# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 3 CW Phosphate Fiber Lasers and Amplifiers
### Er and Yb/Er Co-Doped Phosphate Fiber Lasers and Amplifiers  
Erbium-Ytterbium co-doped system received noticeable interest in the last two decades due to its use for optical communications and eye-safe laser applications.  
In 2004 [PERSON] et al. reported 9.3 W CW 1535 nm multi-mode output from a 7.0 cm short-length single-cladding Er-Yb phosphate fiber [7]. The fiber featured a D-shaped cladding and an off-centered core 130 and 20 \(\upmu\)m in diameter, respectively. A slope efficiency of 29% and a power per unit length (p.u.l.) as high as 1.33 W/cm were achieved. From another 7.1 cm of Er-Yb co-doped fiber laser, 4.0 W single transverse mode output was generated with lower power per unit fiber length (0.56 W/cm), due to the less efficient pump absorption exhibited by the smaller core.  
In the early 2000's the first Er\({}^{3+}\)-doped fiber amplifiers based on heavily Erbium-doped phosphate fibers were demonstrated [53; 54]. They were core-pumped by single-mode laser diodes emitting at 980 nm. For example, [PERSON] et al. [54] reported a high concentration (3.5 wt %) Er\({}^{3+}\)-doped phosphate fiber amplifier. An internal gain of 23 dB and a gain p.u.l. of 3 dB/cm were achieved in a 71 mm-long Er\({}^{3+}\)-doped phosphate fiber core-pumped by a laser diode at around 980 nm. The fiber, manufactured by the rod-in-tube technique, displayed a single-mode behavior at 1.55 \(\upmu\)m and showed a quite high loss of 0.28 dB/cm at 1.3 \(\upmu\)m. The gain obtained was lower than that predicted by theoretical calculations, due to cooperative up-conversion phenomena between the Er\({}^{3+}\) ions and the small absorption cross-section of Erbium at the pumping band of 980 nm. Thus, soon after the same group proposed the realization of the amplifier using a phosphate fiber doped with a lower amount of Er\({}^{3+}\) (3 wt %) [55], to prevent Erbium concentration quenching, and with Yb\({}^{3+}\) sensitization (2 wt %), to increase the pump absorption. An internal gain of 18 dB and a gain p.u.l. of 5 dB/cm were obtained for small-signal input at 1535 nm, using 3.6 cm-long co-doped fiber pumped at 980 nm in a co-propagating configuration.  
In light of limiting the amplifier cost and power scale the output power, single-cladding phosphate fibers were replaced by DC fibers pumped by broad area high power multi-mode laser diodes instead of single-mode pump sources. Thanks to these improvements, in 2003 an Er\({}^{3+}\)-doped phosphate fiber amplifier showing net gain values of 41, 27 and 21 dB at 1535 nm, 1550 nm and over the C-band, respectively, was demonstrated with only 8 cm-long fiber. The fiber was an all-glass single-mode DCfiber with a loss of 0.09 dB/cm at 1.3 \(\upmu\)m [6], which was significantly lower compared to the previously fabricated fibers.  
In 2008 5 cm-long single-mode heavily Yb\({}^{3+}\)/Er\({}^{3+}\) co-doped fiber was used to demonstrate optical amplification of signal in the range 1525-1565 nm [56]. The fiber, doped with 1 mol% of Er\({}^{3+}\) and 2 mol% of Yb\({}^{3+}\), was dual-pumped by two 976 nm fiber pigtailed laser diodes. A peak net gain of 16.5 dB at 1534 nm was achieved, that corresponds to a gain p.u.l. of 3.3 dB/cm (internal gain p.u.l. of 9.1 dB/cm). The fiber loss was evaluated to be 0.06 dB/cm at 1.3 \(\upmu\)m.  
In 2015 novel high concentration Yb/Er co-doped phosphate glasses (1.08 mol% of Er\({}_{2}\)O\({}_{3}\) and 1.08 mol% of Yb\({}_{2}\)O\({}_{3}\)) were employed to manufacture DC fibers for optical amplification. Core-pumped and cladding-pumped amplifiers were demonstrated, reaching gains p.u.l. of 4.0 and 2.3 dB/cm, respectively [57].  
The main results obtained with highly RE-doped CW phosphate fiber lasers are summarized in Table 2.