# 40 W All-Fiber Er/Yb MOPA System Using Self-Fabricated High-Power Passive Fiber Components
## 1 Introduction
Present impressive output power is 297 W at a wavelength of 1567 nm in the case of fiber laser (pump power of 1.2 kW at 975 nm, a slope efficiency of approximately 40% at low power and 19% at high power, respectively) [12], and 151 W at a wavelength of 1563 nm in the case of MOPA setup (pump power of 470 mW at 975 nm, a slope efficiency of approximately 35% at low power and 29% at high power, respectively) [13]. Most recent result showed that 110 W at 1556 nm was achieved with pump power of 250 W at 940 nm with a slope efficiency of approximately 46% [14]. In Reference [14], a free-space setup was used to monitor signals and couple seed signals from single-frequency laser with a passive SM fiber at the output with an SM fiber at the amplifier setup. However, because of that free-space solution, only 1.2 W from 2 W power of seed signal was reaching the active fiber. Although all above setups [7; 12; 13; 14] are using bulk optics components to take the full advantage of MOPA configuration, one should avoid those kinds of components in optical paths, which constitute the entire setup in the so-called all-in-fiber configuration. In this type of configurations, all bulk optics components (e.g., mirrors, lenses) are replaced by passive fiber components, which make setups significantly less complex, robust and immune to external factors, such as vibrations, contamination and long-term thermal drift of optomechanic components; therefore, it does not require regular adjustments. It is also much easier to maintain high beam quality in all-fiber setups, because it is not leaving the waveguide (e.g., fiber core) in the whole MOPA setup [15; 16; 17; 18; 19; 20; 21; 22]. It has been shown that optical power of 100 W at a wavelength of 1566.5 nm (pump power of 360 W at 965 nm) with a slope efficiency of approximately 29% was achieved in an all-fiber laser configuration [23]. In the case of the all-fiber MOPA configuration, 56.4 W at 1550 nm (pump power of 150 W at 976 nm) with a slope efficiency of approximately 37.0% was also obtained [24]. A single-stage Yb-free Er-doped amplifier setup was presented in Reference [25] with impressive power of 103 W at 1585 nm (pump power of 275 W at 976 nm) with a slope efficiency of 37%. Table 1 presents the most important parameters of systems mentioned above. The most important components of all-fiber construction techniques of high-power sources are mode field adaptors (MFAs) and fiber power combiners [20; 21; 22; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40]. MFAs enable signal transmission between two fibers with different mode-field diameters. In general, it means that such a component connects two amplification stages based on different types of fibers (e.g., SM-DC and LMA-DC). Power combiners are very effective components, which are delivering the pump radiation into the inner clad of the active DC fiber and sending the signal into its SM- or LMA-type core. In the case of a power combiner used in a laser setup, it consists of N multimode fibers at the input and one passive DC fiber at the output, which forms a configuration N \(\times\) 1. In the case of a power combiner used in an amplifier setup, additional signal feed-through fiber is necessary at the input configuration (N + 1) \(\times\) 1. Reliable design and fabrication processes of those components are essential for efficiency of fiber lasers and amplifiers, and their reliability and stable operation in different working conditions. Currently available commercial components are often not compatible with geometrical dimensions of fibers used in experimental amplifiers and laser setups. A splice between non-matched fibers can cause leaking of high power radiation, leading to point overheating and even damaging the fiber and the setup. It is often a problem to match commercial components to the fibers used in the experimental setup, which is still under investigation by many research teams [20; 21; 22; 26; 27; 28; 29; 30].