# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 2 Fluoroindate Fibers--Design and Material Properties  
Fluoride glass fibers were discovered at Rennes University in 1974 [72], and since then, these media have made tremendous progress toward commercialization. They are well-suited for high-power applications in the 2-5 \(\upmu\)m wavelength range and have mainly been used as active media for laser oscillators [73; 74; 75; 76], nonlinear media for SC generation [39; 41; 77], and laser power delivery [78; 79]. The most widespread heavy-metal fluoride glass is ZBLAN (ZrF\({}_{4}\)-BaF\({}_{2}\)-LaF\({}_{3}\)-AlF\({}_{3}\)-NaF), which is a highly stable material for high-quality optical fibers [80]. ZBLAN fibers are able to transmit more than 20 W in a single-mode core [41], but the maximum broadening is limited to \(\sim\)4.7 \(\upmu\)m, as already mentioned [81]. To overcome this limitation and extend the spectrum LWE to over 5 \(\upmu\)m, fluoroindate glass fibers have been considered the best choice. Fluoroindate or indium fluoride (InF\({}_{3}\)) glass fibers have a maximum phonon energy of \(\sim\)510 cm\({}^{-1}\)[82; 83] compared with \(\sim\)579 cm\({}^{-1}\) for fluorozirconate glasses [84; 85], and thus, they can provide a transmission window with an LWE extended to \(\sim\)5.5 \(\upmu\)m. They also exhibit a higher damage threshold than ChG fibers [83]. Moreover, fluoroindates have a glass transition temperature (Tg) of \(\sim\)30 \({}^{\circ}\)C [57; 81]; that is, higher than those of ZBLANs (Tg \(\sim\) 260 \({}^{\circ}\)C [86; 87]) and ChG glasses (Tg \(\sim\) 185 \({}^{\circ}\)C for As\({}_{2}\)S\({}_{3}\); Tg \(\sim\) 178 \({}^{\circ}\)C for As\({}_{2}\)Se\({}_{3}\)) [88]. The refractive index of these glasses is within the range of 1.47-1.53 [89; 90], comparable to that of silicates, indicating that the Fresnel loss is less than 4%. The nonlinear refractive index \(n_{2}\) has been recently determined to be 3.2-4.3 \(\times\) 10-20 m\({}^{2}\)/W [91]; that is, 1.5\(\times\) higher than that for ZBLAN fibers and an advantage in the context of SC generation. Currently, InF\({}_{3}\) glass can be drawn into high-quality optical fibers. There are three commercial manufacturers of single-mode fluoride fibers (Le Verre Fluore, Thorlabs, and FiberLabs); however, only two offer single-mode InF\({}_{3}\) fibers. Figure 2 shows the attenuation curves of InF\({}_{3}\) fibers obtained from two different producers as a function of the wavelength. The commercially available fiber offered by Thorlabs exhibits losses \(<\) 0.2 dB/m for all wavelengths between \(\sim\)1.6 and 4.6 \(\upmu\)m with a minimum value of \(\sim\)0.03 dB/m at \(\sim\)3.6 \(\upmu\)m. However, the attenuation of the custom-made fibers could be even lower. For instance, Le Verre Fluore provides InF\({}_{3}\) fibers with losses as low as 0.02 dB/m within the wavelength band of \(\sim\)1.9-4.1 \(\upmu\)m. For all the presented fibers, for wavelengths over 4.8 \(\upmu\)m, the material losses significantly increase, reaching more than 0.6 dB/m at 5 \(\upmu\)m. The main contribution to absorption is made by the OH\({}^{-}\) groups at a wavelength of \(\sim\)2.9 \(\upmu\)m [92]. Multiphonon absorption is predominant beyond 4 \(\upmu\)m.  
The currently available single-mode fluoroindate fibers have cores with diameters ranging from 7 to 16 \(\upmu\)m and an NA of 0.14-0.3, and they are drawn by applying the mainstream preform technique. Drawing parameters yielding high-quality optical fibers are critical; however, they are difficult to control [56]. In particular, impurities and moisture in the atmosphere during the preparation of fluoride glasses can easily induce crystallization, leading to the generation of scattering centers and increased losses [95].