C for Te0Ge to 420 • C for Te35Ge. This rising trend was also observed for glasses in the 65TeO 2 -20ZnO-5Na 2 CO 3 -5TiO 2 -5GeO 2 -1Tm 2 O [21] and (95 − x)TeO 2 -5WO 3 -xGeO 2 systems [21]. The linear increase in T g with changing TeO 2 /GeO 2 molar ratio might be explained as follows: transition temperature corresponds to a change in viscosity, which is sensitive to the chemical compositions [40]. The bond strength of Ge-O (363 kJ/mol) [41] is stronger than Te-O (~250 kJ/mol) [42]. Moreover, the Ge-O bond lengths are shorter than Te-O bonds [32]. Therefore, a higher temperature is required to obtain the same viscosity with the replacement of TeO 2 by GeO 2 , which increases T g . Table 1. The studied samples' glass transition (T g ) and crystalization (T x ) temperatures.
Te0Ge 356  -Te5Ge  361  -Te10Ge  368  -Te15Ge  375  -Te20Ge  380  618  Te25Ge  395  625  Te30Ge  410  620  Te35Ge 420 630 It is also evident that above 15 mol% of GeO 2 , the thermal stability decreased, and in the DSC curves of the Te20Ge, Te25Ge, Te30Ge, and Te35Ge glasses, the crystallization peak was observed at 618, 625, 620, and 630 • C, respectively (Figure 2, and Table 1). To analyse the crystallization tendency of the above glasses, a heat-treatment procedure was performed on glass samples at the T x . The diffraction patterns of the selected heated specimens were presented in Figure 3. Besides the typical halo, several diffraction peaks were observed. Compared with the standard diffraction pattern for the crystalline phase, all diffraction lines might be assigned to the barium gallium germanium oxide BaGa 2 Ge 2 O 8 (PDF 04-009-4175) ( Figure 3). Based on the calculation of the unit cell parameters of the BaGa 2 Ge 2 O 8 crystal structure in the Te35Ge glass, it was found that europium ions were not precipitated in the structure of barium gallium germanium oxide [32]. According to the crystallographic parameters in the reference pattern no 04-009-4175, the value of the lattice parameters lengths of the cell edges a, b, c was 9.3490 Å, 9.9030 Å, and 8.7700 Å, respectively. Calculated parameters a, b, and c of the unit cell of BaGa 2 Ge 2 O 8 crystals in the Te35Ge sample were: a = 9.3485 Å, b = 9.9015 Å, c = 8.7500 Å.

MIR Spectra
MIR spectra of tellurite and tellurite-germanate glasses in the 1100-400 cm −1 range were shown in Figure 4. The two broad bands were observed in the range of 1100-600 cm −1 and 600-400 cm −1 . As shown in Figure 4, the maxima and intensity of the bands strongly depend on the glass composition. Additionally, the position of bands in the abovementioned ranges was shifted toward higher frequencies, and this behavior demonstrated the change in the strength of the chemical bonds due to the addition of the GeO 2 [12]. The differences in the MIR spectra of glasses related to their composition occurred in the medium range (1100-400 cm −1 , in elliptic selections) characteristic of stretching and deformation vibrations of different tellurites and germanates as well gallate units. The overlapping bands due to the vibrations of different bonds makes the FTIR and Raman bands broader. Additionally, the changes in the b building units' bond angles/bond lengths due to network modifications also make the bands broader [48].  To fully comprehend the changes in the network of the glasses, the MIR spectra of the Te0Ge, Te15Ge, and Te35Ge glasses have been decomposed and shown in Figure 5. The assignment of the components bands was presented in Table 2.  The decomposed MIR spectra of the tellurite (Te0Ge) and tellurite-germanate (Te15Ge, Te35Ge) glasses showed four, five, and six bands, respectively. The bands at 500 cm −1 , 615 cm −1 , 681 cm −1 , and 760 cm −1 were observed in the decomposed spectrum of the Te0Ge glass. There were bands at 454 cm −1 , 512 cm −1 , 687 cm −1 , 799 cm −1 , and 886 cm −1 in the decomposed MIR spectrum of the Te15Ge glass. The component bands at 479 cm −1 , 532 cm −1 , 571 cm −1 , 673 cm −1 , 783 cm −1 , and 912 cm −1 presented in the decomposed MIR spectrum of the Te35Ge glass.
It might be seen in the Te15Ge and Te35Ge samples that the addition of the GeO 2 (up to 15 mol%) caused the appearance of additional bands at 455 cm −1 and 479 cm −1 due to bending vibrations of Te-O-Ge bonds. Moreover, IR bands at~886 cm −1 and~909 cm −1 were found for the Te15Ge, Te35Ge glasses. These bands could be due to the [GeO 4 ] units attributable to the asymmetric stretching vibrations of Ge-O-Ge. The IR band associated with trigonally coordinated tellurium ions [TeO 3 ]tp/[TeO 3+1 ] (band at 670-680 cm −1 ) was found for all glasses. Still, its intensity increased as the GeO 2 was increased to 15 mol% and decreased above 15 mol% GeO 2 . The intensity of the symmetrical stretching vibrations of [TeO 3 ]tp/[TeO 3+1 ] units with non-bridging oxygens (band at 760-790 cm −1 ) was changed in the case of GeO 2 addition. For the 15 mol%, GeO 2 showed a decrease in the intensity of this band. On the other hand, above 15 mol% GeO 2 , the band's intensity at 799 cm −1 was increased. Moreover, in the decomposed MIR spectrum of the Te35Ge glass, a band appeared at~571 cm −1 , indicating the [GeO 6 ] octahedral units existed in the glass network ( Figure 5). This suggests that the glass network modification has occurred in the tellurite and germanate subnetworks due to the TeO 2 /GeO 2 molar ratio variation. Based on presented results, the compositional evolutions of the glass network can be explained as follows: with increasing GeO 2 content from 15 mol% up to 35 mol%, the compositional evolution was followed by a conversion of Ge(IV) to Ge(VI), and network continuity broke down with the formation of more significant numbers of NBO. Comparing our investigation results on the coordination number of germanium ions with data for germanate and TeO 2 -GeO 2 based glasses, there was a germanate anomaly in the present glass systems. According to the adopted model of the mechanism, the germanate anomaly is the change of germanium from 4-coordinated to higher coordinated species, five-fold and six-fold Ge [58].

Raman Spectra
The Raman spectra of Eu 3+ -doped glasses in the 1100-200 cm −1 range were presented in Figure 6. The obtained results are very similar to those published in [27,39]. All Raman spectra have identical features: four-banded maximums at around 320, 460, 800, and 850 cm −1 , respectively. Differences between the intensity of the bands in Raman spectra of glasses with the increased amount of germanium dioxide seen in each region of Raman shift. Raman spectra decompositions of Te0Ge, Te15Ge, and Te35Ge glasses doped with Eu 3+ were carried out to evaluate the contribution of various structural units of the Raman spectra in glasses, and they were shown in Figure 7. Raman spectra were first smoothed out and then decomposed. The parameters and assignments of the bands were listed in Table 3. Obtained results showed that there were six bands in the Raman spectra of Te0Ge, Te15Ge, and Te35Ge glasses doped with Eu 3+ , at 305-310 cm −1 , 420-460 cm −1 , 480-510 cm −1 , 670-700 cm −1 , 730-750 cm −1 , and 780-870 cm −1 .    [65]. This band could be overlapped with antisymmetric stretching vibrations of Ge-O − of Ge (2) units [64]. The band at around 870 cm −1 was an assignment to the symmetrical stretching vibrations of Ge-O − in Ge (3) units [48]. Moreover, the range of the 670-870 cm −1 in the Raman spectra might be ascribed to the stretching vibration of O-Ga-O [40,66]. What's more, the band in the 620-650 cm −1 range characteristic for symmetric stretching vibration of Ge-O-Ge bonds in [GeO 6 ] octahedral units was not detected in the decomposed Raman spectra (band at 620-650 cm −1 ) [61,66]. Due to the overlapping of different component bands in the decomposed Raman spectra in the analyzed multicomponent glass system, the correct interpretation of the changes in the integral intensities is impossible. Considering the above, the authors limited the interpretation of decomposed Raman spectra to qualitative analysis. The network of glasses with GeO 2 content consists of [TeO 4 ]tbp, [TeO 3 ]tp/[TeO 3+1 ] units. However, Ge (3) units appeared in the Te35Ge glass, not found in the Te15Ge glass.

Excitation Spectra and Phonon Sideband Analysis
The excitation spectra of the Eu 3+ -doped glasses with various TeO 2 /GeO 2 molar ratios were recorded in the 350-550 nm by monitoring the emission at 611 nm corresponding to the 5 D 0 → 7 F 2 transition. The results were presented in Figure 9. Several bands are assigned to the following transitions: 7 F 0 → 5 D 4 (362 nm), 7 F 0 → 5 G 4 (376 nm), 7 F 0 → 5 G 2 (383 nm), 7 F 0 → 5 L 6 (395 nm), 7 F 0 → 5 D 2 (465 nm), and 7 F 0 → 5 D 1 (527) [69]. Among various transitions, at 465 nm was prominent. In Figure 9, the phonon sideband (PSB) associated with the transition of 7 F 0 → 5 D 2 might be observed. Additionally, the excitation spectra from 445 to 456 cm −1 range were shown in Figure 10. PSB analysis is a useful tool to determine the phonon energy of the glass host and the local environment around the Eu 3+ ions in the glass network. Based on the difference between the positions of the PSB and PET bands, it might be possible to calculate the phonon energy associated with the maximum energy of the vibrational mode around the europium ions [70]. The evaluated phonon energies for the presented glasses increased with the addition of the GeO 2 content and were as follows: 699 cm −1 (Te0Ge) glass, 714 cm −1 (Te5Ge) glass, 702 cm −1 (Te10Ge) glass, 716 cm −1 (Te15Ge) glass, 705 cm −1 (Te20Ge) glass, 726 cm −1 (Te25Ge) glass, 746 cm −1 (Te30Ge) glass, and 749 cm −1 (Te35Ge) glass ( Table 4). The phonon energies values were smaller than heavy metal oxide glasses published in the literature [71,72]. The phonon energies for glasses from the excitation spectrum PSB-PET agreed with the Raman spectra of glasses (Figure 7 and Table 3).

Emission Spectra Analysis
The emission spectra (PL) of all the Eu 3+ -doped glasses were measured under 465 nm excitation. Due to the relaxation from the 5 D 0 state, the intense emission bands were obtained in the 550-750 nm range ( Figure 11). The emission bands at 577 nm, 590 nm (orange), 611 nm (red), 650 nm, and 700 nm correspond to the transitions 5 D 0 → 7 F 0 , 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 , and 5 D 0 → 7 F 4, respectively. The obtained emission spectra are in accordance with glasses in the (60 [74] systems. As might be seen in Figure 11, the intensity of the bands in the emission spectra (PL) of glasses depends on the GeO 2 content. For the glasses with up to 15 mol% of GeO 2 , an increase in the intensity of the bands was observed. However, the further increase in the GeO 2 content caused a decrease in the intensity of the bands of the PL spectra. The possible energy level diagram of the europium ions in the tellurite and telluritegermanate glasses with various TeO 2 /GeO 2 molar ratios was presented in Figure 12. The europium ions were excited at 465 nm wavelengths to the upper levels of 5 D 2 , respectively. The pumped electrons relaxed to the lowest excited level 5 D 0 by non-radiative transition and radiatively relaxed to the ground state 7 F J (J = 0, 1, 2, 3, and 4). The non-radiative multiphonon relaxation to the 5 D 0 level occurred when the europium ions were excited to levels above this level ( 5 D 0 ). Afterwards, intense emission bands appeared due to 5 D 0 → 7F 0 , 7F 1 , 7F 2 , 7F 3 , 7F 4 transitions.
It is well-known that 5 D 0 → 7 F 1 transition in the orange region with ∆J = 1 has been identified as magnetic-dipole (MD) transition. The following red emission at 611 nm ( 5 D 0 → 7 F 2 ) is considered as a hypersensitive transition owing to the selection rule ∆J = 2 and has been classified as the electric-dipole (ED) transition. The changes influence the intensity of this transition in the environment surrounding europium ions. The domination of the ED transition in emission spectra is direct proof that europium ions are situated in non-centrosymmetric sites. If Eu 3+ ions occupy centrosymmetric sites, the probability of the MD transition occurrence is higher than the ED one. The proximity of the europium environment to the centrosymmetry might be characterized by the fluorescence intensity ratio (R) of the electric ( 5 D 0 → 7 F 2 ) to magnetic ( 5 D 0 → 7 F 1 ) dipole transition intensity. Moreover, the R ratio indicates the strength of covalent/ionic bonding between the europium ions and the surrounding ligands [74]. The fluorescence intensity ratio (R) for the Eu 3+ -doped glasses with various TeO 2 /GeO 2 molar ratios are shown in Figure 13. The high fluorescence intensity ratio (R) values indicated that europium ions occupy the low symmetry sites [63,75,76]. Hence, the high R values indicated the covalent nature of the Eu-O bond [77,78]. It was also observed that the asymmetry was decreased with an increase in GeO 2 content up to 15 mol%. However, the addition of germanium dioxide above 15 mol% increased the asymmetry of Eu 3+ ions. The obtained values of fluorescence intensity ratio for the Eu 3+ ions in the tellurite and the tellurite-germanate glasses supported the hypothesis of Wang et al. [79] that the strongly depolymerized network in oxide glasses enhances the regularity of the sites occupied by the lanthanide ions. According to the crystal-chemistry model, the local environments of the rare earth cations are mainly influenced by three factors: (a) the number of coordinating anions around the rare-earth cation; (b) the size of the rare-earth cation, and (c) the degree of the depolymerization of the network. The regularity of the site occupied by the rareearth cation increases as polymerization decreases. In the Te0Ge and Te15Ge glasses, the Eu 3+ ions can coordinate [TeO 3 ] 2− anions (MIR results). For Te35Ge glass, the presence of TeO 3 2− [GeO 6 ] 2− , and Ge-O − of the Ge (3) unit is responsible for disturbing the symmetry in the local framework of Eu 3+ , and thereby the probability of the 5 D 0 → 7 F 2 transition can be enhanced in glasses up to 15 mol%GeO 2 , and a decrease in R/O ratio values above 15 mol% of GeO 2 was observed.

Luminescence Decay Analysis
The luminescence decays from the 5 D 0 state were recorded for λ em = 612 nm ( 7 F 0 → 5 D 2 ) using λ exc = 465 nm ( 7 F 0 → 5 D 2 ). The decay curves registered for glasses were wellfitted by the single-exponential I(t)/I 0 = Aexp(-t/τ m ) function, where τ m corresponds to luminescence decay time [80]. Obtained results were presented in Figure 14. The calculated luminescence lifetimes in glasses were equal to 8.61 ms (Te0Ge), 8.75 ms (Te5Ge), 9.03 ms (Te10Ge), 9.20 ms (Te15Ge), 9.39 ms (Te20Ge), 9.40 ms (Te25Ge), 9.60 ms (Te30Ge), and 9.80 ms (Te35Ge) what might be seen in Figure 14. Values of the luminescence lifetimes for the 5 D 0 state of europium ions increased when GeO 2 in the glass composition replaced TeO 2 . It was confirmed that the lifetime for the 5 D 0 state of europium increases with increasing phonon energy in heavy metal oxide glasses [66]. A similar situation was observed for Eu 3+ ions in our studies. [81]. The measured lifetime for the 5 D 0 state of Eu 3+ enlarged with the increasing phonon energy of studied glasses due to GeO 2 addition. The increase in a lifetime 5 D 0 state of Eu 3+ with increasing phonon energy can be explained with regard to the energy gap law of europium ions. The energy gap between excited level 5 D 0 and lower level 7 F 6 is large and has ∆E = 12,500 cm −1 . Due to non-radiative relaxation, rates for rare-earth are negligibly low. The value of the order of multiphonon relaxation for the 5 D 0 state of Eu 3+ amount 16 in our glasses. The rates of multiphonon relaxation processes at ten rows are negligible [81].

Optical Fiber
In recent years, significant progress has been made in expanding the spectral range of tellurite-based fiber lasers generating at wavelengths above 2 µm. A detailed review on the state of the art of Nd 3+ -, Er 3+ -, Tm 3+ -, Ho 3+ -doped tellurite glass fiber was presented by E. A. Anashkina [82]. In this section of the manuscript, the results of the glass fiber draw were demonstrated. The glass preform was made from the Te35Ge glass composition. Figure 15 presents the result of the PL spectra of the Eu 3+doped glass fiber under the 465 nm excitation. The bands located on the spectra were adequate to the ones given in Figure 11, and they indicate the same transitions: 5 D 0 → 7 F 0 (577 nm), 5 D 0 → 7 F 1 (590 nm), 5 D 0 → 7 F 2 (611 nm), 5 D 0 → 7 F 3 (650 nm), and 5 D 0 → 7 F 4 (702 nm). In the inset of Figure 15

Discussion
Novel tellurite and tellurite-germanate glasses with a GeO 2 content of up to 35 mol% were obtained by the melt quenching method.
Their thermal stability was analyzed to fulfill optical fiber drawing requirements according to the glass compositions. The DSC curves showed a rise in transition temperature with an increase in GeO 2 content in the glass composition from 356 to 420 • C. Moreover, the addition of germanium oxide above 15 mol% content caused a decrease in the thermal stability of the glasses. A crystallization peak was detected in the DSC curves of the glasses with 20, 25, 30, and 35 mol% of GeO 2 . After heat treatment at the maximum temperature, the exothermic peak, BaGa 2 Ge 2 O 8 crystal phase, appeared ( Figure 3). The increased tendency towards crystallization agrees with structural studies (MIR, Raman spectroscopy).
The change in the network of glasses with various TeO 2 /GeO 2 molar ratios had an important impact on the local environment of Eu 3+ ions. This might be explained considering the field strengths and electronegativity of the network-modifying ions. According to Rada et al. [83], the presence of multiple oxides in the glass increased the tendency of the network formers to attract the structural units for compensation of the charge yield. In Figure 5 (1) , Ge (2) , and [GaO 4 ] was found in the glass network of the Te15Ge glass.
On the other hand, in the network of the Te35Ge glass, there appeared to be negatively charged [GeO 6 ] 2− and Ge-O − of the Ge (3) units [85]. In glasses with GeO 2 content above 15 mol%, the europium ions were surrounded by a concentration of negative charge from NBO atoms in tellurite and germanium units to the neutrality of the structure. However, PSB associated with the 7 ions in the Te0Ge, Te5Ge, Te10Ge,  and Te15Ge glasses. In contrast, europium ions in the Te30Ge and Te35Ge glasses were also coupled with higher modes of germanate units ( Table 4). The change in the structure and the local environment of Eu 3+ ions resulted in a change in the luminescence properties of studied glasses.
Moreover, the measured lifetime for the 5 D 0 state of Eu 3+ increased with the phonon energy of studied glasses. The glass composition characterized by the most extended lifetime of europium ions was used as a core material to draw optical fiber. It is also worth pointing out that the value of the refractive index of glasses varies from 1.933 to 1.805 with increasing GeO 2 , which is helpful to adjust glass composition for fiber cladding in the future. The broad refractive index change was consistent with the larger polarizability of the tellurium ion than the other cations [67].

Conclusions
The Eu 3+ -doped glasses in the TeO 2 -xGeO 2 -Ga 2 O 3 -ZnO-BaO-Na 2 O system (x = 0-35 mol%) were investigated to explore their potential application as a host material for RE doping and optical fiber drawing. The thermal, structural, and optical properties have been considered as the main factors. The thermal evaluation showed high glass stability, although a crystallization peak appeared above 15 mol% of GeO 2 . This was correlated with the MIR and Raman results, which indicated that the conversation of the germanium ions from tetrahedral to octahedral coordination is promoted. It is concluded that glasses with 20-35 mol% germanium dioxide content possess a weaker network than more polymerized glasses with 5-15 mol% of GeO 2 . The above structural changes are also seen when analyzing fluorescence intensity ratio (R) for the Eu 3+ ion, and its asymmetry decreased with the GeO 2 content up to 15 mol% and returned to 20 mol%. Despite the above changes, the luminescence behavior of Eu 3+ -doped tellurite and tellurite-germanate glasses showed the differences in emission intensity and the continuous growth of the lifetime, which can be explained by the large energy gap of Eu 3+ ions in the glass matrix, having in mind the increasing phonon energy of studied glasses.
In that aspect, the correlation between the phonon energy of glasses and GeO 2 content was determined based on PSB analysis. The characteristic hypersensitive emission bands corresponding to 5 D 0 → 7 F J (J = 1, 2) transitions in the europium ions showed the R-value increase for the glasses with GeO 2 content up to 15 mol%. In contrast, the R factor was reduced for the glasses with a higher concentration of GeO 2 .
Glass with 35 mol% of GeO 2 was used for optical preform and fiber fabrication. Both are achieved by intensive luminescence originating from Eu 3+ ions, which conformed to the readiness of this glass system for active optical fiber development.  Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.