# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 5 Pulsed Phosphate Fiber Lasers and Amplifiers  
Pulsed fiber laser operation is generally obtained through modulation of the laser cavity using several Q-switching and mode-locking techniques. Owing to the very large spectral bandwidths achievable from RE ions in glass hosts, pulsed fiber lasers can operate in nanosecond, picosecond and even femtosecond regimes using chirped pulsed amplification techniques. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Fiber Type** & **RE Doping** & **\(\uplambda_{\text{pump}}\)** & **\(\uplambda_{\text{signal}}\)** & **Output** & **Slope** & **Gain** & **Linewidth** \\
**[Reference]** & **Concentration** & **(mm)** & **(mm)** & **Power (mW)** & **Efficiency (\%)** & **Length (cm)** & **(kHz)** \\ \hline Yb\({}^{3+}\)-doped [69] & 15.2 wt \% & 976 & 1064 & 400 & 72.7 & 0.8 & \textless{}7 \\ \hline Yb\({}^{3+}\)-doped [20] & 6 wt \% & 915 & 976 & 100 & 25 & 2 & \textless{}3 \\ \hline Yb\({}^{3+}\)-doped [75] & 15.2 wt \% & 976 & 1014 & 164 & 21.9 & 0.5 & \textless{}7 \\ \hline Yb\({}^{3+}\)-doped [70] & 18.3 wt \% & 976 & 1083 & 100 & 29.6 & 1.8 & \textless{}2 \\ \hline Yb\({}^{3+}\)/Er\({}^{3+}\) & 8 wt \% Yb\({}_{2}\)O\({}_{3}\) & 975 & 1538 & 550 & 12 & 7 & \textless{}60 \\ \hline Yb\({}^{3+}\)/Er\({}^{3+}\) & \(8.6\times 10^{20}\) Yb\({}^{3+}\)/cm\({}^{3}\) & 976 & 1550 & 1600 & 5 & - & 5.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main results obtained with CW single-frequency phosphate fiber lasers based on highly RE-doped phosphate glasses. Q-switching is a technique mostly applied for the generation of micro and nanosecond pulses of high energy and peak power, while the mode-locking technique enables ultrafast laser pulses in the picosecond and femtosecond ranges. In 1993 a Q-switched fiber laser based on a multicomponent Nd-doped phosphate glass was reported with 2.2 \(\upmu\)J, 2 ns duration pulses at up to 2 kHz repetition rate with only 22 mW of absorbed pump power at 812 nm from a laser diode [80]. Tunability over a 40 nm tuning range around 1060 nm was demonstrated thanks to the broad emission line of Neodymium in the phosphate glass. More recently, in 2004 [81], a single-frequency all-fiber Q-switched laser at 1550 nm was reported with 25 W of peak power in 12 ns, 0.3 \(\upmu\)J pulses at a repetition rate of 80 kHz with 370 mW of pump power, without using an amplifier. The laser consisted of a 2 cm-long Yb/Er co-doped phosphate glass fiber fusion spliced between two FBGs inscribed in silica fibers. The fiber was actively Q-switched by fast stress-induced birefringence modulation, a technique that allows an all-fiber configuration, thus avoiding the use of bulk components in the cavity responsible for the growth of loss, size and complexity. By using 2 cm-long highly (6 wt %) Yb\({}^{3+}\)-doped phosphate fiber the first compact, single-frequency all-fiber Q-switched laser at 1 \(\upmu\)m was reported in 2007 [82]. The fiber was actively Q-switched by fast stress-induced birefringence modulation [81], a technique previously demonstrated at 1.5 \(\upmu\)m. A repetition rate up to 700 KHz was reported, with a maximum average power of 31 mW. A maximum average pulse energy of 0.4 \(\upmu\)J was obtained. Concerning fiber lasers operating in the picosecond regime, mode-locking techniques are generally used. Passive mode-locking is usually preferred to avoid costly modulators in the laser cavity as those commonly employed in active mode-locking solutions. However, passive mode-locking of fiber lasers allows achieving low repetition rates (tens of MHz), as a results of the long cavity length. Thus, an attractive approach to push the pulse repetition rate into the GHz regime is the use of short Fabry-Perot lasers with high gain RE-doped phosphate fiber and a small saturable absorber that could be integrated with fiber-end. In 2007 [83] a mode-locked short cavity 10 GHz phosphate fiber laser emitting at 1535 nm was reported using a 1 cm-long heavily Yb/Er co-doped phosphate fiber and a saturable absorber constituted by single-wall carbon nanotubes.