5 kW Near-Diffraction-Limited and 8 kW High-Brightness Monolithic Continuous Wave Fiber Lasers Directly Pumped by Laser Diodes

Tandem pumping technique are traditionally adopted to develop >3-kW continuous-wave (cw) Yb
³⁺
-doped fiber lasers, which are usually pumped by other fiber lasers at shorter wavelengths (1018 nm e.g.,). Fiber lasers directly pumped by laser diodes have higher wall-plug efficiency and are more compact. Here we report two high brightness monolithic cw fiber laser sources at 1080 nm. Both lasers consist of a cw fiber laser oscillator and one laser-diode pumped double cladding fiber amplifier in the master oscillator-power amplifier configuration. One laser, using 30-μm-core Yb
³⁺
-doped fiber as the gain medium, can produce >5-kW average laser power with near diffraction-limited beam quality (M
²
<1.8). The slope efficiency of the fiber amplifier with respect to the launched pump power reached 86.5%. The other laser utilized 50-μm-core Yb
³⁺
-doped fiber as the gain medium and produced >8-kW average laser power with high beam quality (M
²
: ∼ 4). The slope efficiency of the fiber amplifier with respect to the launched pump power reach 83%. To the best of our knowledge, this is the first detailed report for >5-kW near-diffraction-limited and >8-kW high-brightness monolithic fiber lasers directly pumped by laser diodes.