# 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
Similarly, a Ge-Sb-Se fiber with a core diameter of 6 \(\upmu\)m and length of 11 cm was pumped at 4.485 \(\upmu\)m with 330 fs pulses, and an SC covering the 2-12 \(\upmu\)m spectral range with an output average power of \(\sim\)17 mW was achieved [117]. Te-based glasses have low phonon energies and high linear refractive indices, which results in wide optical windows and high nonlinear refractive indices [118; 119; 120]. Thus, relatively wider SC generation can be expected from Te-based fibers. However, the metallic characteristic of Te can lead to a greater tendency for crystallite formation, which may prevent the production of low-loss optical fibers due to scattering effects. A second consequence of the metallic characteristic of Te is a low bandgap that results in significant background carrier absorption caused at room temperature by thermally excited charge carriers. However, these drawbacks can be alleviated by substituting a small amount of Te with Se to lower the conductivity and dramatically increase the resistance to crystallization while retaining a wide optical window in the long wavelengths [115]. According to a previous report, Ge-As-Se-Te step-index fibers can be fabricated for long-wave transmissions as high as 12 \(\upmu\)m [121], but the optical fiber loss needs to be decreased to fit the requirements of SC generation and other applications. Zhao et al. [122] fabricated a fiber based on Ge-As-Se-Te with a transmission loss of 2-3 dB/m in the 6.2-10.3 \(\upmu\)m wavelength range, and experimentally demonstrated MIR SC generation spanning from 1.5-14 \(\upmu\)m when pumped by a 4.5 \(\upmu\)m laser with a pulse duration of 150 fs and a repetition rate of 1 kHz. However, the broadening of SC spectra to the longer wavelengths was limited by the multiphoton absorption of Se-Se. Previous work has demonstrated the potential of Ge-Ga-Te [120], Ge-Te-I [118], and Ge-Te-AgI [123; 124] glass systems without Se for FIR applications. Iodine can trap free electrons from tellurium, helping to form more stable glass. Moreover, the high atomic weight of iodine, which is a neighbor of Te in the periodic table, allows the low phonon characteristics of the glass matrix to be maintained and the FIR transparency to be retained, while providing improved rheological properties. It has been shown that the nonlinearity of ChG glasses could be enhanced when doped with Cu or Ag [125]. A broad MIR SC spanning from 2.0 to 16.0 \(\upmu\)m (Figure 5) was successfully achieved when a Ge-Te-AgI fiber with double cladding and a minimal loss of 6 dB/m at 11 \(\upmu\)m was pumped by OPA in the normal dispersion regime at 7 \(\upmu\)m [126]. This is the broadest MIR SC generation obtained experimentally in optical fibers. This demonstration has also revealed that SC spectra generated from Te-based ChG glass fibers can emit across the entire MIR molecular fingerprint region. Table 2 lists the partial results of SC generation in ChG step-index fibers. It can be seen that the spectral bandwidths of SCs generated in ChG step-index fibers are limited when pumped with shorter laser pulses. In contrast, when fibers are pumped with pulses longer than 6.0 \(\upmu\)m, the long wavelength edges of generated SCs can be extended to \(>\)12 \(\upmu\)m. In addition, wider SC spectrums can be obtained in ChG fibers with higher refractive indexes. It is worth noting that the maximum average power of a generated SC is generally low, limited to the order of mW, or even \(\upmu\)W, due to the low pump  
repetition rate [105]. However, SC sources require not only wide bandwidths but also high brightness for practical applications. The output power of generated SCs from ChG fibers is usually limited by the average power of the input pump pulse, and altering the pump source parameters can scale up the input average and further increase the output power. Two common approaches can be used for power scaling: increasing the repetition rate while maintaining pulse duration and peak power, and increasing the pulse duration while maintaining the other parameters.