## 4 Mid-Infrared Fiber Lasers  
Optical fiber has many advantages in numerous fields. This paper mainly discusses the mid-infrared fiber laser with soft glass [fluoride (Er\({}^{3+}\), Ho\({}^{3+}\), Dy\({}^{3+}\)), chalcogenide, telluride] as the gain medium. The description is shown in Figure 11.  
Figure 11: Description diagram of mid-infrared fiber lasers.  
The most-used material for fluoride optical fiber is a multi-component fluoride glass called \"ZBLAN\"; the mid-infrared fiber laser operating at 3-5 um band has a similar outer electron arrangement for gain ions. Energy level transitions between configurations produce abundant emission lines; the gain fiber mainly includes Er3+, Ho3+, Dy3+, and its energy level diagram [75] is shown in Figure 12.  
The chalcogenide glass has excellent mid-infrared transmission, thermal and mechanical properties. Compared with fluoride glass fiber, its phonon energy is lower, which makes up for the defect that ZBLAN is hindered to work at wavelengths exceeding 4 um due to the reduction of high-energy states caused by multi-phonon transitions. In the context of the chalcogenide glass fiber lasers, the ions that have received the most attention are praseodymium and terbium. The energy level diagram [76] is shown in Figure 13.  
For the glass fibers of fluoride, chalcogenide and tellurite, their physical and chemical properties are different, as shown in Table 9.  
Figure 12: Energy level diagram.  
Figure 13: Energy level diagram.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Properties** & **Fluoride** & **Chalcogenide** & **Tellurite** \\ \hline The lowest loss (dB/m) & 0.45 \(\times\) 10\({}^{-3}\) & 0.023 & 0.02 \\ Max. phonon energy (cm\({}^{-1}\)) & 560 & 300–450 & 700 \\ Transparency (um) & 0.4–6 & 1–16 & 0.5–5 \\ Nonlinear refractive index & 2–3 & 300–500 & 59 \\ (\(\times\)10\({}^{-20}\) m\({}^{2}\)/W) & 265 & 250 & 500 \\ Melting point (\({}^{\circ}\)C) & 265 & 250 & 500 \\ Durability & poor & good & good \\ Toxicity & high & high & safe \\ \hline \hline \end{tabular}
\end{table}
Table 9: Physicochemical properties of various soft glass fibers.  
Compared with chalcogenides, the fluoride glass has lower loss but higher phonon energy, and its transparency range is far inferior to chalcogenide's. However, compared with tellurite glass, the fluoride glass and chalcogenide glass are more toxic. Three kinds of glass optical fibers are the best choice for mid-infrared transmission. Their low optical loss and high-power damage threshold make many applications possible.  
The fiber lasers with different gain media have unique advantages and characteristics. The developments are shown in Table 10.  
is relatively mature. However, the realization of mid-infrared laser output with high conversion efficiency and the output energy still needs further development; due to the limited manufacturing process of InF\({}_{3}\) and the telluride, there are still difficulties in general commercial use; chalcogenide glass has excellent transmission performance in the mid-infrared band due to its low material dispersion, so it has an indispensable application value at 3-5 \(\upmu\)m. For the future, it is necessary to optimize the gain fiber, to increase the pump power and to achieve a higher power mid-infrared laser output.