# A Review of Mid-Infrared Supercontinuum Generation in Chalcogenide Glass Fibers
## 5 Toward All-Fiber SC Sources Based on ChG Fibers  
Many distinguished achievements have been made in MIR SC generation by using ChG fibers, and most of this work has employed free-space systems. However, these systems are generally unsuitable for practical applications, since OPA and OPO are typically complex and expensive [107; 108]. Therefore, it is necessary to achieve all-fiber and alignment-free broadband SC sources with high-power spectral densities. To achieve this goal, much effort has been put into developing all-fiber systems. Fiber-based SC generation based on amplifier pumping sources or fiber lasers has the advantages of having a much smaller size, lower weight, and power consumption, and much more rugged packaging. Thus, it has the potential to offer robust, compact, and low-cost solutions. MIR laser sources with good compactness and beam quality show great potential in all-fiber and broadband SC generation. On one hand, multistage master oscillator power amplifiers (MOPAs) have been successfully used as pump sources for all-fiber MIR SC generation in ChG fibers. A typical schematic of such a pumping system is shown in Figure 12, in which a 1.55 \(\upmu\)m seed laser is amplified in multiple stages and Raman-shifted in highly nonlinear fibers into the IR range to pump ChG fibers which are used in the last stage. By adopting this type of pumping scheme, Shaw and Gattass et al. [98; 99] achieved SCs with spectral coverages of 1.5-5 \(\upmu\)m (140 mW output power) and 1.9-4.8 \(\upmu\)m (565 mW output power), respectively, by using 2-m-long step-index As\({}_{2}\)S\({}_{3}\) fibers with a core/ cladding diameter of 10/140 \(\upmu\)m and an NA of 0.3. On the other hand, MIR lasers can usually be obtained from non-silica  
Figure 11: (**a**) and (**c**) are spectrum evolutions in two tapered fibers with different structures; (**b**) and (**d**) are the corresponding degrees of coherence (reproduced with permission from [170], copyright AIP Publishing, 2016). fibers doped with rare-earth ions. By doping Er\({}^{3+}\), Ho\({}^{3+}\), Pr\({}^{3+}\), or Dy\({}^{3+}\) ions into fluoride fibers, laser transitions of around 3.0 [175; 176; 177; 178], 3.5 [179] and 3.9 \(\upmu\)m [180] have been realized. Hudson et al. [181] recently achieved an SC generation (1.8-9.5 \(\upmu\)m) with an average power of more than 30 mW by pumping an As\({}_{2}\)Se\({}_{3}\) taper with a 4-m-long Ho\({}^{3+}\)/ Pr\({}^{3+}\) co-doped ZBLAN fiber laser which emitted a stable fs pulse train of 2874 nm, with an average power of 140 mW, at a repetition rate of 42 MHz. Moreover, MIR lasers can also be achieved using nonlinear gain through optical frequency conversions in passive non-silica fibers [182]. For example, by using a low-loss Fabry-Perot cavity, formed by a pair of Bragg gratings written into a single-mode As\({}_{2}\)S\({}_{3}\) fiber, Raman fiber lasers at a wavelength of 3.34 \(\upmu\)m and a peak power of 0.6 W, and a wavelength of 3.77 \(\upmu\)m and a peak power of 112 mW were reported by Martin Bernier et al. in refs. [183] and [184], respectively. These achievements represent an important step towards the development of compact laser sources operating in the MIR. However, for fiber lasers doped with rare-earth ions, many wavelengths cannot be accessed by using rare-earth gain alone [185]. For Raman fiber lasers, the Raman lasing process is highly restricted to the Raman gain of host glasses and reflected wavelengths of fiber Bragg gratings [186]. Once Raman fiber lasers are fabricated, the output wavelengths cannot be changed. Furthermore, the peak powers of Raman lasers are too low to reach the SC broadening thresholds. These disadvantages limit the practical flexibility and wavelength tunability of Raman fiber lasers as pump sources in the MIR region. Alternatively, cascading approaches have been demonstrated, where a fiber-based SC is used to pump ChG fibers to further extend the spectral coverage [187; 188; 189; 190; 191].