# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 4 CW Single-Frequency Phosphate Fiber Lasers and Amplifiers
### Yb-Doped Single-Frequency Phosphate Fiber Lasers  
In 2004 [PERSON] et al. [68] reported for the first time a DBR laser based on 1.5 cm-long Yb\({}^{3+}\)-doped phosphate fiber that was fusion spliced with two FBGs to form the resonator. 200 mW of single-frequency output power at 1064.2 nm was achieved (gain p.u.l. equal to 2 dB/cm), with a linewidth of approximately 3 kHz. In 2011 [PERSON] et al. [69] demonstrated over 400 mW of emission at 1063.90 nm from a DBR Yb\({}^{3+}\)-doped phosphate fiber laser based on 0.8 cm-long active fiber. The fiber was heavily doped (15.2 wt %) in order to obtain a very short resonator cavity and thus ensure laser operation on single-longitudinal mode. A slope efficiency of about 73% versus launched pump power was obtained, with a net gain coefficient p.u.l. of 5.7 dB/cm. This work reports the highest slope efficiency and output power obtained so far in this kind of single-frequency fiber lasers. Besides the typical lasing wavelength at 1064 nm, Yb\({}^{3+}\)-doped phosphate fibers have been successfully exploited to obtain single-frequency operation at more exotic wavelengths [20; 70; 71], thanks to the broad Yb\({}^{3+}\) emission from 970 to 1200 nm. In particular, during last years, interest for high power single-frequency lasers below 1 \(\upmu\)m has increased, since they are excellent pump sources for second and fourth harmonic generation in the blue and deep UV, which have found several scientific and industrial applications [72]. Yb\({}^{3+}\)-doped phosphate fibers have been successfully employed to achieve single-frequency laser emitting at 976 nm, through the transition from the lowest level of the excited state \({}^{2}\)F\({}_{5/2}\) manifold to the lowest level of the ground state \({}^{2}\)F\({}_{7/2}\) manifold (quasi-three-level operation) (see Figure 1a). Yb\({}^{3+}\)-doped phosphate fibers, compared to Yb\({}^{3+}\)-doped silica fibers, exhibit a larger difference between the absorption and emission cross-sections at shorter wavelengths, making them more suitable for short wavelength laser emission [73]. The first demonstration of an Yb\({}^{3+}\)-doped phosphate fiber laser emitting at 976 nm was reported by [PERSON] et al. [32] in 2006 using 2 cm-long highly doped fiber (1 \(\times\) 10\({}^{21}\) Yb\({}^{3+}\) ions/cm\({}^{3}\)). 250 mW of output power was achieved, with an efficiency versus launched pump power of about 40%. In 2012 the first 976 nm single-frequency all-fiber DBR fiber laser was developed with a 2 cm-long highly (6 wt %) Yb\({}^{3+}\)-doped phosphate fiber [20]. 100 mW of linearly polarized output power was generated with a linewidth of less than 3 kHz. By using this fiber laser as a seeder in a MOPA configuration, a single-frequency Yb\({}^{3+}\)-doped phosphate fiber amplifier was realized. A linearly polarized output power of 350 mW was obtained by core-pumping a 4 cm-long polarization maintaining (PM) phosphate fiber [74]. The birefringence of the PM fiber was made possible by inserting two high stress elements around the fiber core. A small signal net gain of 25 dB, corresponding to a unit gain over 6 dB/cm, was achieved. With the aim to investigate the power scaling of a single-frequency laser operating at 976 nm, [PERSON] et al. fabricated an Yb\({}^{3+}\)-doped phosphate fiber amplifier based on 7 cm-long Yb\({}^{3+}\)-doped DC phosphate fiber cladding-pumped by high power multi-mode laser diodes. 3.41 W single-frequency output power was obtained, with a relatively low slope efficiency of \(\sim\)7% due to the reduced spatial overlap of the pump and the doped fiber core in the cladding-pumped arrangement [73]. Single-frequency emission from an Yb\({}^{3+}\)-doped phosphate fiber was also demonstrated at 1014 nm, which is an interesting wavelength for optical lattice clock applications [75]. A compact fiber laser of 5 mm was reported, with a CW single transverse and longitudinal mode characterized by an output power over 164 mW, low noise and a linewidth lower than 7 kHz. At the wavelength of 1014 nm [PERSON] et al.