## 2 Materials and Experimental Methods  
1000 ppmw (parts per million by weight) Pr\({}^{3+}\)-doped Ge-As-Ga-Se glass rods were prepared using the melt-quenching method; more details can be found in [12, 29]. In the experiment, an unstructured fiber approximately 450 \(\upmu\)m in diameter and 70 mm long was used. The rare earth ion-doped chalcogenide glass multimode fiber fabrication is described in more detail in [12]. The experimental set-up for measuring the output power, photoluminescence (PL) spectrum and PL decay was similar to that described in [45]. The Pr\({}^{3+}\)-doped selenide-chalcogenide glass fiber was placed in an aluminum V-groove on a XYZ translation stage. The output PL power was measured in the spectral region above 3 \(\upmu\)m by excitation of the Pr\({}^{3+}\) fiber, using each of the three different laser diode pumping sources at NIR wavelengths of: 1.47 \(\upmu\)m (SemiNex 4 PN-127), 1.511 \(\upmu\)m (SemiNex 4-PN-109) and 1.69 \(\upmu\)m (QPC Laser PR-6017-0000). For achieving efficient collection of the PL signal from the fiber end, a lens with a short focal length and high numerical aperture (NA) is preferred. This is because, due to the high refractive index of the unstructured chalcogenide glass fiber, the emitted beam divergence is large [26]. Here, three different ways of collecting PL from the fiber end were tested. In the first case: (i) the output from the fiber end was collimated using a 1.87 mm focal length (NA = 0.86) black diamond lens (Thorlabs), which was AR-(anti-reflection)-coated for the 3-5 \(\upmu\)m range. In case (ii), a black diamond lens was again used but the beam was subsequently focused using a germanium aspheric lens, AR-coated for 3-12 \(\upmu\)m, of focal length: 12.7 mm and NA = 1 (Edmund Optics Stock #89-607). In the case of (iii), a pair of germanium aspheric lenses, each of focal length: 12.7 mm, were used to collimate and then focus the light on the optical power meter detector. Germanium lenses also provided some optical filtering, with a cut-on around 2 \(\upmu\)m, thus the pump signal was fully attenuated before reaching the power meter. The output power was measured using a sensitive thermal power meter (Thorlabs S401C) combined with a 3 \(\upmu\)m cut-on long-pass filter. For measuring the PL spectra, a monochromator (MSH-150 Quantum Lot; focal length: 150 mm) operating with either a 4000-nmblazed: 150 line/mm, or a 9000-nm blazed: 100 line/mm, grating was used. The PL was collected from the fiber end using a pair of germanium aspheric lenses, AR-coated for 3-12 \(\upmu\)m, of focal length = 12.7 mm and NA = 1. In this case, the pump beam was electronically chopped with a low frequency as the reference signal for a lock-in amplifier, in order to reduce any thermal background noise. The PL signal was detected in the spectral region spanning 3.0 to 9 \(\upmu\)m, using a thermoelectrically cooled (to 200 K) MCT (mercury cadmium telluride) detector (Vigo System PVI-4 TE-8), which was connected to a lock-in amplifier (Scitec Instruments Inc., Trowbridge, UK). In the measurements, one of three long pass filters was used: a long-pass filter with cut-on wavelength around 3 \(\upmu\)m (Spectrogon 71M09339, Taby, Sweden), a long-pass filter with a cut-on wavelength around 3.6 \(\upmu\)m (Edmund Optics, Barrington, USA) or a long-pass filter with a cut-on wavelength of 6.15 \(\upmu\)m (Spectrogon 71M09001). The PL decay rates were measured at the fiber end using the MCT detector and directly modulating the pump lasers. The decay rates were measured at the wavelengths: 6.5 \(\upmu\)m, 7 \(\upmu\)m, and 7.4 \(\upmu\)m. The time response of the detector and the preamplifier used in the PL decay measurements was < 3 \(\upmu\)s. In order to select the measured wavelengths, a monochromator was used to act as a tunable bandpass filter. In all of the considered cases, the 6.15 \(\upmu\)m filter was placed in front of the monochromator to remove the pump signal and any unwanted second order diffraction.