## 3 Experimental Results and Discussion
### Output Power Measurements from the Pr\({}^{3+}\) Doped Spontaneous Emission Fiber Source
Mid-Infrared Emission from the Pr\({}^{3+}\) Doped Selenide-Chalcogenide Glass Fiber in the Spectral Region: 3.5-8 \(\mu\)m  
In this subsection, the experimental investigation of the PL properties of 1000 ppmw Pr\({}^{3+}\)-doped selenide-chalcogenide glass fiber is discussed. Figure 5 shows the room temperature mid-infrared emission bands of Pr\({}^{3+}\)-doped GeAsGaSe glass fiber recorded in the wavelength range 3-6.5 \(\upmu\)m under excitation at 1.47 \(\upmu\)m. The PL emission was collected from the fiber end using the MCT detector operating in the spectral region 3-9 \(\upmu\)m, long pass filters with cut-on wavelengths of 3 \(\upmu\)m and 3.5 \(\upmu\)m, and a diffraction grating blazed at 4 \(\upmu\)m. The recorded emission band is mainly associated with the (\({}^{3}\)F\({}_{2}\),\({}^{3}\)H\({}_{6}\))\(\rightarrow\)\({}^{3}\)H\({}_{5}\) and \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) transitions (see Figure 1), however, emission from (\({}^{3}\)F\({}_{4}\),\({}^{3}\)F\({}_{3}\))\(\rightarrow\)(\({}^{3}\)F\({}_{2}\),\({}^{3}\)H\({}_{6}\)) can also contribute to the mid-infrared emission [12; 19]. It can be clearly seen from Figure 5 that the emission at wavelengths longer than 6 \(\upmu\)m is non-negligible. Therefore, in order to investigate emissions above 6 \(\upmu\)m, a diffraction grating blazed at 9 \(\upmu\)m and a long pass filter with cut-on wavelength of 6.15 \(\upmu\)m were introduced into the set-up. Figure 6 displays the room temperature mid-infrared emission of the 70-mm-long Pr\({}^{3+}\)-doped GeAsGaSe fiber in the wavelength range: 6.15-8 \(\upmu\)m, under pumping at 1.47 \(\upmu\)m. This emission band most likely can be attributed to the long tail of the \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) transition and to the emission from the  
Figure 4: 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 1.69 \(\upmu\)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 5: Normalized MIR PL spectrum from the 70-mm-long Pr\({}^{3+}\)-doped Ge-As-Se-Ga glass fiber across the spectral region 3-6.5 \(\upmu\)m, pumped with a 300 mW diode laser operating at 1.47 \(\upmu\)m. The dip observed in the fluorescence spectra at 4.26 \(\upmu\)m can be attributed to ambient CO\({}_{2}\) absorption in the optical path. \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition (see Figure 1). Recently, a similar emission in the spectral region 6-8.5 \(\upmu\)m was measured in Pr\({}^{3+}\)-doped bromide crystals [36] and in the bulk glass: Pr\({}^{3+}\)-doped Ge-Ga-Se [19]. This emission was attributed to the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition [19; 36]. In order to investigate further the emission across the 6.1-8 \(\upmu\)m spectral region in the Pr\({}^{3+}\) selenide-chalcogenide glass fiber here, the PL decays at selected wavelengths were measured (see Figure 7). Figure 7 presents the PL decay characteristics at the output of a 70-mm-long Pr\({}^{3+}\)-doped Ge-As-Ga-Se glass fiber measured at: 6.5 \(\upmu\)m (Figure 7a), 7 \(\upmu\)m (Figure 7b), 7.4 \(\upmu\)m (Figure 7c) and 1.6 \(\upmu\)m (Figure 7d), under 1.47 \(\upmu\)m excitation. In the measurement of PL lifetime, each presented decay plot was collected from a fiber sample and averaged over several hundred measurements to improve the signal-to-noise ratio. The measured PL decay at 6.5 \(\upmu\)m was best fit by using a two-exponential function. The fast decay (\(\tau_{1}\) = 0.3 ms) can be attributed to the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition whilst the slow decay \(\tau_{2}\) = 6.8 ms is most likely a contribution from the \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\) transition.