# Brief Review of Recent Developments in Fiber Lasers
## 3 Rare Earth-Doped Fiber Lasers
### Power Evolution of RE-Doped Fiber Lasers  
The rapid development of active RE-doped optical fibers has led to the development of high-power lasers with output powers of tens or hundreds of watts, sometimes even several kilowatts, from a single fiber. High-power laser sources are widely used in industrial precision processing. They can serve as a new platform for strong-field physics research using peak power over petawatt.  
As noted above, Yb\({}^{3+}\) ions are widely used in fiber lasers. Since they have only two manifolds, they are free from a number of cooperative effects. The quantum efficiency of Yb\({}^{3+}\)-doped fiber lasers is close to 100% due to the low energy difference between the pump and laser photons. The Yb\({}^{3+}\) ions possess a broadband absorption spectrum ranging from \(\sim\)850 nm to \(\sim\)1080 nm, with the peak in the range between \(\sim\)915 nm and \(\sim\)980 nm. The window of emission wavelengths from Yb\({}^{3+}\)-doped fiber lasers is limited to a small band (1050-1160 nm). At this junction, all-fiber laser oscillators operating at \(\sim\)1000 nm are well developed. Their power evolution throughout the past several years is presented in Figure 3 [37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48].  
As can be seen in Figure 3, the maximum power \(\sim\)8.5 kW was reached in 2020. Fiber lasers with wavelengths of 980 nm, 1007 nm, 1018 nm, and 1150 nm have attracted significant interest since they are very useful in nonlinear frequency conversion and efficient pumping of other RE-doped fibers which have absorption bands located near these wavelengths. A 1.3 kW, 1018 nm fiber laser with a beam quality \(M^{2}\) less than 1.1 was reported by IPG Photonics [49].  
The Er\({}^{3+}\)-doped and Er\({}^{3+}\)-Yb\({}^{3+}\) co-doped fiber lasers partly complement the emission range of 1530-1590 nm. The power evolution of these fiber lasers at \(\sim\)1500 nm throughout the past several years is presented in Figure 4 [50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61]. The highest power achieved with Er\({}^{3+}\)-doped fibers with a relatively multimode operation (\(M^{2}\)\(\sim\) 10.5) is 656 W [56].  
The Tm\({}^{3+}\)-doped and Tm\({}^{3+}\)-Ho\({}^{3+}\) co-doped fiber lasers have an emission range between \(\sim\)1900 nm and \(\sim\)2100 nm. The output power \(\sim\)1.2 kW at \(\sim\)2000 nm was obtained with these lasers in 2018. Their power evolution is presented in Figure 5 [62; 63; 64; 65; 66; 67; 68; 69; 70; 71].  
Recent experiments have demonstrated the single-mode Ho-doped fiber laser providing 407 W of output power [72].  
The Er\({}^{3+}\)- and Tm\({}^{3+}\)-doped lasers are significantly limited in power and efficiency compared to Yb\({}^{3+}\)-doped fiber lasers. As one can see in Figures 4 and 5, the erbium lasers are limited to a few hundred watts; the thulium lasers have achieved power levels of around a kilowatt. Greater quantum defects, difficulty in increasing the ion concentration, excited state absorption, and cooperative effects prevent the erbium and thulium dopants from competing in the output power with ytterbium.  
Although fiber laser systems exceeding 100 kW are commercially available [73], there is still a lot of room for further development of the RE-doped fiber lasers with excellent optical properties.  
Figure 4: Power evolution of Er\({}^{3+}\)-doped and Er\({}^{3+}\)-Yb\({}^{3+}\) co-doped fiber lasers operating at \(\sim\)1.5 \(\upmu\)m [50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61].  
Figure 3: Power evolution of all-fiber laser oscillators operating at \(\sim\)1000 nm [37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48].