Fig. 1. Laser structure and experimental results. (a) Schematic diagram of the 6 kW fiber laser pumped by long-wavelength LDs; (b) output power and optical-to-optical conversion efficiency; (c) measurement in time-domain and frequency-domain at the operation of 6 kW laser; (d) output spectra of seed laser and 6 kW laser; (e) beam quality and beam profile of seed laser and 6 kW laser