## 3 Fabrications of Chalcogenide Fibers  
In order to achieve a broad-bandwidth Mid-IR supercontinuum generation, chalcogenide glass has to be synthesized under vacuumed conditions and purified to avoid optical absorption caused by the presence of chemical bonds such as O-H, Se-H, and As-O, or impurities such as CO\({}_{2}\) and H\({}_{2}\)O. To date, several purification methods have been reported, such as the purification of raw material before synthesis [23] or using microwave treatment before synthesis [24]. Also, distillation of the glass after synthesis in the presence of chemical getters proved to be efficient in reducing optical losses in fibers [25]. This last process consists of the addition of halides (such as TeCl\({}_{4}\)) and metals (such as Mg or Al) to the charge before synthesis. During the melting of the charge, metals will react with oxygen-based pollutants, while halides will react with hydrogen and carbon impurities. The byproducts of this reaction are either refractory or volatile, allowing for their removal by distillation of the glass.  
Standard chalcogenide fibers exhibit a step-index profile that can be achieved by implementing two different fabrication methods: the \"rod-in-tube\" method and the double-crucible method [26; 27; 28]. The first technique allows a better control of the fiber-core size, and consequently of the core-clad  
Figure 3: Cross section of a chalcogenide microstructured optical fiber. d: hole size, \(\Lambda\): distance between the hole (also called “pitch”).  
ratio [21; 29; 30]. The second one permits a better control of the core-clad interface [31; 32], and commercial step-index chalcogenide fibers have been fabricated by using this technique. Microstructured optical fibers have been obtained with a variety of different glasses, including chalcogenide glasses [33; 34]. In 2000, the first chalcogenide MOF, with a Ga-La-S composition, was successfully fabricated by using the \"stack-and-draw\" technique originally utilized for making silica MOF, yet guidance of light was not observed [33]. Later on, light propagation in chalcogenide MOFs based on sulfur and selenide was realized [35; 36]. Then, in 2006, single-mode propagation in a chalcogenide MOF with Ge-Ga-Sb-S composition was reported [34]. Propagation losses of the fiber were quite high, between 15 and 20 dB m\({}^{-1}\) at 1.55 \(\upmu\)m; this was attributed to the poor quality of chalcogenide glass preforms prepared by the \"stack-and-draw\" method [37]. Indeed, it has been shown that important defects at the interfaces between the chalcogenide capillaries constituting the stack induce strong optical losses [37].  
Consequently, other fabrication methods had to be developed for chalcogenide glasses. For example, the geometry of chalcogenide MOF can be realized by the \"drilling method\" or \"casting method\". The \"drilling method\" exploits mechanical drilling in chalcogenide glass preforms to create various geometrical structures. In order to avoid structure destruction, the position of the holes and the friction between glass and drills have to be precisely controlled. Besides, for the protection of the inner side of the holes, it is necessary to optimize the rotation speed and the force applied by the drills [38]. For the \"casting method\", a mold made of silica capillaries threaded in hexagonal silica guides is designed with the desired distribution of holes. Then, the chalcogenide glass, which has been heated to a quasi-liquid state, is poured into the mold. Afterward, the mold-glass ensemble is quenched in air and annealed. The silica tube and capillaries, which are still inside the MOF preform at this stage, are removed with a diamond tool and soaking in 40% concentrated hydrofluoric acid [39]. Both methods have been tested for making monolithic preforms suitable for obtaining microstructured optical fibers [38; 39; 40].