# A Review of Mid-Infrared Supercontinuum Generation in Chalcogenide Glass Fibers
## 5 Toward All-Fiber SC Sources Based on ChG Fibers
This cascading approach is mainly based on the SC spectrum that contains many solitons with fs pulses and high peak powers in the long wavelength range from the first fiber with anomalous dispersion [2]. As long as the second fiber has a ZDW below the SC long wavelength edge, the solitons can continue to redshift in the second fiber when two fibers are cascading, which can then lead to wide SC generation. In 2014, Kubat et al. [187] numerically proposed a cascading scheme for SC generation in ChGs pumped with MIR SC sources (Figure 13), where the SC was extended to 9 \(\upmu\)m in a 10-cm-long As\({}_{2}\)Se\({}_{3}\) MOF with a core diameter of 5 \(\upmu\)m by pumping with a 0.9-4.1 \(\upmu\)m SC generated from a ZBLAN fiber. The ZBLAN fiber was chosen as the first fiber in the cascading scheme because of its following excellent properties: high damage threshold; ability to generate SC sources with >3.5 \(\upmu\)m long wavelength edges extending above the ZDW of typical step-index ZBLAN fibers (around 1.6 \(\upmu\)m); and high output powers of 13 [192], 15.2 [190], and even 21.8 W [193]. Petersen et al. [189] experimentally demonstrated an SC spectrum which was extended to 7 \(\upmu\)m with an output power of 6.5 mW in an As\({}_{38}\)Se\({}_{62}\) SCF with a 4-\(\upmu\)m-core, pumped by an SC source with along wavelength edge of 4.4 \(\upmu\)m and power of 51.4 mW, from a 5-m-long ZBLAN fiber with a carefully designed dispersion. Yin et al. [194] adopted a thulium-doped fiber amplifier (a 2-2.5 \(\upmu\)m SC source with a maximum output power of ~1 W) to generate a 2-4.2 \(\upmu\)m SC source in a 12-m-long ZBLAN fiber, which was then used as the pump light for an As\({}_{2}\)S\({}_{3}\) step-index fiber. Spectrally flat 2-5 \(\upmu\)m SC generation with a high power of 57.6 mW was eventually realized. Furthermore, Robichaud et al. [188] adopted a 3-4.2 \(\upmu\)m SC source from a 1-m-long ZrF\({}_{4}\) fiber pumped with an Er-doped ZrF\({}_{4}\) fiber amplifier to excite a low-loss As\({}_{2}\)Se\({}_{3}\) fiber. As a result, a MIR SC source spanning from 3 to 8 \(\upmu\)m with an output power of 1.5 mW was demonstrated. Figure 12: A typical schematic of fiber-based SC generation based on amplifier pumping sources. HNLF, highly nonlinear fiber. Recently, Martinez et al. [195] utilized a MOPA to pump cascading ZBLAN, As\({}_{2}\)S\({}_{3}\), and As\({}_{2}\)Se\({}_{3}\) fibers. Figure 14a shows the corresponding setup for SC generation. The As\({}_{2}\)S\({}_{3}\) fiber was excited by the SC sources that were extended to 4.5 \(\upmu\)m from the ZBLAN fiber. Actually, the As\({}_{2}\)S\({}_{3}\) fiber played an exceptional role as an intermediary between the ZBLAN and As\({}_{2}\)Se\({}_{3}\) fibers because of its high power handling capability. When the pulse's repetition rate was scaled up to 800 kHz, the cascading SC generation shown in Figure 14b spanned a broadband spectrum, from 1.6 to \(>\)11 \(\upmu\)m, from the As\({}_{2}\)Se\({}_{3}\) step-index fibers, and produced a high output power of 417 mW. Such an SC generation exhibited the wide spectral coverage and high average power required for practical applications. All-fiber MIR SC generation based on ChG fibers is the development trend for the future. However, although levels of hundreds of mW have been reached, the output powers of SC sources generated from ChG fibers still need to be improved. On one hand, the pump power injected into ChG fibers needs to be increased by adjusting the repetition rate and pulse width of the seed sources to reach a W level average power range. On the other hand, the propagation losses of the all-fiber system  
Figure 14: (**a**) Setup of MOPA and cascading scheme for all-fiber SC generation. (**b**) Measured SC spectra from ZBLAN (black), AS\({}_{2}\)S\({}_{3}\) (red), and As\({}_{2}\)Se\({}_{3}\) (blue) fibers (reproduced with permission from [195], copyright Optical Society of America, 2018).