In a high-power fiber amplifier, a frequency-chirped seed interrupts the coherent interaction between the laser and Stokes waves, raising the threshold for stimulated Brillouin scattering SBS. Moving the external mirror of a vertical cavity surface-emitting diode laser 0.2 m in 10 s can yield a frequency chirp of 5 1017 Hzs at a nearly constant output power. Opto-electronic feedback loops can linearize the chirp and stabilize the output power. The simple and deterministic variation of phase with time preserves temporal coherence, in the sense that it is straightforward to coherently combine multiple amplifiers despite a large path-length mismatch. The seed bandwidth, as seen by the counterpropagating SBS, also increases linearly with fiber length, resulting in a nearly length-independent SBS threshold. Experimental results at the 1.6-kW level with a 19-m delivery fiber are presented. A numerical simulation is also presented.