Cascaded Stokes waves generation due to stimulated Brillouin scattering (SBS) of coherent optical pulses in a double-cladding Er/sup 3+/-Yb/sup 3+/ codoped fiber amplifier is reported. The highest attainable output power strongly depends on the amplifier pumping arrangement. A maximum of 40-W peak power has been obtained in counterpumping configuration. The highest energy extracted from the single-mode fiber amplifier in 1-/spl mu/s pulses is limited SBS to 15 /spl mu/J. Theses results have been theoretically confirmed using coupled-waves SBS model.