# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 2 Rare-Earth Active Ions for Phosphate Fiber Lasers  
Among the different RE ions, Ytterbium is one of the most commonly used for the fabrication of phosphate fiber lasers. The electronic transition \({}^{2}\)F\({}_{5/2}\rightarrow{}^{2}\)F\({}_{7/2}\) (see Figure 1) can generate laser emission over a band of wavelengths ranging from \(\sim\)970 to \(\sim\)1200 nm, thus allowing a wide tuning ability of the output beam in the 1 \(\upmu\)m window and short pulse amplification (down to fs regime) [22]. The absorption band is also very broad, from \(\sim\)850 to \(\sim\)1070 nm, and covers a range where laser diodes have their highest output power and efficiency. Since the pump wavelength is very close to the laser emission one, an Yb\({}^{3+}\)-doped fiber laser displays always a small quantum defect, potentially allowing for very high power efficiencies and reduced overheating effects in high-power regimes [23]. Ytterbium is well-known for the simplicity of its energy level diagram, based on a simple two levels system that reduces the incidence of detrimental processes such as multiphonon relaxation and excited state absorption (ESA). Figure 1a shows a typical Yb\({}^{3+}\) ions energy level diagram with indicative splitting of the Stark sublevels. This splitting allows the ion to work in a three or four levels system, according to the choice of the pump and lasing wavelengths. Figure 1b displays typical absorption and emission spectra of a multicomponent Yb\({}^{3+}\)-doped phosphate glass. Table 1 summarizes the typical absorption and emission cross-sections reported in literature, alongside typical lifetime value of the \({}^{2}\)F\({}_{5/2}\) excited state level. The Neodymium is featured by a much more complex structure. Figure 2a reports the schematic of the energy levels of Nd\({}^{3+}\) ions with the transitions of interest with respect to this review. Although the radiative decay can occur to all the lower lying \({}^{4}\)I\({}_{1}\) levels, the strongest transition is the one to the \({}^{4}\)I\({}_{11/2}\) level, that leads to a maximum peak at a wavelength of 1053 nm in most phosphate glasses. Thanks to the numerous absorption bands in the visible and near-infrared, Nd could be pumped by flash lamps, which were the oldest and most common pumping solution for this ion in phosphate glass host. The first fiber laser in the 1960's was indeed demonstrated in a Nd-doped fiber excited by means of flash lamps [1; 2]. More recently, with the availability of laser diodes at the requested wavelengths, a more efficient and compact pumping scheme has been achieved through the use of AlGaAs pump diode lasers operating at around 808 nm to excite the transition \({}^{4}\)I\({}_{9/2}\rightarrow{}^{2}\)H\({}_{9/2}+{}^{4}\)F\({}_{5/2}\) (see Figure 2a). Despite the large number of closely spaced excited states, Nd\({}^{3+}\) ion does  
Figure 1: (**a**) Typical energy level diagram of Yb\({}^{3+}\) ions in phosphate glasses; (**b**) Typical absorption and emission spectra of Yb\({}^{3+}\) ions in an in-house fabricated phosphate glass. not suffer from ESA. Even though the \({}^{4}\)F\({}_{3/2}\rightarrow{}^{2}\)D\({}_{5/2}\) ESA transition occurs, it is resonant with the photon energy of the 800 nm wavelength laser pump, it is negligible and does not significantly affect the 800 nm pumping [24]. Thanks to the several absorption bands in the visible, an unusual pumping scheme for Nd-doped phosphate lasers by solar radiation was also investigated in the framework of the research on solar pumped lasers [25; 26]. Figure 2b reports typical absorption and emission spectra of Nd\({}^{3+}\) ions in phosphate glasses. Typical cross-section values are reported in Table 1, along with lifetime of the \({}^{4}\)F\({}_{3/2}\) excited state in phosphate glass.