# 40 W All-Fiber Er/Yb MOPA System Using Self-Fabricated High-Power Passive Fiber Components
## 1 Introduction  
High-power fiber sources, operating in a 1.55 \(\upmu\)m spectral range in a master oscillator power amplifier (MOPA) configuration, are very attractive alternatives to solid-state lasers in many applications. They have many advantages, such as operation in eye-safe spectral regions, low-transmission losses in atmosphere and telecommunication fibers, and integration of easily available and cost-effective fiber communication components. Furthermore, they can be found in applications, e.g., free-space communications, light detection and ranging systems (LiDAR), range finding and remote sensing [1; 2; 3]. Coherent light sources built in MOPA configurations are advantageous in comparison with classical laser configurations. In this type of the configuration, a low-power high-beam-quality signal laser (master oscillator) is used. Its radiation is gradually amplified by a cascade of fiber amplifiers (typically two or three stages). In such a configuration, almost all parameters of MOPA systems are determined by seed laser, while output power or pulse energy depends on amplification stages. Especially in fiber technology, one can use a very low power seed laser and a few high-gain fiber amplification stages to get high and easily controllable output power. Typically, in a three-stage MOPA system, the first stage for low-power amplification (output power below 1 W) is utilizing active single-mode single-clad (SM-SC) fibers. In this case, an input seed signal and pump power are coupled into the core of the active fiber. The second amplifier stage for medium-power amplification (output power of several watts) can be still based on a single-mode double-clad fiber (SM-DC). Here, the amplified signal is launched into the active core, with the pump power coupled into the inner cladding, of which a cross section area is much larger than the core size. This allows using higher levels of pump power with simultaneously lower optical power density per unit area than that in the case of the active SM-SC fiber. This is why in the third amplifier stage for high-power amplification (output power of dozens or more watts), a large-mode-area double-clad (LMA-DC) fiber with a larger inner cladding should be used. Such a fiber is capable to handle both: high-power pump and signal transmission. In order to achieve required performance (beam quality, line width, wavelength tuning range, pulse duration, modulations, etc.) in the case of standard fiber-laser approaches, high-power/high-energy components should be used, which can be difficult to fabricate, especially for the use in pulse applications. It is because of technological limitations of fiber components, such as modulators, couplers and isolators, due to their high-power/high-energy pulse handling capability. Therefore, it is very profitable to modulate the low-power signal laser or use a modulator and any additional components between the signal laser and the first amplifier stage, where power intensities are much lower and have no impact on system efficiency. Emission in the eye-safe spectral range can be achieved by using active Er-doped SM-DC fibers; however, the output power of setups using this type of active fibers is limited, due to effects, such as clustering of erbium ions, excited state absorption, pair-induced quenching etc. [4; 5; 6]. To overcome these problems, active LMA-DC fibers can be used, which will result in efficient output power scaling. In Reference [7], [PERSON] et al. have shown that using active Er-doped LMA-DC fibers allowed achieving continuous-wave (CW) output power of 67 W at 1570 nm. In the case of purely Er-doped active fibers used in laser- and amplifier-enabled setup, further scaling of their output power can be achieved by resonant pumping [8; 9; 10]. However, the most common approach is co-doping active Er-doped fibers with ytterbium ions. This well-known nowadays co-doping technique allows achieving significantly higher output power in comparison with purely Er-doped fiber lasers and amplifiers [11].