## 3 Results and Discussion
The Raman spectra of glasses in Figure 1b are normalized to the band at \(\sim\)1170 cm\({}^{-1}\), thus all the discussed intensity changes are expressed relatively to this main peak. The Raman spectra of the glasses exhibit bands at 300, 500, \(\sim\)700, 1170 and 1280 cm\({}^{-1}\) and several bands between 800 and 1110 cm\({}^{-1}\). As for the IR spectra, those Raman spectra are similar to those reported in [5]. The band at 300 cm\({}^{-1}\) can be attributed to ZnO\({}_{4}\) units and the bands in the 400-600 cm\({}^{-1}\) range can be related to bend mode of phosphate polyhedral and to Zn-O vibration [17]. The band centered at 700 cm\({}^{-1}\) can be related to the symmetric stretching of bridging \(\
u_{\rm sym}\)(P\(-\)O\(-\)P) of Q\({}^{2}\) groups and the band at 1020 cm\({}^{-1}\) to the symmetric stretching \(\
u_{\rm sym}\)(P\(-\)O) of terminal Q\({}^{1}\) groups [12]. The bands at \(\sim\)1170 and 1280 cm\({}^{-1}\) can be attributed to symmetric and asymmetric stretching of non-bridging \(\
u\)(PO\({}_{2}\)) of Q\({}^{2}\) groups, respectively [18]. As seen in [5], there is no evidence of Q\({}^{3}\) units, usually appearing in wavenumber larger than 1300 cm\({}^{-1}\), although the concentration of ZnO was increased up to 10 mol %. With the addition of ZnO, the bands in the 200-1100 cm\({}^{-1}\) range increase in intensity compared to that of the main band. Before discussing the IR and Raman spectra, it is crucial to recapitulate the structure of a phosphate glass. As explained in [11], in ultraphosphate glasses ([P\({}_{2}\)O\({}_{5}\)] > 50 mol %), the structural units are Q\({}^{3}\) and Q\({}^{2}\) tetrahedra, the Q\({}^{3}\) units forming the 3D network, while the Q\({}^{2}\) units assist the chain formation. With a decrease in P\({}_{2}\)O\({}_{5}\) content, the structure of the glass becomes metaphosphate formed by Q\({}^{2}\) units with chains of (P\(-\)O\(-\)P)-bridges and rings. Further reduction in P\({}_{2}\)O\({}_{5}\) content leads to the formation of polyphosphate glasses formed by shorter chains of Q\({}^{2}\), which are terminated by Q\({}^{1}\) units. Although the ZnO content was increased up to 10 mol %, the IR and Raman spectra still clearly indicate the presence of a metaphosphate structure. The presence of the Raman bands at \(\sim\)700 cm\({}^{-1}\) indicates that the chains are present with different lengths. We confirm our former assumption in [5] that up to 10 mol %, ZnO enters as a modifier leading to a depolymerization of the network and to a less cross-linked network, as seen by an increase of the Q\({}^{1}\) units at the expense of the expense of the Q\({}^{2}\) units. This is in agreement with the decrease in the T\({}_{\rm g}\) seen with an increase in x (Table 1). The absorption spectra and the normalized emission spectra of the investigated glasses are shown in Figure 2a,b, respectively. The absorption spectra exhibit several bands, which are characteristics of the Er\({}^{3+}\) ions 4f-4f transitions from the ground state to various excited levels [19]. The absorption band at 975 nm can be also related to the Yb\({}^{3+}\) 4f-4f (\({}^{2}\)F\({}_{7/2}\)\(\rightarrow\)\({}^{2}\)F\({}_{5/2}\)) transition, which overlaps with the weak absorption band due to the Er\({}^{3+}\) 4f-4f (\({}^{4}\)I\({}_{15/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{11/2}\)) transition. An increase in x leads to a shift of the band gap to longer wavelength probably due to the depolymerization of the phosphate as explained in [5]. The absorption cross-section at 975 nm and 1.5 \(\upmu\)m were calculated from the absorption coefficient using Equation (1) and are presented in Table 2.  
Within the error of the measurement, the absorption cross-section at 1.5 \(\upmu\)m are similar to those reported in [5]. However, the absorption cross-section at 975 nm of the investigated glasses are larger than those reported in [5] due to the Yb\({}^{3+}\) codoping.