# Highly Doped Phosphate Glass Fibers for Compact Lasers and Amplifiers: A Review
## 5 Pulsed Phosphate Fiber Lasers and Amplifiers
Stable pulse trains with an output power as high as 30 mW and a repetition rate as high as 10 GHz were achieved. In 2012 another solution for the integrated saturable absorber was proposed: a graphene film deposited on one tip of the active fiber [84]. The obtained laser operated in a stable mode-locked regime with fundamental repetition rate of 7 GHz but with an output power of only 1.2 mW. In 2014 [85] a repetition rate of 12 GHz was demonstrated from a compact and stable all-fiber fundamentally mode-locked fiber laser based on an Yb/Er co-doped phosphate fiber and a semiconductor saturable absorbing mirror (SESAM). A short length of 0.8 cm of high gain (6 dB/cm at 1535 nm) PM fiber was used to form the laser cavity. Several applications like coherent LIDAR, range finding or active remote sensing require, for high precision measurements, not only high peak power laser pulses in the nanosecond time regime but also a narrow linewidth. However, neither directly Q-switching a single-frequency fiber laser nor modulating a CW single-frequency fiber laser with an external modulator can deliver sufficient energy/power for practical applications [86]. A MOPA system is thus required to boost the power of a single-frequency low power seed laser. Power scaling of narrow linewidth fiber laser and amplifiers using single-mode silica fibers is challenging due to the onset of nonlinearities in the fiber, primarily SBS, that build up strongly in case of pulses >1 ns [87]. Therefore, increasing SBS threshold for fiber amplifiers is crucial and several solutions have been proposed to fulfill this scope [88; 89; 90]. One possible approach is to focus on the optical fiber itself, with a reduction of the active fiber length and the increase of the core size, conditions that can be achieved more easily by using active fibers made of phosphate glass. In 2009 [PERSON] et al. [21] proposed the use of a single-mode PM large core highly Er/Yb co-doped phosphate fiber for engineering the final power amplifier stage of a monolithic all-fiber pulsed MOPA laser operating in the C band. Single-mode operation was achieved in such a large core (15 \(\upmu\)m) thanks to the accurate control of the refractive indices of the core and the cladding obtainable in phosphate glasses (core NA = 0.053). The fiber length was 12 cm only, thanks to the high doping level used in the phosphate glass core (15% Yb and 3% Er). Pulse energy of 54 \(\upmu\)J and peak power of 332 W without SBS effects for 153 ns pulses at 1538 nm were demonstrated. In 2010 the same research group, by using 15 cm of a new active fiber with larger single-mode core (25 \(\upmu\)m, core NA = 0.0395) for the power amplifier section, was able to demonstrate a peak power of 1.2 kW for 105 ns pulses at 1530 nm, equivalent to a pulse energy of 0.126 mJ, with transform-limited linewidth and diffraction limited beam quality [91]. In 2012 a pulse energy of 0.38 mJ and a peak power of 128 kW were demonstrated using a multiple stage, short, large core Yb/Er co-doped phosphate fiber, in which a short pulse width of few nanoseconds was used to further enhance the SBS threshold [92]. In fact, SBS is an acousto-optic non-linear effect due to the interaction between photons and phonons in the fiber, therefore employing pulses with duration shorter then phonon lifetime, in applications in which this is allowed, is an additional method to increase the SBS threshold. Further increase in power amplifier performances was reported in 2014 by NP Photonics [93]. An all-fiber pulsed MOPA system with 1 W average power, 200 \(\upmu\)J energy, >10 kW peak power and 20 ns of duration at 1550 nm was demonstrated using a seed phosphate single-frequency fiber laser, amplified by several Er-doped phosphate fibers, with a final power amplifier stage consisting in 11 cm of highly Yb/Er co-doped PM phosphate gain fiber. The same group reported also a pulsed all-fiber MOPA system in the nanosecond regime emitting in the 1064 nm band.