A high-power narrow-linewidth fiber laser system includes a seed oscillator with multiple resonant cavities and an amplifier stage. The seed oscillator includes a gain fiber, a pump source to introduce pump light into the gain fiber, a single-mode output fiber arranged at the end of the active gain fiber, a first resonant cavity including the active gain fiber, and a second resonant cavity including the active gain fiber. The first and second resonant cavities cooperate to minimize the synchronization of longitudinal modes and thereby reduce modal beating. The amplifier preferably includes an active multimode gain fiber capable of supporting a single fundamental mode at the signal wavelength, wherein the single mode output fiber of the seed oscillator and the multimode gain fiber of the amplifier are mode-matched and coupled without a mode converter.