# A Review of Mid-Infrared Supercontinuum Generation in Chalcogenide Glass Fibers
## 3 MIR SC Generation in ChG Fibers
### SC Generation in ChG Tapered Fibers  
Another important method for tailoring the dispersion property is the tapering of fibers, which can engineer the overall dispersion from normal to zero or even into the anomalous region and enable a dramatic increase in nonlinearity by reducing the effective mode area. Using a ChG tapered fiber with an As\({}_{2}\)Se\({}_{3}\) core and As\({}_{2}\)S\({}_{3}\) cladding as an example, the dispersion, \(\gamma\), and A\({}_{\text{eff}}\) as a function of the fiber core diameter are shown in Figure 9. Owing to the improvement of fiber fabrication and tapering techniques, ChG tapered fibers have been used in many applications, such as frequency conversion, chemical sensing, and SC generation. In particular, by using tapered fibers, the pump power required for SC generation can be decreased, and the pump wavelength can be shortened. The use of ChG tapered fibers for SC generation was first demonstrated in 2007 by Magi et al. [70]. An As\({}_{2}\)Se\({}_{3}\) ChG fiber with a cladding diameter of 165 \(\upmu\)m was tapered down to a waist diameter  
Figure 8: Measured SC generation in 25-mm-long fresh (**a**) and 7-month aged (**b**) As\({}_{2}\)S\({}_{3}\) fiber pumped with various wavelengths at average power of 190 mW (reproduced with permission from [85], copyright Springer Nature, 2016). Figure 9: Dispersion (**a**), nonlinear coefficient (\(\gamma\)), and effective mode area (A\({}_{\text{eff}}\)) (**b**) of a ChG tapered fiber with an As\({}_{2}\)Se\({}_{3}\) core and As\({}_{2}\)S\({}_{3}\) cladding as a function of the fiber’s core diameter. of 1.2 \(\upmu\)m with a nonlinear coefficient (\(\gamma\)) of 68,000 W\({}^{-1}\)km\({}^{-1}\), which was 62,000 times larger than that of standard single-mode silica fibers. A mode-locked fiber laser with a pulse duration of 1.48 ps, repetition rate of 4 MHz, and wavelength of 1545 nm was used as the pump source to excite the tapered fiber. The pump pulses were extended at the pump peak power of 5.69 W by the nonlinear process of SPM. Given the longer dispersion length compared with the length of the sample, the dispersion did not significantly contribute to the spectral broadening. In 2011, Hudson at al. [68] increased the \(\gamma\) to 12,400 W\({}^{-1}\) km\({}^{-1}\) in an As\({}_{2}\)S\({}_{3}\) tapered fiber with a taper waist diameter of 1.3 \(\upmu\)m and a mode area of 0.8 \(\upmu\)m\({}^{2}\), exhibiting a ZDW around 1.4 \(\upmu\)m. Pump pulses with a duration of 250 fs and a repetition frequency of 38.6 MHz were generated from an Er-fiber-based, mode-locked laser. By carefully tailoring the input pulse chirp and utilizing the robust power handing capabilities, a -20 dB bandwidth spanning from 970 to 1990 nm was achieved from the fiber taper using a pump pulse energy of only 77 pJ (3 mW average power). Rudy et al. [148] adopted a Tm-doped fiber laser and amplifier system as the pump source, which produced sub-100-fs pulses with a center wavelength of 2.04 \(\upmu\)m at a repetition rate of 75 MHz. An As\({}_{2}\)S\({}_{3}\) single mode fiber with a core/cladding diameter of 7/160 \(\upmu\)m was tapered to a diameter of 2 \(\upmu\)m using the in situ tapering method to shift the dispersion to the anomalous region. A ~1 to 3.7 \(\upmu\)m SC was obtained from this tapered fiber at a pulse energy of 300 pJ. OPA has also been used as a pump source to excite ChG tapered fibers to broaden SCs to longer wavelengths. Sun et al. [34] fabricated an As\({}_{2}\)Se\({}_{3}\)-As\({}_{2}\)S\({}_{3}\) fiber with high numerical aperture (NA) of 1.45. A 15-cm-long tapered fiber with a core diameter at the taper waist of 1.9 \(\upmu\)m was prepared for SC generation. This tapered fiber exhibited a ZDW of 3.3 \(\upmu\)m and a low anomalous dispersion at longer wavelengths. When pumped by pulses from an OPA with a pulse duration of 100 fs and a peak power of 500 kW at 3.4 \(\upmu\)m, the generated SC in this tapered fiber had a 20 dB spectrum covering from 1.5 to longer than 4.8 \(\upmu\)m (limited by the measurement range of the spectrometer).