## 3 Experimental Results and Discussion
### Output Power Measurements from the Pr\({}^{3+}\) Doped Spontaneous Emission Fiber Source
From the coefficients of the calculated fitting function, one can extract that the contributed intensities at 6.5 \(\upmu\)m of the \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) and \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transitions are in the ratio of approximately 1.3:1. In the case of the PL decay measured at 7 \(\upmu\)m, a similar two-exponential behavior can be observed, but in this case the \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) and \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transitions 'contributed intensities' ratio was 1:1.7, while for the PL decay recorded at 7.4 \(\upmu\)m, the ratio of contributed intensities between the \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) and \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transitions was 1:5.3. Thus, these results show that, at longer wavelengths, the contribution from the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition starts to dominate. In order to provide further evidence showing that the observed emission comes from the \({}^{3}\)F\({}_{3}\) level, the emission at 1.6 \(\upmu\)m wavelength was measured. The emission at 1.6 \(\upmu\)m can be attributed only to the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) transition and was successfully fit with a single exponential function with a lifetime of 0.27 ms. In comparison with the literature data of Pr\({}^{3+}\)-doped chalcogenide-selenide glasses, the measured lifetime of 0.27 ms for \({}^{3}\)F\({}_{3}\) level obtained in this work is in good agreement with the measured lifetime of 0.25 ms for the \({}^{3}\)F\({}_{3}\) level [15]. Thus, the measured fast decay at 7.4 \(\upmu\)m and the decay measured at 1.6 \(\upmu\)m have similar time constants. These results indicate that both emissions originate from the same \({}^{3}\)F\({}_{3}\) manifold. To the Authors' best knowledge, this is the first observation of 7.4 \(\upmu\)m emission attributed mainly to \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) in a Pr\({}^{3+}\) selenide chalcogenide fiber and supports the observation by [PERSON] et al. of a similar emission in Pr\({}^{3+}\)-doped bulk selenide-chalcogenide glass [19]. It should be pointed out that, using this same transition, room temperature laser action at 7.2 \(\upmu\)m in Pr\({}^{3+}\): LaCl\({}_{3}\) was obtained [37]. It is well known that rapid thermalization takes place within each of the two pairs of closely-lying upper states [\({}^{3}\)F\({}_{4}\) with \({}^{3}\)F\({}_{3}\)] and [\({}^{3}\)F\({}_{2}\) with \({}^{3}\)H\({}_{6}\)] Pr\({}^{3+}\) levels in the chalcogenide glass host, with occupancy in the two lower states of at least -90% [15; 16]. Thus, under pumping at 1.47 \(\upmu\)m from the ground state to the \({}^{3}\)F\({}_{4}\) level, most of the ions will immediately  
Figure 6: Normalized MIR PL spectrum from the 70-mm-long Pr\({}^{3+}\)-doped Ge-As-Ga-Se fiber across the spectral region: 6–8 \(\upmu\)m, pumped with a 300 mW diode laser operating at 1.47 \(\upmu\)m, and the PL was measured after passage through a long-pass filter with cut-on at 6.15 \(\upmu\)m.  
drop down to level \({}^{3}\)F\({}_{3}\), whilst only a small fraction of ions will occupy the \({}^{3}\)F\({}_{2}\) level. Therefore, a laser operating on the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transitions, in a selenide-chalcogenide glass host, should act as a true four-level laser system. Additionally, it is noted that the minimum loss in a selenide-chalcogenide glass has been recorded at approximately 6.6 \(\upmu\)m, which further predestines the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition for a successful realization of the first chalcogenide glass fiber laser operating at a wavelength exceeding 4 \(\upmu\)m [47].