Backward Raman amplification in a plasma waveguide.

Backward Raman amplification of a short laser pulse in a plasma waveguide is demonstrated. With a guided seed pulse of $0.8\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{J}$ energy and a pump pulse of 345-mJ energy in a 9-mm-long optically preformed plasma waveguide, 910-fold energy amplification is achieved. Heating of the plasma by the long pump pulse is identified to be a key issue for plasma-waveguide-based backward Raman amplifiers.