## 3 Experimental Results and Discussion
### Output Power Measurements from the Pr\({}^{3+}\) Doped Spontaneous Emission Fiber Source  
In this subsection, the results of the output power measurements from the Pr\({}^{3+}\)-doped selenide-chalcogenide glass fiber, under different pumping conditions, are presented. Figure 2 shows the measured dependence of MIR output power on the 1.47 \(\upmu\)m pump power. In this case, the maximum measured output power in the spectral range 3-8 \(\upmu\)m was around 1 mW. Measured results are very similar for all of the three different collection approaches described in Section 2, which suggests that the achieved results are repeatable and reliable. In this experiment, a 70-mm-long fiber was used. The pump absorption cross-section was around \(1.7\times 10^{-20}\) cm\({}^{2}\) at 1.47 \(\upmu\)m and the Pr\({}^{3+}\) concentration was 1000 ppmw (\(1.9\times 10^{19}\) cm\({}^{-3}\)) [29]; thus, 70 mm of fiber would have provided around 10 dB absorption of the 1.47 \(\upmu\)m pump. A longer fiber length of \(\sim\)150 mm was also tested but the measured MIR output power was lower than in the case of the 70-mm-long fiber sample, which most likely can be attributed to reabsorption of the MIR output signal, plus attenuation of the output signal due to intrinsic fiber loss. Figure 3 shows the measured dependence of the MIR output power on the 1.511 \(\upmu\)m pump power. The results obtained for this second case were very similar to the first case when 1.47 \(\upmu\)m pumping was used. This can be explained by the fact that the absorption cross-section of the Pr\({}^{3+}\) ions at 1.511 \(\upmu\)m is \(\sim\)1.9 \(\times\) 10\({}^{-20}\) cm\({}^{2}\), which is very similar to their absorption cross-section at 1.47 \(\upmu\)m. Figure 4 presents the dependence of the output MIR power on the 1.69 \(\upmu\)m pump power. It this case, more pump power was needed to achieve 1 mW output power, due to the lower value of the cross-section at this wavelength, i.e., 0.9 \(\times\) 10\({}^{-20}\) cm\({}^{2}\) at 1.69 \(\upmu\)m. Efficiency of approximately 0.1% implies that only a small fraction of the pump power feeds into the MIR beam collected at the end of the fiber. The remaining pump power either feeds into the generation of light at shorter wavelengths, dissipated via fiber loss, passed unabsorbed through the fiber and filtered out, or feeds into MIR emission radiated outside of the output lens aperture. To summarize these experiments,for all considered cases, a MIR output power of, at or above 1 mW was obtained. To the Authors' best knowledge, this is the highest output power achieved from rare earth ion-doped chalcogenide fiber spontaneous emission sources in the spectral region above 3 \(\upmu\)m. Such broadband sources can be widely applied, for example for gas spectroscopic detection [24; 25; 26]. Further, commercially available multimode laser diodes operating at wavelengths of 1.47 \(\upmu\)m, 1.511 \(\upmu\)m and 1.69 \(\upmu\)m can generate output powers in the range of tens of Watts, so further power scaling of this MIR fiber spontaneous emission source is possible, considering that the pump powers used to obtain the results shown in Figures 1-3 did not exceed 2 W. In this experiment, the pump power was limited below 2 W in order to avoid possible damage of the fiber sample [46]. Figure 3: Measured dependence of MIR output power on pump power for a Pr\({}^{3+}\)-doped Ge-As-Ga-Se spontaneous emission fiber source of length 70 mm. The pump wavelength was 1.511 \(\upmu\)m. Output power was measured after passage through a long-pass filter with cut-on at 3 \(\upmu\)m. BD is black diamond lens. Figure 2: Measured dependence of MIR output power on pump power for a Pr\({}^{3+}\)-doped Ge-As-Ga-Se spontaneous emission fiber source of length 70 mm. The pump wavelength was 1.47 \(\upmu\)m. Output power was measured after passage through a long-pass filter with cut-on at 3 \(\upmu\)m. BD is black diamond lens.