# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 3 CW Phosphate Fiber Lasers and Amplifiers  
RE-doped glass fibers are excellent gain media for obtaining high power lasers with enhanced brightness. The large surface-to-volume ratio of active fibers favors heat dissipation to the surrounding, allowing active fibers to operate at very high powers. The heat dissipation further increases by using efficient RE dopants, such as Ytterbium, which allows operation with pump wavelengths close to the signal wavelength [23]. As a consequence, beam distortion is negligible and the beam quality is mainly affected by the fiber geometry. In addition, the waveguide structure confines the pump light over the entire fiber length, enabling noticeably large single-pass gains to be achieved due to the long interaction path of the light with the active medium. This leads to a very efficient operation of the fiber lasers, showing very high gain and low pump threshold values. Nowadays, high power fiber lasers and amplifiers are typically implemented using RE-doped double-cladding (DC) optical fibers, a concept which was introduced in 1988 by [PERSON] et al. [36] and has allowed in last decades a significant enhancement in scaling fiber lasers to higher power levels. DC fibers consist in a RE-doped core surrounded by a much larger inner cladding, itself surrounded by an outer cladding (see Figure 4). The pump light is launched and confined into the inner cladding and spatially overlaps the core. The pump power is then absorbed over the entire length of the fiber through multiple interaction with the core, thus exploiting the fiber geometry. The benefit of this design resides in the possibility to use multi-mode high power laser diodes with relatively poor beam quality and to obtain a single-mode fiber radiation with outstanding beam quality. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Ytterbium** & **Neodymium** & **Erbium** \\ \hline Absorption cross-section (cm\({}^{2}\)) & \({}^{2}\)F\({}_{7/2}\)\(\rightarrow\)\({}^{2}\)F\({}_{5/2}\) & \({}^{4}\)I\({}_{9/2}\)\(\rightarrow\)\({}^{2}\)H\({}_{9/2}\)\(+\)\({}^{4}\)F\({}_{5/2}\) & \({}^{4}\)I\({}_{15/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{13/2}\) \\  & \({}^{2}\)F\({}_{7/2}\)\(\rightarrow\)\({}^{2}\)F\({}_{5/2}\) & \({}^{4}\)I\({}_{9/2}\)\(\rightarrow\)\({}^{2}\)H\({}_{9/2}\)\(+\)\({}^{4}\)F\({}_{5/2}\) & \({}^{4}\)I\({}_{15/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{13/2}\) \\ \hline Emission cross-section (cm\({}^{2}\)) & \({}^{1}\).4 \(\times\)\(10^{-20}\) & \({}^{2}\).4 \(\times\)\(10^{-20}\) & \({}^{-}\).7 \(\times\)\(10^{-21}\) \\ \({}^{2}\)F\({}_{5/2}\)\(\rightarrow\)\({}^{2}\)F\({}_{7/2}\) & \({}^{4}\)F\({}_{3/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{11/2}\) & \({}^{4}\)I\({}_{13/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) \\ \hline Doping level (ions/cm\({}^{3}\)) & 1.4 \(\times\)\(10^{21}\) & \(\sim\)2.7 \(\times\)\(10^{20}\) & 4.0 \(\times\)\(10^{20}\) \\ \hline Excited state lifetime (ms) & 1.2 & 0.3 & 8.0 \\ \({}^{2}\)F\({}_{5/2}\)\(\rightarrow\)\({}^{2}\)F\({}_{7/2}\) & \({}^{4}\)F\({}_{3/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{11/2}\) & \({}^{4}\)I\({}_{13/2}\)\(\rightarrow\)\({}^{4}\)I\({}_{15/2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical spectroscopic parameters of common rare-earth (RE) ions in highly doped phosphate glasses [8; 11; 14; 25; 32; 33; 34; 35]. Thanks to DC fiber geometry, power scaling of silica-based fiber lasers has experienced noticeable progress in the last two decades, reaching 100 kW of output power in the 1 \(\upmu\)m wavelength region from a multi-mode Yb\({}^{3+}\)-doped fiber [37] and 10 kW from a single-mode fiber laser [38]. In eye-safe region, at 1.5 \(\upmu\)m, 297 W from an Yb\({}^{3+}\)-Er\({}^{3+}\)-doped fiber laser was demonstrated [39], while at 2 \(\upmu\)m 1 kW of emitted power was reported from a Tm\({}^{3+}\)-doped fiber laser [40]. Power scaling of silica fiber lasers is ultimately limited by two main mechanisms: nonlinear effects (especially SBS) induced by the long path lengths of the tightly confined light in the fiber core and photodarkening.