# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 4 CW Single-Frequency Phosphate Fiber Lasers and Amplifiers  
Fiber lasers considered in the previous Sections display relatively broad linewidth, usually in the 1-10 nm range. However, several applications such as interferometric gravitational-wave detection [58], coherent Light Detection and Ranging (LIDAR) [59], laser nonlinear frequency conversion [60], coherent beam combining [61] and laser spectroscopy [62] require laser emissions with narrower linewidth. This feature can be provided by single-frequency fiber lasers, which are lasers that operate with only a single-longitudinal mode and thus can emit quasi-monochromatic radiation with a very narrow linewidth and low noise.  
Different configurations have been exploited to demonstrate single-longitudinal mode operation. One typical design is the distributed feedback (DFB) fiber laser, in which a FBG is directly written in the core of the active fiber and is responsible for the generation of a phase change in the middle of the grating area.  
Another common configuration is the distributed Bragg reflector (DBR) fiber laser, in which the laser resonator is formed by the RE-doped fiber placed between two FBGs. This configuration is generally preferred for the fabrication of single-frequency fiber lasers thanks to its compactness and robustness [63].  
When an output power higher than 1 W is demanded, a MOPA configuration is used, with a single-frequency seed laser followed by a series of amplifier stages to achieve the power amplification. This configuration is able to power scale the output power while preserving the spectral characteristics of the seed laser.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Fiber Type [Reference]** & \begin{tabular}{c} **RE Doping** \\ **Concentration** \\ \end{tabular} & \begin{tabular}{c} **\(\uplambda_{\text{pump}}\)** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **\(\uplambda_{\text{signal}}\)** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **Output** \\ **Power (W)** \\ \end{tabular} & \
\begin{tabular}{c} **Slope** \\ **Efficiency (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Gain** \\ **Length (cm)** \\ \end{tabular} \\ \hline Yb\({}^{3+}\)-doped & \(12\) wt \% & 940 & 1064 & 20 & 34.4 & 84.6 \\ Yb\({}^{3+}\)-doped & \(12\) wt \% & 977 & 1064 & 57 & 56.7 & 71.6 \\ single-mode [43] & \(3.5\times 10^{20}\) Nd\({}^{3+}\)/cm\({}^{3}\) & 795 & 1053 & 2.9 & 44.7 & 26 \\ Nd\({}^{3+}\)-doped & \(3.5\times 10^{20}\) Nd\({}^{3+}\)/cm\({}^{3}\) & 808 & 1053 & 1.4 & 34.1 & 21 \\ Nd\({}^{3+}\)-doped & \(3\) wt \% & 793 & 1053 & 15.5 & 57 & 25 \\ single-mode PCF [52] & \(8.6\times 10^{20}\) Yb\({}^{3+}\)/cm\({}^{3}\) & 976 & 1535 & 9.3 & 39 & 7.0 \\ Yb\({}^{3+}\)/Er\({}^{3+}\) co-doped & \(8.6\times 10^{20}\) Yb\({}^{3+}\)/cm\({}^{3}\) & 976 & 1535 & 4.0 & 39 & 7.1 \\ single-mode [7] & \(1.1\times 10^{20}\) Er\({}^{3+}\)/cm\({}^{3}\) & \multicolumn{1}{c}{} & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Main results obtained with continuous wave (CW) fiber lasers based on highly RE-doped phosphate glasses.  
The use of highly doped phosphate glass fibers for the manufacturing of single-frequency fiber lasers is particularly advantageous since allows the realization of a short cavity that creates large longitudinal mode spacing, helping to maintain lasing on a single-longitudinal mode and thus a single-frequency behavior.  
Power scaling of single-frequency fiber lasers is ultimately reduced by SBS and is much more arduous than that of a multi-longitudinal mode CW fiber laser. Several different methods have been implemented to limit the SBS in silica-based fiber lasers [64; 65; 66]. The use of phosphate glass-based optical fiber represents an alternative option to overcome this limitation.  
CW single-frequency phosphate-based fiber lasers operating at 1 and 1.5 \(\upmu\)m have reached a relatively high maturity level and are commercially available [67].