## 2 Materials and Methods  
Different glasses with the system (98-x) (0.50P\({}_{2}\)O\({}_{5}\)-0.40 SrO-0.10 Na\({}_{2}\)O) -0.5 Er\({}_{2}\)O\({}_{3}\)-1.5 Yb\({}_{2}\)O\({}_{3}\)-xZnO (in mol %) with x = 0, 2.5, 5 and 10 (labeled Ref., 2.5 Zn, 5 Zn, 10 Zn) were prepared using a standard melting method in air. NaPO\({}_{3}\), SrCO\({}_{3}\), (NH\({}_{4}\))\({}_{2}\)HPO\({}_{4}\), Yb\({}_{2}\)O\({}_{3}\), Er\({}_{2}\)O\({}_{3}\) and ZnO were used as raw materials for the batch preparation. Sr(PO\({}_{3}\))\({}_{2}\) was separately prepared from the mixture of SrCO\({}_{3}\) and (NH\({}_{4}\))\({}_{2}\)HPO\({}_{4}\) using slow heating rate up to 850 \({}^{\circ}\)C. The 10 g batches were mixed in a mortar, and melted in platinum crucible for 30 min between 1150 \({}^{\circ}\)C to 1525 \({}^{\circ}\)C, depending on the glass composition. To ensure the homogeneity of the molten glass, the glass melt was mixed before quenching. After quenching, the glasses were annealed at 40 \({}^{\circ}\)C below their respective glass transition temperature for 5 h to release the stress from the quench. Finally, the glass samples were polished and also crushed into powder. A scanning electron microscope (Carl Zeiss Crossbeam 540) equipped with Oxford Instruments X-MaxN 80 EDS detector was used to check the composition of the glasses and within the accuracy of the analysis (\(\pm\)1.5 mol %), the composition of all the glasses was found to be in agreement with the nominal one. The glass transition temperature (T\({}_{\mathrm{g}}\)) and crystallization temperature (T\({}_{\mathrm{p}}\)) were measured by differential thermal analysis (DTA) using a Netszch JUPITER F1 instrument. The measurement was carried out in a Pt crucible at a heating rate of 10 \({}^{\circ}\)C/min. T\({}_{\mathrm{g}}\) was determined as the inflection point of the endotherm obtained by taking the first derivative of the DTA curve, while T\({}_{\mathrm{p}}\) was taken as the maximum peak of the exotherm. The onset of the crystallization peak, T\({}_{\mathrm{x}}\), was also measured. All measurements were performed with an error of \(\pm\)3 \({}^{\circ}\)C.  
The density of the glasses was measured using Archimedes' method with an accuracy of \(\pm\)0.02 g/cm\({}^{3}\), using ethanol as immersion liquid. The absorption spectra of the bulk glasses were measured using UV-Vis-NIR spectrophotometer (UV-360 Plus, Shimadzu) from 200 to 1750 nm with the resolution of 0.5 nm. The absorption cross-section \(\sigma_{\mathrm{abs}}\) (\(\lambda\)) is calculated using Equation (1). \[\sigma_{Abs}(\lambda)=\frac{2.303}{NL}\log\biggl{(}\frac{I_{0}}{I}\biggr{)} \tag{1}\]  
where log(\(I_{0}\)/\(I\)) is the absorbance, \(L\) is the thickness of the sample (in cm) and \(N\) is the concentration of rare-earth ions (ions/cm\({}^{3}\)). The concentration of Er\({}^{3+}\) ions was calculated from the measured glasses density. The accuracy of absorption cross-section measurement was \(\pm\)10%. The IR spectra were measured using a 'Perkin Elmer Spectrum one FTIR (Fourier-transform infrared spectroscopy) spectrometer' in Attenuated Total Reflectance (ATR) mode in mid infrared region 600-1400 cm\({}^{-1}\). The resolution used was 2 cm\({}^{-1}\) and the spectra were obtained from the accumulation of 8 scans. The Raman spectra of the samples were measured using Thermo ScientificTM DXRTTM 2 xi Raman imaging microscope. The wavelength of the laser was 532 nm and the power at the sample was 2.0 mW. The exposure time was 0.2 s per pixel and data was averaged four times. The emission spectra in the 1400-1700 nm range were measured on sample crushed into powder with a Jobin Yvon iHR320 spectrometer equipped with a Hamamatsu P4631-02 detector and a filter (Thorlabs FEL 1500). Emission spectra were obtained at room temperature using an excitation monochromatic light at 976 nm, emitted by a single-mode fiber pigtailed laser diode (CM962 UF76P-10R, Oclaro).