# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 3 CW Phosphate Fiber Lasers and Amplifiers
### Nd-Doped Phosphate Fiber Lasers and Amplifiers  
As the first fiber laser was realized using a Nd\({}^{3+}\)-doped fiber, in that case made of silicate glass [1], the first phosphate fiber laser reported in literature was also manufactured with a Nd\({}^{3+}\)-doped phosphate fiber [45]. A length of 10 mm of single-mode and single-cladding optical fiber, manufactured by rod-in-tube technique and highly doped with 3 wt % of Nd\({}_{2}\)O\({}_{3}\) (3.1 \(\times\) 10\({}^{20}\) Nd\({}^{3+}\) ions/cm\({}^{3}\)), was used to demonstrate laser emission at 1.054 \(\upmu\)m, when core-pumped by an 808 nm laser diode. A low threshold of 1 mW and an efficiency of \(\sim\)50% were measured. Lasing was also demonstrated at 1.366 \(\upmu\)m, although with lower performance both in threshold and slope efficiencies, as expected for this less efficient neodymium emission. In 1996 [PERSON] et al. [46] reported a core-pumped Nd-doped multi-mode phosphate fiber, although with a quite low output power of 130 mW at 1053 nm. An efficiency of 31% with respect to the launched pump power was achieved at 807 nm. Concerning power scaling of CW Nd\({}^{3+}\)-doped phosphate fiber lasers, in 2011 [PERSON] et al. [47] reported an output power of 2.87 W at 1053 nm from a length of 26 cm of heavily Nd\({}^{3+}\)-doped (3.5 \(\times\) 10\({}^{20}\) Nd\({}^{3+}\)/cm\({}^{3}\)) DC fiber, with diameters of 14/285/360 \(\upmu\)m for the core, inner cladding and outer cladding, respectively. Average slope efficiencies of 26.2% and 44.7% with respect to the launched and absorbed pump powers at 795 nm were obtained. The reported loss of 1.5 dB/m at 1053 nm is the lowest ever measured for a Nd\({}^{3+}\)-doped phosphate fiber. More recently [48], single-mode operation was reported in a cladding-pumped Nd\({}^{3+}\)-doped fiber laser having an output power of 1.42 W and an efficiency of 34% with respect to the absorbed pump power at 808 nm. A 21 cm-long DC fiber was used, with a small core diameter of 5.3 \(\upmu\)m to allow single-mode operation at 1.053 \(\upmu\)m. In view of improving the coupling efficiency between the pump diode and the active fiber, a novel design for local cooling by fluid sealing of the coupling point was exploited. In 2015 a Nd\({}^{3+}\)-doped phosphate fiber laser was demonstrated by using a DC fiber with an hexagonal inner cladding made by a stack-and-draw technique, that is commonly used for the realization of microstructured optical fibers [42]. Single-mode operation was achieved with a core diameter of 35 \(\upmu\)m, thanks to the low NA of the fiber core. A maximum output power of 2.87 W and a slope efficiency of 19% were achieved. The poor laser performance was due to the high propagation loss of the fiber, as high as 3.8 dB/m, and to the mismatch between the pump wavelength (793 nm) and the Nd-doped glass peak absorption (802 nm). An alternative solution that was recently proposed for the power scaling of Nd\({}^{3+}\)-doped fiber lasers and amplifiers relies in the use of short-length core/cladding canes as active elements. A 60 mm length of cane, with a core diameter of 270 \(\upmu\)m and a cladding diameter of 800 \(\upmu\)m, was able to emit 2 W of output power at 1054 nm with slope efficiencies of 30% and 40% with respect to the launched and absorbed pump powers, respectively. [PERSON]'s analysis revealed that a slope efficiency of 55% over the absorbed power could be achieved in optimal conditions, matching the highest results obtained so far [27]. Although the latter configuration did not operate in single-mode regime, similar can configuration can be employed as a booster amplifier in a MOPA configuration to reach high power while preserving high beam quality [49]. Significant results have been obtained in last years from photonic crystal fibers (PCFs). In 2012 the first non-silica all-solid Nd\({}^{3+}\)-doped PCF was demonstrated, with an hexagonal shaped inner cladding, to achieve high absorption coefficient of pump power [50].