High power fiber lasers have been recently demonstrated at the kilowatt level. The spectral linewidths of these lasers oscillators can exceed 20 nm. Whereas, such broad spectra are fine for many applications, such as materials processing where raw power is the primary requirement, other applications, including coherent beam combination, harmonic generation, or gravitational wave detection, require high powers beams with much narrower linewidths. Amplification of narrow linewidth signals in optical fibers is limited by stimulated Brillouin scattering (SBS). We discuss novel fiber designs that limit SBS allowing the amplification of narrow linewidth signals to kilowatt power levels.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.