Benefitting from advanced fiber fabricating processes, fiber device manufacturing processes, and double-cladding pumping technology, high-power fiber lasers have been developed rapidly[1,2]. Since the discovery of transverse mode instability (TMI) in 2010, stimulated Raman scattering (SRS) and TMI have been the most important factors limiting the power scaling of fiber lasers[3–8]. It is necessary to increase the core diameter and shorten the fiber length for SRS suppression, but the suppression of TMI needs a reduction in the core diameter and increase in the fiber length. Thus, it is difficult to balance TMI and SRS at the same time. In order to realize the simultaneous suppression of TMI and SRS, substantial research work has been carried out, including the design of new fiber structures[9–14] and the optimization of laser system parameters and structures[15–18]. From the perspective of laser structure, high-power fiber lasers are mainly divided into two types: fiber amplifiers based on a master oscillation power amplification (MOPA) structure and fiber laser oscillators based on a Fabry–Pérot (FP) cavity structure. For the fiber amplifier, a simple structure with fewer fiber devices in the amplifying stage produces a higher optical-to-optical (O–O) efficiency. As early as 2009, IPG Photonics realized a 10-kW near-single-mode fiber laser based on the MOPA structure. However, the fiber amplifier is more sensitive to the back reflected light, and it is difficult for such lasers to be deployed in industrial applications such as laser cutting, with the presence of strong reflections from the target. In contrast, the fiber laser oscillator based on FP-fiber cavity has better anti-back-reflected light ability, but the existence of the device insertion loss and output coupler grating loss leads to a relative lower O–O efficiency. Currently, the highest output power of the all-fiber laser oscillator is 8 kW with an O–O efficiency of 81%, and the beam quality BPP (beam parameter product) value is 0.5 mm·mrad.
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