# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 1 Introduction  
Fiber lasers are nearly as old as the glass laser itself. The first demonstration of a fiber laser dated back to the 1960s, when [PERSON] and [PERSON] demonstrated laser action in a neodymium-doped fiber [1, 2]. Nowadays, fiber lasers are not only one of the fastest growing photonic technologies, but they are also a sound industrial reality that is rapidly eroding the markets shares of other laser technologies. They find applications in a variety of fields ranging from industrial material processing, medical applications, semiconductor device manufacturing, remote sensing and scientific instrumentation. The key advantages of fiber laser reside in their waveguide geometry. Indeed, a low threshold and a high conversion efficiency are possible thanks to the tight confinement of the pump and the generated signal. First of all the tight confinement of both pump and laser light results in a low threshold and a high optical conversion efficiency. Moreover, optical fibers exhibit an enhanced heat dissipation capability thanks to their high surface-to-volume ratio and the distribution of the thermal load over a considerably long length, which facilitates unprecedented power scaling capacity. The whole laser resonator can be realized using fiber components only, such as Fiber Bragg Gratings (FBGs) and fibercouplers. This all-fiber configuration avoids the use of free-space optics, thus allowing a robust and compact system design that facilitates the usability of fiber lasers outside the laboratory. Another advantage of fiber laser is that beam handling and beam deliveries become inherently simple thanks to the fiber based configuration [3,4]. Another important aspect of fiber laser technology is the fact that is extremely versatile and can be adapted to different laser applications, by customizing the output power, beam quality and dimension, spectral properties, output stability and temporal output properties. Moreover, the very large spectral bandwidths achievable from rare-earth (RE) ions in glass allow fiber lasers to operate from the continuous wave (CW) regime down to pulse durations of few femtoseconds. The fiber geometry is indeed the origin of the main advantages of fiber lasers; however, it is also responsible for the main drawbacks of such devices. The enforcement of deleterious nonlinear effects, such as self-phase modulation (SPM), stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), is due to the confinement of the laser radiation in a small area and to the long interaction length. These effects constitute the main restriction of RE-doped fiber laser systems [3]. Optical fibers for lasers and amplifiers are drawn from glasses and several glass systems suitable for photonic applications are well-known, with a variety of chemical, physical and optical properties [5]. However, most of the research work performed so far on fiber lasers has been performed on doped silica glasses, thanks to their outstanding properties. Silica is featured by the lowest propagation losses (0.2 dB/km), can withstand high temperatures and shows exceptional mechanical strength and chemical durability. However, a major drawback of silica glass is the poor solubility of RE ions, and the consequent tendency to cluster. This causes deleterious effects on the spectroscopic properties of the material. Multicomponent phosphate glasses have demonstrated in last years to be a promising contender to silica glass as a fiber material, especially for the realization of compact active devices [6; 7; 8]. The main reason is that, thanks to the very open and disordered matrix structure [9; 10], they can withstand very high doping level of rare earth-ions (up to \(10^{21}\) ions/cm\({}^{3}\)), typically about 50 times higher than in the more rigid glass matrix of silica glass.