# A Review of Mid-Infrared Supercontinuum Generation in Chalcogenide Glass Fibers
## 3 MIR SC Generation in ChG Fibers
### SC Generation in ChG Step-Index Fibers
Moreover, the use of ChG step-index fibers in SC generation was limited by the lack of high peak power pump sources in the MIR until the discovery that optical parametric amplifiers (OPAs) and oscillators (OPOs) could be successful employed as MIR pump sources to excite fibers in the anomalous dispersion regime [107; 108]. An ultrabroad MIR SC, spanning from 1.4 to 13.3 \(\upmu\)m, was generated experimentally when ~100 fs pulses with a central wavelength of 6.3 \(\upmu\)m and a repetition rate of 1 kHz were launched into an 85-mm-long step-index fiber made of As\({}_{2}\)Se\({}_{3}\) core and Ge\({}_{10}\)As\({}_{23,4}\)Se\({}_{66,6}\) cladding. The pump peak power was estimated to be 2.29 MW, and the pump wavelength was located in the anomalous dispersion regime, just above the fiber ZDW of 5.83 \(\upmu\)m [105]. Figure 4 shows the setup used for SC generation and the measured SC result. This was the first time that an SC source with a spectral width of >10 \(\upmu\)m had been achieved in ChG fibers, thus representing a breakthrough in broadband MIR light source research. The transparent atmospheric windows of 3-5 \(\upmu\)m and 8-13 \(\upmu\)m, and the significant part of the fingerprint region of molecular vibrational resonances were covered. Since then, the generation of broadband SC sources in ChG step-index fibers has made rapid development, driven by the large number of potential applications in MIR technology. Researchers have subsequently focused on the enhancement of SC broadening. ChG fibers with short lengths, small core diameters, high nonlinearity, and optimized pump conditions have been designed and used to generate SC spectra with good performance [17; 55; 109; 110; 111; 112]. For example, an As-Se fiber with a length of 3 cm and a ZDW of 5.5 \(\upmu\)m was pumped by pulses of 9.8 \(\upmu\)m with a pulse duration of 170 fs and a repetition rate of 1 kHz, produced from a difference in frequency  
Figure 4: (a) Setup for supercontinuum (SC) generation in the ChG fiber. BD-2, black-diamond-2 aspheric lenses; CM, concave mirror; LPF, long-pass filter; MCT, mercury cadmium telluride; NDF, neutral density filter; NDFG, noncollinear difference frequency generation; OPA: optical parametric amplifier. (b) Measured SC generation in the ChG fiber pumped at 6.3 \(\upmu\)m (reproduced with permission from [105], copyright Springer Nature, 2014). generation. When the peak power was 2.89 MW, the generation of SC spanning from 2 to 15.1 \(\upmu\)m was obtained [113]. Meanwhile, researchers started to explore novel glass systems of ChG fibers to achieve MIR SC. Ou et al. [114] reported the fabrication of a fiber with the core and cladding made of Ge\({}_{15}\)Sb\({}_{25}\)Se\({}_{60}\) and Ge\({}_{15}\)Sb\({}_{20}\)Se\({}_{65}\) glasses, respectively. An SC generation spanning from 1.8 to 14 \(\upmu\)m was achieved in a 20-cm-long fiber with a core diameter of 23 \(\upmu\)m by pumping at 6.0 \(\upmu\)m in the anomalous dispersion regime. The optical nonlinearity of the core glass (19 \(\times\) 10\({}^{-18}\) m\({}^{2}\)/W) was greater than that of As\({}_{2}\)Se\({}_{3}\) because of the replacement of As by more the metallic Sb [110; 115; 116]. Furthermore, Ge-Sb-Se glasses exhibit higher thermal and mechanical durability and have been demonstrated to be suitable for molded IR-transmitting lenses. The most important factor is that visible damage arises at a beam intensity of 3674 GW/cm\({}^{2}\) for Ge-Sb-Se glass at 3.0 \(\upmu\)m, a value more than twice than that for As\({}_{2}\)Se\({}_{3}\) glass (1524 GW/cm\({}^{2}\)) [114]. This indicates that Ge-Sb-Se fibers can endure a relatively higher peak power. Such a large nonlinearity and a high laser-induced damage threshold of the glass gives Ge-Sb-Se fibers great potential to be used with an ultra-broad MIR SC with high output power.