In the recent years many optical devices have been replaced by new architectures. In particular, fiber lasers have been the focus of intense researches due to their advantages spanning from high beam quality, large wavelength tunability to robustness and low cost. Work on fiber lasers started at the Southampton University in England in 1985 by Sir David Payne and co-workers. Subsequently, the need for even higher output power has pushed the scientific community towards the use of optical fibers doped with rare-earth elements such as ytterbium and thulium. The fiber technology has grown mature and can provide a platform for fabricating robust laser systems.