Forward and backward seeded continuous-wave Raman fiber amplifiers based on multimode fibers

We report on the performance of seeded Raman fiber amplifiers based on multimode gradient index fiber. The "beam cleanup" improvement in beam quality of the Stokes beam over that of input pump beam (previously observed in unseeded fiber amplifier geometries using stimulated Raman scattering in multimode graded index fiber) is shown here to be limited by the beam quality of the input seed beam for seeded amplifier configurations. The amplifiers are characterized in terms of their capacity for beam-cleanup, their ability to amplify the seed and in terms of their output spectra. The advantages and disadvantages of a backward-pumped geometry versus a forward-pumped geometry are discussed. Depending on the geometry, amplified power can be readily distributed to a cascade of Stokes frequencies (unseeded forward-pumped geometry) or can be mostly contained in the seed frequency (seeded backward-pumped geometry).