8kW single-mode ytterbium fiber laser with beam quality M2-parameter < 1.1 and SRS-to-signal ratio < -40dB using direct diode pumping

We report an 8 kW single-mode (SM) ytterbium (Yb)-doped fiber laser with a beam quality parameter M2 < 1.1 and an SRS-to-signal ratio below -40 dB, achieved using direct diode pumping in a compact 4U rack. To the best of our knowledge, this represents the highest output power demonstrated for fiber lasers with diffraction-limited beam quality using direct diode pumping, as well as the highest laser beam brightness achieved for both direct and tandem pumping configurations. The SM Yb fiber laser consists of a Yb fiber generator operating in a Fabry-Perot (FP) configuration at 1070 nm with an output power of 450 W, followed by a high-power fiber amplifier. Both stages are bidirectionally pumped with IPG laser diodes. The laser includes a 3 m output delivery fiber terminated with an IPG HLC-16 connector. The electro-optical efficiency of the laser exceeds 37%. At the maximum output power of 8.2 kW, the SRS-to-signal ratio remains below -40 dB, and the linewidth at the -3 dB level is 6.6 nm. The measured beam quality parameter M2 remains below 1.1 over the entire power range.