# A Review of Mid-Infrared Supercontinuum Generation in Chalcogenide Glass Fibers
## 3 MIR SC Generation in ChG Fibers
### SC Generation in ChG Tapered Fibers
Many researchers have attempted to extend the spectrum into the MIR region and increase the output power from ChG tapered fibers. However, the long wavelength edges of SCs obtained in ChG tapered fibers have been limited to 5 \(\upmu\)m, with output powers lower than 15 mW. This phenomenon can be attributed not only to the low input power, but also to the energy loss in the transition region of the tapered fiber due to the decrease in fiber size. Apart from increasing pump power, enhancing nonlinearity and reducing power consumption in higher-order modes of fibers, both are effective methods to broaden SC bandwidth and improve the output power in ChG tapered fibers. Tapered MOFs with large-mode areas can also be used for SC generation; thus, a high coupling efficiency and damage threshold for the pump power injected to the fiber end facet will be obtained, while the tapered waist offers strong nonlinearity and anomalous dispersion at the pump wavelength. Moreover, the MOF structure can ensure single-mode propagation, which improves the beam quality and reduces losses in the taper due to higher-order mode stripping. Peterson et al. [149] tapered a 15.1 \(\upmu\)m core Ge\({}_{10}\)As\({}_{22}\)Se\({}_{68}\) PCF with a large mode area into a core diameter of 5.9 \(\upmu\)m at the taper waist. The higher-order modes excited in the large-core fiber were stripped during propagation in the transition region. By using an OPA source with a pulse duration of 250 fs and a repetition rate of 21 MHz at 4 \(\upmu\)m, the broadest bandwidth (from 1 to 11.5 \(\upmu\)m) was obtained with an average output power of 35.4 mW, and the maximum output power of 57.3 mW had a spectral bandwidth spanning from 1 to 8 \(\upmu\)m in the tapered PCFs. Several improvements were also derived to guide future optimization of tapered fibers. Relatively shorter taper waists, a higher pitch-to-hole ratio, and shorter fiber lengths before the taper are all advantageous for extending SC to longer wavelengths. Furthermore, other structural parameters of tapered fibers, such as taper waist diameter and transition region length can affect the SC results. Some researchers have demonstrated that wider SC generation can be achieved in a tapered fiber with a smaller diameter [150; 151; 152]. A small fiber diameter produces a small \(A_{\mathit{eff}}\) in ChG tapered fibers, which increases the laser power intensity at the taper waist. Nonlinear power intensity dependent effects, such as SPM and SRS, are significant, and can lead to wide SC generation. However, broadband SC generation from the tapered fiber is often obtained at the cost of low output power. The effects of the transition region length of an As-S tapered fiber on SC spectral behavior were investigated by Wang et al. [153]. As shown by the experimental results in Figure 10, when the diameter of the transition region changes slowly with a relatively longer transition length, this is always accompanied by a lower number of higher-order modes, and thus, a higher transmission efficiency of light, resulting in a higher output power and a larger spectral broadening from the tapered fiber. SC generation spanning 1.4-7.2 \(\upmu\)m with an average power of 1.06 mW was achieved from a 12-cm-long, As-S tapered fiber with a transition region length of 10 mm, pumped by an OPA with a pulse duration of 150 fs and a repetition rate of 1 kHz at 3.25 \(\upmu\)m.  
Owing to the requirements of dispersion control and nonlinearity, fibers are usually tapered down to extremely small diameters of a few micrometers or even submicrons [96; 151]. This phenomenon results in the very low mechanical strength of tapered fibers during preparation, optical testing, and practical applications. To overcome this obstacle, polymer jackets with large diameters have been used to coat ChG fibers for convenient handling. The commonly used polymers are PMMA [69; 95], PES [61; 151], and PEI [151], which are thermally compatible with ChG fibers and can be tapered without removal.