## 3 Results and Discussion
### Emission Spectroscopy and Analysis  
Figure 2a shows the fluorescence decay signal of the \({}^{3}\)H\({}_{4}\) level and \({}^{3}\)F\({}_{4}\) level, corresponding to the 1860-nm and 1460-nm transitions of the two TeO\({}_{2}\)-K\({}_{2}\)O-Nb\({}_{2}\)O\({}_{5}\) samples with Tm\({}_{2}\)O\({}_{3}\) concentrations of 1.0 and 0.125 mol %. The fluorescence signal has a sharp peak following an exponential decay. As can be seen from Figure 2b, the curves corresponding to the 1860-nm transition have a smooth peak in comparison with the other transition. That is mainly due to increase in the population in \({}^{3}\)H\({}_{4}\) level resulting from the decay from the \({}^{3}\)F\({}_{4}\) to \({}^{3}\)H\({}_{4}\) level as illustrated in the energy level diagram in Figure 1a. Similar behavior was observed for the other glass samples with different glass modifiers. In the case of 0.125 mol % doping concentration for the 1860-nm transition (Figure 2b), the fluorescence signal exhibits an initial rise followed by an exponential decay. This can be attributed to the slow decay rate from the \({}^{3}\)F\({}_{4}\) to \({}^{3}\)H\({}_{4}\) level, filling the population in the \({}^{3}\)H\({}_{4}\) level. In the case of the sample with 1.0 mol % doping concentration, this effect is negligible due to the faster decay rate from \({}^{3}\)F\({}_{4}\) to \({}^{3}\)H\({}_{4}\). The fluorescence lifetimes (\(\tau_{F}\)) were obtained by fitting a single exponential decay curve to the tail of the measured signal, where the cross-relaxation effect is negligible. From the calculated radiative lifetimes and experimentally measured fluorescence lifetimes, the luminescence quantum efficiency (\(\eta\)) can then be determined by using \(\eta=\tau_{F}/\tau_{R}\). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**J-O Parameters**} & \(\Omega_{2}\) & \(\Omega_{4}\) & \(\Omega_{6}\) \\ \cline{2-4}  & **(10\({}^{-20}\) cm\({}^{2}\))** & **(10\({}^{-20}\) cm\({}^{2}\))** & **(10\({}^{-20}\) cm\({}^{2}\))** \\ \hline TeNbkTm & 4.51 \(\pm\) 0.79 & 0.76 \(\pm\) 0.46 & 1.13 \(\pm\) 0.67 \\ TeZnTm & 4.02 \(\pm\) 0.56 & 0.93 \(\pm\) 0.33 & 1.12 \(\pm\) 0.47 \\ TeNbTm & 4.09 \(\pm\) 0.03 & 0.69 \(\pm\) 0.01 & 1.11 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Judd–Ofelt (J-O) intensity parameters for three different glass hosts with 1 mol % Tm ion concentration. Figure 1: **(a)** Energy level diagram of the Tm\({}^{3+}\) ion. CR: Cross-relaxation **(b)** Absorption spectra of two Tm\({}_{2}\)O\({}_{3}\): TeO\({}_{2}\)-Nb\({}_{2}\)O\({}_{5}\) glass samples with Tm\({}_{2}\)O\({}_{3}\) concentrations of 1.0 and 0.25 mol % in the range of 450–2050 nm. The corresponding absorption bands from the ground state (\({}^{3}\)H\({}_{6}\)) are indicated. Figure 2: Measured fluorescence decay curves and the exponential fits of the **(a)** \({}^{3}\)F\({}_{4}\) level and **(b)** \({}^{3}\)H\({}_{4}\) level for the two Tm\({}_{2}\)O\({}_{3}\): TeO\({}_{2}\)-K\({}_{2}\)O-Nb\({}_{2}\)O\({}_{5}\) (TeNbKTm) samples with Tm\({}_{2}\)O\({}_{3}\) concentrations of 1.0 and 0.125 mol % [29]. Table 2 summarizes the obtained fluorescence lifetimes and quantum efficiencies of the glass samples for the two transitions peaking at 1860 nm and 1460 nm. With increasing Tm\({}_{2}\)O\({}_{3}\) concentration from 0.125 to 1.0 mol %, we observed a decrease in the fluorescence lifetime due to the increasing role of nonradiative relaxation mechanisms. The Te\({}_{2}\)O\({}_{3}\)-Nb\({}_{2}\)O\({}_{5}\) glass with 0.25 mol % Tm\({}_{2}\)O\({}_{3}\) doping had the longest fluorescence lifetime (893 \(\upmu\)s) for the 1860-nm emission band among the glass samples investigated in this paper. The highest quantum efficiency for the sample TeZnTm was obtained as 32% for the 1860-nm emission band and 89% for the 1460-nm emission band when the ion concentration was 0.125 mol %. We obtained the highest quantum yields of 32% and 76% for the 1860 and 1460 nm emission bands, respectively, in the sample TeNbTm, when the ion concentration was 0.25 mol %.