# Recent Development of Mid-Infrared Supercontinuum Generation in Fluoroindate Glass Fibers
## 1 Introduction
In the case of SC sources pumped with longer pulses (of the order of picoseconds and nanoseconds) at a wavelength corresponding to the anomalous dispersion regime, the emission of new frequencies is initialized with SPM and modulation instability (MI) [29], manifesting as a disintegration of the long pulses into a distributed spectrum of femtosecond sub-pulses during propagation in the fiber core. In the second step, the ultrafast sub-pulses are red-shifted toward the mid-IR region via soliton dynamics, such as soliton fission and soliton frequency shift, owing to the Raman effect [25; 29]. The dominant physical mechanisms underlying SC generation in a fiber pumped with longer pulses in the normal dispersion region are essentially FWM and Raman scattering [29; 30]. The choice of a nonlinear material that can be considered for SC generation is determined by a number of parameters, such as the transmission band, nonlinearity, ability to handle high optical power, availability of materials of sufficient quality, and maturity of fiber manufacturing technology that is particularly important, considering practical aspects. To date, silica-based fibers are the predominant nonlinear media for SC generation, mainly owing to their high strength, low losses, high resistance to atmospheric conditions, and above all, their mature drawing technology. Such systems utilizing both passive and active silica fibers have been widely reported over the last decade (see [31; 32; 33] for examples). However, the longest emission wavelength that can be generated in this medium is \(\sim\)2.8 \(\upmu\)m, which results from the maximum phonon energy of silica glass (1100 cm\({}^{-1}\)) [34]. The use of silica glass fibers with high-GeO\({}_{2}\)-doped cores has extended the transmission beyond 3.4 \(\upmu\)m [35; 36]. However, to extend the spectrum toward the mid-IR further, the use of soft-glass fibers (with wider infrared transmission windows), such as fluoride [37; 38; 39; 40; 41; 42; 43; 44; 45], tellurite [46; 47; 48], and chalcogenide (ChG) (e.g., As\({}_{2}\)S\({}_{3}\) and As\({}_{2}\)Se\({}_{3}\)) [49; 50; 51; 52; 53; 54; 55] fibers, is manda  
Figure 1: Schematic setup for SC generation. tory. Each of these media offers certain advantages and disadvantages, such as ease of manufacture, physical strength, wavelength transmission band, corrosion resistance, and thermal damage. Fluorozirconate (ZBLAN) fibers have been used for SC generation with an average output power of several tens of watts; however, the output spectrum tends to be limited to \(\sim\)4.7 \(\upmu\)m on the long-wavelength side [38; 39; 41], which results from intrinsic fiber losses at longer wavelengths. Fluoroindate (InF\({}_{3}\)) fibers with lower phonon energy can guarantee an extended transmission window (to \(\sim\)5.5 \(\upmu\)m [56; 57]) and the delivery of high average power output beams [58; 59; 60; 61]. The recent progress in the drawing technology of single-mode tellurite fibers supporting light transmission in a band of \(\sim\)0.4-5 \(\upmu\)m allows considering them theoretically as an alternative to fluoride fibers [46; 62; 63]. Nevertheless, the current literature reports show that increased material absorption is related to water retention, and, consequently, a reduced damage threshold still hinders the scaling up of the SC power in this medium. Recently, an SC output power of 22.7 W generated from a fluotellurite fiber was also demonstrated [64], but in this case, the spectrum long-wavelength edge (LWE) was limited to 3.95 \(\upmu\)m. For all aforementioned soft-glass fibers, the LWE of the spectrum is determined by the multiphonon absorption of the nonlinear medium. To achieve emission at wavelengths beyond this edge, short fiber lengths and pumping with high-intensity pulses are required. This approach is typically implemented in a laboratory environment.