With an increasing output power, the power scaling of monolithic fiber laser oscillators faces the severe limitations of stimulated Raman scattering (SRS) and the transverse mode instability (TMI) effect. In this work, we report a high power monolithic fiber laser oscillator with a maximum output power of 5.2 kW, which is realized with a trade-off design between the SRS and TMI. The monolithic fiber laser oscillator is constructed with ytterbium-doped fiber with a core/inner cladding diameter of 25/400 µm and corresponding home-made FBG. High-power 915 nm laser diodes are employed as a pump source and are distributed in a bidirectional-pump configuration. By optimizing the bidirectional pump proportion, the monolithic fiber laser oscillator is scaled up to 5.2 kW with a slope efficiency of ~63%. Operating at 5.2 kW, the intensity of the Raman stokes light is ~22 dB below the signal laser and the beam quality (M2-factor) is ~2.2. To the best of our knowledge, this is a record high power for monolithic fiber laser oscillators.

With the increasing output power of the monolithic fiber laser oscillators, the stimulated Raman scattering (SRS) effect becomes one of the main limitations of power scaling. Employing fiber with a larger mode area is an effective technique to mitigate the SRS, but, for the monolithic fiber laser oscillators, the difficulty of the inscription of the high-reflection fiber Bragg gratings (FBGs) increases with the fiber mode area. In this work, we demonstrated a high-power monolithic fiber laser oscillator based on the home-made large mode area FBGs and ytterbium-doped fiber (YDF) with 25 μm core diameters. A maximum output power of 4.05 kW is achieved at the central wavelength of ∼1080 nm with a total 915 nm pump power of ∼6.7 kW. At the operation of 4.05 kW, the intensity of the Raman Stokes light is ∼25 dB below the signal laser, and the beam quality (M 2-factor) is ∼2.2. To the best of our knowledge, this is the first detailed report of the monolithic fiber laser oscillator with an output power beyond 4 kW.

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