## 2 Materials and Methods  
We developed our step-index tellurite fibers for mid-IR SC studies through the built-in-casting and the rod-in tube techniques which are well described in the literature [18; 27]. A compatible core/cladding glass pair was chosen based on their optical, thermal and mechanical compatibility (Table 1) for the built-in-casting process, and their suitable refractive index difference [27]. The compositions are respectively 80 TeO\({}_{2}\)-5 ZnO-10 Na\({}_{2}\)O-5 ZnF\({}_{2}\) (TZNF, molar fraction) and 60 TeO\({}_{2}\)-5 ZnO 20 Na\({}_{2}\)O-15 GeO\({}_{2}\) (TZNG). The synthesis and quenching of the glass preform are performed in a glove box under dry atmosphere to avoid any water contaminations from the environment and thus reducing the amount of residual OH groups in the glass matrix. To further reduce the presence of hydroxyl compounds and limit the mid-IR absorption of the fibre, zinc fluoride is added to the core composition for dehydration of the bulk glass. TZNF and TZNG glasses are synthesised from high purity precursors (5N for TeO\({}_{2}\) and GeO\({}_{2}\), 5N for ZnO and 6N for Na\({}_{2}\)CO\({}_{3}\), all from Fox Chemical and 4N from Alfa Aesar for ZnF\({}_{2}\)). The powders are weighed and mixed in platinum crucibles and then placed in the same electric furnace, one above the other, for 1 h at 650 \({}^{\circ}\)C to allow the decarbonation process of the Na\({}_{2}\)CO\({}_{3}\) to take place and then at 900 \({}^{\circ}\)C for 2 h (resp. 1 h 15) for the TZNF glass (resp. TZNG glass). There is a constant flow of dry O\({}_{2}\) (2 L\(\cdot\)min\({}^{-1}\)) in the synthesis furnace to control the oxidation conditions of the batch during both the decarbonation and the melting steps. To limit the evaporation of some of the precursors, a platinum cap is used and thus avoids some variations of the glasses' compositions. Both melted glasses are quenched in a cylindrical brass mould pre-heated at 240 \({}^{\circ}\)C and then annealed for 8 h at 270 \({}^{\circ}\)C. At the end of this process, we obtain a 45 mm long and 16 mm large cylindrical preform inside which it is easy to differentiate the two different compositions; this is shown in Figure 1.  
One part of the step-index glass preform obtained thereby is heated above the vitreous transition temperature T\({}_{\text{g}}\) in order to be drawn into a large core step-index fibre with a core to cladding diameter ratio of roughly 50% for loss measurement; this is shown in Figure 2. The fiber exhibits background losses below 1 dB/m between 1 and 2.8 \(\upmu\)m. Thanks to our dehydration process, the glass OH content is very low, namely the water-related absorption band between 3 and 4 \(\upmu\)m is almost eliminated, before losses increase when reaching the multiphonon absorption edge around 4 \(\upmu\)m. Even at such wavelength, the fiber losses still remain moderate when considering cm-long fiber segments for SC studies in the following. \begin{table}
\begin{tabular}{c c c c} \hline  & **n (633 nm)** & **Tg (\({}^{\circ}\)C)** & **\(\alpha\) (\(\times\)10\({}^{-6.^{\circ}}\)C\({}^{-1}\))** \\ \hline TZNF & 2.037 & 280 & 16.7 \\ TZNG & 1.905 & 272 & 13.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Principal properties of the core and cladding glasses used for built-in-casting. Figure 1: Pictures of the cross-section (**a**) and side-section (**b**) of the step-index preform synthetized with the built-in-casting method. The second part of the step-index glass preform is drawn into capillaries with an outer diameter of 800 \(\upmu\)m by drastically reducing the drawing speed, and then used for the next rod-in-tube stage. A second cladding glass preform used as a tube is made following the same protocol as previously described and drilled mechanically. The step-index rod is then inserted into the hole of the second cladding preform and another drawing stage is performed to obtain small-core step-index fibers (see Figure 3).