High power fiber laser systems have attracted extensive attention due to compactness, good beam quality, efficient heat dissipation and high conversion efficiency. They are widely used in industrial processing, military, medical treatment and other fields. Over the past two decades, the development of double cladding fiber and high-brightness laser diodes has greatly improved the output power of fiber lasers. Unfortunately, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), limit the further enhancement of the output power of fiber lasers. Apparently, increasing the core diameter is the most common way to suppress NLEs in the fiber, but this causes another limiting factor i.e. mode instability (MI), resulting in deterioration of the beam quality and limitation of the power scaling. Therefore, it is important and urgent topic for suppression of NLEs and MI simultaneously in fiber lasers. The M-type fiber, by designing refractive index profile, breaks the stringent trade-off between mode area and numerical aperture (NA), so it possesses a larger mode area compared with the step index fiber, which helps to avoid NLEs and expand the power range. The M-type ytterbium doped double-clad fiber was fabricated by the modified chemical vapor deposition (MCVD) process with solution doping technology (SDT), the core/cladding diameter is 25/400 μm. The NA of high index ring and index dip in the core are 0.054 and 0.025, respectively. To test the performance of the M-type fiber during high-power operation, a 976 nm bidirectional pumped all-fiber amplifier was constructed. As a result, a maximum output power of 2285 W was achieved with an optical-to-optical conversion efficiency of 66.5% under bidirectional pumping scheme, and the measured M2 factor is 1.42, the central wavelength and 3 dB linewidth of output laser are 1080 nm and 3.01 nm, respectively. To the best of our knowledge, this is the highest output power in a continuous-wave fiber laser employing a M-type fiber at present. However, the MI effect was observed at an output power of 2252 W. Future work will optimize the structure of the M-type fiber to achieve a stable laser output with higher power and higher efficiency.
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