## 1 Introduction  
Mid-infrared (MIR) fiber sources have been the subject of extensive research in recent years. The main reason is the broad range of potential applications for such sources in fields of biomedical sensing, environmental monitoring, free space communication and so on [1; 2; 3; 4; 5]. When compared to other types of MIR light sources, fiber sources have advantages of good beam quality, compactness, reliability, and a simple construction [1; 2; 3; 4; 5]. Recently, continuous wave (CW) 3.92 \(\upmu\)m laser action at room temperature has been observed in Ho\({}^{3+}\)-doped fluorodinate glass fibers [6] and photoluminescence beyond 4 \(\upmu\)m in Dy\({}^{3+}\)-doped fluorodinate glass fibers [7]. Bearing in mind that it has taken 23 years to push the fiber lasing wavelength from 3.90 \(\upmu\)m [8] only to 3.92 \(\upmu\)m i.e., [6], to access longer wavelengths, beyond 4 \(\upmu\)m, host glass materials with lower phonon energy are needed [5]. Potentially excellent candidate host glasses are chalcogenide-selenide glasses. Chalcogenide glass fibers have already proven their usefulness in the development of ultra-broadband MIR supercontinuum sources to 15 \(\upmu\)m [9; 10]. In addition, Raman lasers operating beyond 3.5 \(\upmu\)m have been successfully realized in chalcogenideglass fibers [11]. However, the making of lanthanide ion-doped MIR fiber lasers beyond, 4 um has so far proved elusive, despite large efforts. To date, spontaneous emission in the range 3-6 um has been observed in chalcogenide glasses doped with Pr\({}^{3+}\), Tb\({}^{3+}\), Dy\({}^{3+}\) or Nd\({}^{3+}\)[12; 13; 14; 15; 16; 17]. Also, PL (photoluminescence) in bulk chalcogenide glasses has been observed in the 7-8 um region in Tb\({}^{3+}\) and Pr\({}^{3+}\), respectively, [18; 19]. Recently, spontaneous emission between 7-9 um has been reported in chalcogenide glass fibers doped with Tb\({}^{3+}\) or Sm\({}^{3+}\), respectively [20; 21]. Nonetheless, the failure to achieve MIR laser action in chalcogenide glass fibers doped with lanthanides is in part, at least, due to residual loss around 4.6 um wavelength because of Se-H extrinsic impurity in the glass host [22]. Still, rare earth ion-doped chalcogenide glass multimode fibers show great promise to act instead as MIR spontaneous emission fiber light sources. A MIR spontaneous emission fiber source comprises a rare earth ion-doped chalcogenide fiber, a pump laser and focusing optics. Importantly, in the case of spontaneous emission sources, a population inversion is not required. This is because the MIR light is produced via the spontaneous emission phenomenon. Work to date has shown their suitability in gas sensing [23; 24; 25; 26; 14]. Moreover, absorption bands arising from vibrational excitations relevant to biological and medical applications cover the wavelength range from 2.8 to 3.7 um and from 5.7 to 7.3 um[4] and both are covered by a MIR spontaneous emission fiber source. Further, most FTIR (Fourier transform infrared) spectrometers use as a MIR light source, a Globar. A Globar is silicon carbide rod of 5 to 10 mm width and 20 to 50 mm length that is electrically heated up to 1000 to 1650 \({}^{\circ}\)C. A Globar requires water-cooling and also special safety precautions. The MIR rare earth-doped spontaneous emission source developed in this contribution operates, on the other hand, at room temperature and is easy to handle. Praseodymium ion-doped, selenide host glasses exhibit strong absorption bands at 1.55 um and 1.94 um, which are suitable for pumping with readily commercially available, high power, and cost-effective laser diodes. A simplified, energy-level diagram of Pr\({}^{3+}\) in chalcogenide glass with indicated possible emission paths for wavelengths above 3 um is presented in Figure 1. Broadband MIR emission across the 3.5-6 um wavelength range from Pr\({}^{3+}\)-doped chalcogenide glasses has already been reported by several research groups [12; 13; 27; 28; 29; 30; 31; 32; 33; 34; 35].