## 1 Introduction
This MIR emission from Pr\({}^{3+}\) ions can be attributed to emissions from several transitions that overlapped, including: (\({}^{3}\)F\({}_{4}\),\({}^{3}\)F\({}_{3}\))\(\rightarrow\)(\({}^{3}\)F\({}_{2}\),\({}^{3}\)H\({}_{6}\)), (\({}^{3}\)F\({}_{2}\),\({}^{3}\)H\({}_{6}\)\(\rightarrow\)\({}^{3}\)H\({}_{5}\) and \({}^{3}\)H\({}_{5}\)\(\rightarrow\)\({}^{3}\)H\({}_{4}\)). Results to date demonstrate the potential of Pr\({}^{3+}\)-doped chalcogenide fiber to be developed into MIR spontaneous emission sources. This is the subject of this paper. Figure 1: Schematic energy level diagram for Pr\({}^{3+}\)-doped selenide chalcogenide with indicated possible emission paths for wavelengths above 3 μm under 1.47 μm pumping (adapted from [19]). Of relevance is the recent report on MIR emission from 6.5 to 8.5 \(\upmu\)m in a Pr\({}^{3+}\)-doped bromide crystal [36], which is attributed to the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition. Furthermore, this emission was exploited in realizing MIR lasing in Pr\({}^{3+}\)-doped crystalline materials, lasing at wavelengths: 7.2 \(\upmu\)m (\({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\)), and 5.2 \(\upmu\)m (\({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)H\({}_{6}\)), respectively, in a Pr\({}^{3+}\): LaCl\({}_{3}\) crystal. Such laser action utilizing the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition exemplifies a true four-level laser system [37, 38]. The maximum phonon energy of LaCl\({}_{3}\) is around 260 cm\({}^{-1}\), which is comparable to that of a chalcogenide selenide glass at 250-300 cm\({}^{-1}\)[39]. Despite the observation of spontaneous emission from the (\({}^{3}\)F\({}_{4}\),\({}^{3}\)F\({}_{3}\)) degenerate levels to the \({}^{3}\)H\({}_{4}\) ground state at ca. 1.6 \(\upmu\)m wavelength, which have already been reported in a Pr\({}^{3+}\)-doped selenide chalcogenide glass [40, 41, 42, 43, 44], there is only very recent evidence for emission from this transition in the MIR region beyond 6.5 \(\upmu\)m in the literature to date [19]; hence, this a focus of this contribution. The motivation of this paper is the application of praseodymium ion-doped chalcogenide selenide glass fibers, pumped using near-infrared (NIR) laser diodes, as MIR light sources. The key factors considered here are power-scaling and a large emission spectral-bandwidth. Thus, in this contribution, it is demonstrated that an output power \(>\) 1 mW in the MIR wavelength region is achievable from multimode Pr\({}^{3+}\)-doped selenide chalcogenide fiber, under pumping using either 1.47 \(\upmu\)m, 1.51 \(\upmu\)m or 1.69 \(\upmu\)m laser diodes. To the best of our knowledge, this is the largest output power achieved to date in a rare earth ion-doped chalcogenide fiber spontaneous emission source in the spectral region beyond 3 \(\upmu\)m. These results clearly show the power scaling potential of the MIR spontaneous emission fiber source studied. Moreover, emission at \(\sim\)7.2 \(\upmu\)m, attributed to the \({}^{3}\)F\({}_{3}\)\(\rightarrow\)\({}^{3}\)F\({}_{2}\) transition in the Pr\({}^{3+}\) selenide chalcogenide fiber, is reported. To the best of the Authors' knowledge, this is the first experimental demonstration of 7.2 \(\upmu\)m emission in a Pr\({}^{3+}\)-doped chalcogenide fiber, demonstrating also that the spontaneous emission from a Pr\({}^{3+}\) selenide chalcogenide fiber can stretch up to 8 \(\upmu\)m. This is an important result from the point of view of realization of the first fiber laser beyond 4 \(\upmu\)m, due to the fact that the transition centered at around 7.2 \(\upmu\)m has the potential to operate as a four-level laser system [37].