# 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  
The study of SC generation in ChG step-index fibers started in 2005. Wei et al. [16] demonstrated SC generation using a 1.5-m-long As\({}_{2}\)S\({}_{3}\) single-mode step-index fiber pumped in the normal dispersion regime. A passively 1.5 \(\upmu\)m mode-locked Er-doped fiber laser with a pulse duration of 100 fs at a repetition rate of 20 MHz was used as the pump source. Using the maximum fiber-input power of 16.4 mW, the output spectrum was broadened with a \(-15\) dB bandwidth of 310 nm. Although the spectral width of the SC generation was not wide, the observed spectral broadening was very attractive, and it confirmed the potential for ChG fibers to be used for MIR SC generation. Highly nonlinear ChG fibers are therefore promising media for compact broadband MIR sources designed for different potential applications. In the following years, some researchers reported SC generation in ChG step-index fibers which were excited by fiber lasers at short wavelengths [98; 99; 100]. However, broadband MIR SC generation was barely achieved in ChG step-index fibers pumped with a normal dispersion regime with a short pump wavelength from a fiber laser. This is because during the process of spectral extension in the normal dispersion regime, the nonlinear effects of SPM, optical wave break (OWB), and stimulated Raman Scattering (SRS) play important roles [63]. The SPM effect during the initial propagation in the fiber makes the broadened pump pulse spectra maintain almost symmetric shapes with typical oscillatory structures, which is induced by spectral interference at different temporal locations within the pulse where the same spectral components exist. Then, OWB occurs on the trailing and leading edges of the pulse, which is responsible for the additional broadening on both sides of the spectrum [20]. Moreover, the slightly asymmetric extension of the generated spectrum and the slightly increased energy of the relatively longer wavelengths are attributed to the SRS [16]. However, these nonlinear effects are limited by the pump peak power and dispersion slopes, inevitably impairing spectral broadening. Generally, efficient and broadband SC generation can be obtained by pumping in the anomalous dispersion regime close to the ZDW of the fiber. In this case, SC generation in fibers is dominated by soliton dynamics, such as soliton fission and soliton self-frequency shift in the long wavelengths, and dispersion waves in the short wavelength [101; 102; 103; 68]. First, the pump pulse transforms into a higher-order soliton, which shows periodic spectral and temporal evolution over a soliton period. Higher-order dispersion and SRS are the two most important effects that can perturb the ideal periodic evolution of the higher-order soliton and induce pulse breakup into multiple fundamental solitons through soliton fission. Then, each soliton experiences a continuous shift to longer wavelengths due to the Raman self-frequency shift [2]. Solitons that are ejected earlier from the input pulse have relatively shorter durations and faster propagation group velocities, leading to a greater frequency red-shift [2; 105]. Under higher-order dispersion perturbing, solitons emit dispersion waves via the Cherenkov radiation at wavelengths that are phase-matched with solitons in the normal dispersion regime [104; 106]. Finally, solitons trap the dispersion waves by group velocity matching, and causethem to shift to short wavelengths. Additional frequency components can also be generated by the cross-phase modulation and four wave mixing, and these effects can further broaden the spectral bandwidth of the achieved SC. However, depending partly on the material dispersion, the ZDWs of ChG step-index fibers are usually located at long wavelengths (>5 \(\upmu\)m). Therefore, it is difficult to match the ZDWs of ChG fibers with the operation wavelengths of commercial fiber lasers.