High brightness 1018  nm monolithic fiber laser with power scaling to >500  W

We have studied 1018 nm high power monolithic fiber lasers to be used as pump sources for multi-kilowatt (kW)-level in-band master oscillator power amplifier (MOPA) systems. The performance of the cavity in terms of wavelength selectivity, optical efficiency, and robustness against amplified spontaneous emission (ASE)-induced parasitic lasing is analytically investigated with respect to doped fiber length and cavity parameters. Experimentally, we have demonstrated a 1018 nm fiber laser with an output power of 502 W from a 15/130 µm Yb³⁺-doped double-cladded fiber, enabling, to our knowledge, the highest reported brightness and radiance density as well. The laser cavity has a slope efficiency of ∼76%, ASE suppression of >50dB, and a near-diffraction-limited beam quality of M²∼1.15. Owing to high brightness with a small core size (i.e., high radiant density), the proposed laser is promising to enable high-power in-band pump integration via fused combiners for multi-kW in-band MOPA systems.